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01 cognitive neuroscience introduction
- 1. Introduction to Cognitive Neurophysiology
- 4. Spectrum of Cognition Nano Meta Para Macro Micro
- 5. Cognitive Science Neuro Science Psycology Philosophy Computer Science Genomics
- 6. Philosophy : Mind behind Mind Psychology : Mind Neuroscience: Brain Three States of Cognition
- 7. Evolution & Cognition “ Cognition is survival instinct a consequence of carefully crafted modules dedicated to solving specific evolutionary problems”
- 13. Galen 130-200 AD
- 14. Brain as hollow organ : Nemesius (circa 320),
- 15. Leonardo Da Vinci April 15, 1452 – May 2, 1519
- 16. Andreas Vesalius (1514-1564 CE)
- 17. Phrenology 1806
- 18. Lobar Localization Paul Broca 1868
- 19. Brodmann’s area
- 23. Imaging of brain CT Scan
- 24. MRI Brain
- 25. fMRI
- 26. PET scan
- 27. Descartes, Brain and Mind (1596-1650)
- 29. Discovery of Neuron Ramony Cajal and Camillo Golgi 1906 Noble
- 30. Nerve Cell
- 34. 1 Functional Organization of NS
- 35. 1 Structural Organization of NS
- 36. 2 Nerve signal processing
- 37. 3. Sensory Signal Processing Laws of specific sense energies – Muller 1826 “ Each nerve fiber is activated primarily by a certain type of stimulus and each makes specific connections to structures in the central nervous system whose activity gives rise to specific sensations”
- 38. 4. Motor Control Voluntary Involuntary
- 39. 5. Consciousness, Sleep, Emotion Reproduction
- 40. 6. Higher Cognitive Functions: Language, Memory
- 41. 7. Brain Development and Genetics
- 44. Selected Reading
Notes de l'éditeur
- The Major Goal of Cognitive Neural Science Is to Study the Neural Representations of Mental Acts. This lecture is an introduction to Cognitive neurophysiology for Autumn Semester for the International Institute of Information technology Hyderabad 2010. Cognitive neuroscience is a burgeoning field of Neuro and Cognitive science, biological basis of cognition. Overview Perhaps the major reason that neuroscience remains such an exciting field is the wealth of unanswered questions about the fundamental structure and functions of the human brain. To understand this remarkable organ (and the rest of the nervous system), the myriad cell types that constitute the nervous system must be identified, their interconnections traced, and the physiological role of the resulting circuits defined. Adding to these several challenges is the fact that a specialized anatomical vocabulary has arisen to describe the structure of the nervous system, as well as a specialized set of physiological terms to describe its functions. In light of these conceptual and semantic difficulties, comprehending the brain and the rest of the nervous system is greatly facilitated by a general picture of the organization of the nervous system, and by a review of the basic terms and anatomical conventions used in discussing its structure and function.
- Cognition is not a specific term and definition varies. If you look at the dictionary meaning of Cognition, you can find various definition, most of them are related to acquiring and processing knowledge and higher mental process of reasoning, judgment. MeSH (Medical Subject Heading) has defined cognition as an Intellectual or mental process whereby an organism become aware of or obtain knowledge. However C ognitive disorder is defined as disturbances in the mental process related to thinking, reasoning, and judgment.
- There are various synonyms for cognition, most of them related to knowledge and perception.
- Cognition can be subatomic or nano-cognition, still very little is known and we are trying to understand more. Micro-cognition or molecular level cognition is what we see in unicellular organism without brain at genetic and molecular level learning. Macrocognition is at tissue level and nervous tissue has specially developed for this purpose. Meta-cognition is more complex and higher level cognition probably developed in human to think about how we think. Paracognition is more metaphysical cognition and very little is known about the basis but can be realized. By parcognition we create new ideas and theory while metacognition is manipulating information already existed.
- Cognitive science is like an elephant with different specialties look at it from different angle but do communicate with each other to have overview.
- There is close correlation between neuroscience psychology and philosophy like three states of matter. Neuroscience looks for the biologic correlation of behavior. Psychology is scientific principle to study behavior and Philosophy is to logically understand what is not understood.
- “ Cognition is survival instinct a consequence of carefully crafted modules dedicated to solving specific evolutionary problems” There is close interaction between brain gene and environment and evolution is the result of it. Introduction to Evolutionary Cognitive Neuroscience All organisms were and continue to be subject to the pressures of natural and sexual selection. These pressures are what formed all biological organs and hence also carefully crafted animal nervous systems—the seat of animal and human behavior, and the means by which organisms employ information-processing programs to adaptively deal with their environment. This theory was first formalized by Darwin (1859) in his seminal book, On the Origin of Species by Natural Selection . Unlike the theoretical work of early psychologists and behavioral scientists such as Skinner and Watson, which envisioned organisms as “blank slates” capable of making an infinite number of associations, evolutionary metatheory is beginning to shed light on this flawed theoretical approach to behavior analysis (see Barkow, Cosmides, & Tooby, 1992; Buss, 2005; Cosmides & Tooby, 2005). In fact, many of the emerging studies are contending directly with the standard social science model of psychology, namely, that organisms possess general-purpose learning mechanisms and that biology plays little if any role in the manifestation of behavior.
- Some of the first psychological studies to demonstrate that learning is not mediated by general-purpose learning mechanisms were conducted several decades ago and mark what might be considered the beginning of evolutionary thinking in psychology; they also contributed greately to what has become known as the cognitive revolution . In his landmark study, Garcia discovered that animals learned to avoid novel food products that made them ill in as little as one learning conditioning trial, something that had not been demonstrated with any other stimulus class previously. Labeled conditioned taste aversion , this effect describes an adaptive problem that has since been demonstrated in almost every species tested (the exception to this rule appears to be crocodilians; see Gallup & Suarez, 1988). This adaptation serves an important function: don’t eat food that makes you ill, or you might not survive to reproduce and pass on your genes. In other words, being ill could result in a number of fitness disadvantages such as death, inability to avoid predation, inability to search and secure mates, and loss of mate value. In a similar discovery, Seligman demonstrated what he referred to as prepared learning. Prepared learning is a phenomenon in which it is easier to make associations between stimuli that possess a biological predisposition to be conditioned because of a role these stimuli played in an organism’s evolutionary history. Seligman and his colleagues demonstrated that it was much easier for humans (and animals) to form conditioned emotional responses and associative fear responses to evolutionarily relevant threats such as snakes, insects, and heights than it took to condition fear to present-day threatening stimuli that subjects were much more likely to be have encountered and be harmed by, such as cars, knives, and guns. In other words, it was easier to condition humans to fear snakes, spiders, and heights than it was to condition them to fear guns, cars, and knives. These two series of studies demonstrated that psychological traits, like the design of bodily organs, were crafted by evolutionary forces into adaptations that allowed our ancestors to flourish. That is, the information- processing mechanisms designed to deal with situations such as poisonous food or potential threats to survival evolved as part of our ancestors’ recurrent experience with such situations. These studies refute a key premise of the standard social science model, emphasizing that there is no general-purpose learning mechanism. Rather, all learning is a nsequence of carefully crafted modules dedicated to solving specific evolutionary problems (see Barkow, Cosmides, & Tooby, 1992; Pinker, xiv Preface 2002). Our brains have evolved to be efficient problem solvers, and the problems they are designed to solve are those that our ancestors recurrently faced over human evolutionary history. Hence, those among our ancestors who were psychologically adaptated to solve these problems survived and passed the genes for those traits on to offspring.
- Brain is considered as the seat of cognition or biological basis of cognition. All research has concentrated around this enigmatic organ of intelligence. Let us see from evolution of brain to the evolution of concept of linking brain to mind. Mankind has been linking mind to the brain for a long time. Human skulls with holes deliberately made in them were found in sites more than 10.000 years old. Probably, those holes were made in order to grant a way out for the bad spirits that should be tormenting those brains [ 4 ]. The link between brain and mental functions was a natural one to achieve, because primitive people in all ages could easily observe that strong blows to the skull resulted in loss of consciousness and of memory, and even convulsions, which often led to significant alterations of perception and behavior.
- The best and most important documental proof about this knowledge comes from the famous Surgical Papyrus, discovered by archeologist Edwin Smith [ 6 ], and which was written around 1.600 BC in Egypt. It contains the first known descriptions of cranial sutures, the external brain surface, brain liquor (CSF) and intracranial pulsation. Its author describes further 30 clincal cases of head and spine trauma, noting how the several brain injuries were associated to changes in the function of other parts of the body, especially in the lower limbs, such as hemiplegic contractures, paralysis, miction and ejaculation and priapism, due to trauma inflicted to the spinal medulla.
- Alcmaeon of Croton (5th century B.C) was possibly the first one to put in the brain the site of sensations. According to him, the optic nerves, supposed to be hollow, carried the information to the brain, where each sensory modality had its own localization. Alcmaeon of Croton, one of the most emlinent natural philosophers of antiquity, was a native of Croton in Magna Graecia. His father's name was Pirithus, and he is said to have been a pupil of Pythagoras , and must therefore have lived in the latter half of the sixth century before Christ. (Diog. Laërt. viii. 83.) Nothing more is known of the events of his life. His most celebrated anatomical discovery has been noticed in the Dict. of Ant. p. 756, a; but whether his knowledge in this branch of science was derived from the dissection of animals or of human bodies, is a disputed question, which it is difficult to decide. Chalcidius, on whose authority the fact rests, merely says (Comment. in Plat. "Tim." p. 368, ed. Fabr.), " qui primus exsectionem aggredi est ausus," and the word cxsectio would apply equally well to either case. He is said also (Diog. Laërt. l. c.; Clemens Alexandr. Strom. i. p. 308) to have been the first person who wrote on natural philosophy (Phusikon logon), and to have invented fables (fabulas, Isid. Oriy. i. 39). He also wrote several other medical and philosophical works, of which nothing but the titles and a few fragments have been preserved by Stobaeus (Eclog. Phys.) Plutarch (De Phys. Philos. Decr.), and Galen . (Histor. Philosophy.) A further account of his philosophical opinions may be found in Menage's Notes to Diogenes Laërtius, viii. 83, p. 387; Le Clerc, Hist. de la Méd. ; Alfons. Ciacconius ap. Flbric, Biblioth. Graec. vol. xiii. p. 48, ed. vet.; Sprengel, Hist. de la Méd. vol. i. p. 239; C. G. Kühn, De Philosoph. ante Hippocr. Medicinae Cultor. Lips. 1781, 4to., reprinted in Ackermann's Opusc. ad Histor. Medic. Pertinentia, Norimb. 1797, 8vo., and in Kühn's Opusc. Acad. Med. et Philol. Lips. 1827-8, 2 vols. 8vo.; Isensec, Gesch. der Medicin. [W.A.G] Although Alcmaeon is termed a pupil of Pythagoras, there is great reason to doubt whether he was a Pythagorean at all; his name seems to have crept into the lists of supposititious Pythagoreans given us by later writers. (Brandis, Geschichte der Philosophie, vol. i. p. 507.) Aristotle ( Metaphys. 1 ) mentions him as nearly contemporary with Pythagoras, but distinguishes between the stoicheia of opposites, under which the Pythagoreans included all things, and the double principle of Alcmaeon, according to Aristotle, less extended, although he does not explain the precise difference. Other doctrines of Alcmaeon have been preserved to us. He said that the human soul was immortal and partook of the divine nature, because like the heavenly bodies it contained in itself a principle of motion. (Arist. de Anima, i. 2, p. 405; Cic. de Nat. Deor. i. 11.) The eclipse of the moon, which was also eternal, he supposed to arise from its shape, which he said was like a boat. All his doctrines which have come down to us, relate to physics or medicine; and seem to have arisen partly out of the speculations of the Ionian school, with which rather than the Pythagorean, Aristotle appears to connect Alcmaeon, partly front the traditionary lore of the earliest medical science. (Brandis, vol. i. p. 508.) Alcmaeon First published Thu Apr 10, 2003; substantive revision Mon Apr 28, 2008 Alcmaeon of Croton was an early Greek medical writer and philosopher-scientist. His exact date, his relationship to other early Greek philosopher-scientists, and whether he was primarily a medical writer/physician or a typical Presocratic cosmologist, are all matters of controversy. He is likely to have written his book sometime between 500 and 450 BC. The surviving fragments and testimonia focus primarily on issues of psychology and epistemology and reveal Alcmaeon to be a thinker of considerable originality. He was the first to identify the brain as the seat of understanding and to distinguish understanding from perception. Alcmaeon thought that the sensory organs were connected to the brain by channels (poroi) and may have discovered the poroi connecting the eyes to the brain (i.e. the optic nerve) by excising the eyeball of an animal, although it is doubtful that he used dissection as a standard method. He was the first to develop an argument for the immortality of the soul. He used a political metaphor to define health and disease: The equality (isonomia) of the opposing powers which make up the body (e.g., the wet, the dry, the hot, the cold, the sweet, the bitter etc.) preserve health, whereas the monarchy of any one of them produces disease. Alcmaeon discussed a wide range of topics in physiology including sleep, death and the development of the embryo. It is unclear whether he also presented a cosmology in terms of opposing powers, but we do have some testimonia concerning his views on astronomy. Alcmaeon had considerable impact on his successors in the Greek philosophical tradition. Aristotle wrote a treatise responding to him, Plato adopted his argument for the immortality of the soul, and both Plato and Philolaus accepted his view that the brain is the seat of intelligence. 1. Life and Works 1.1 Medical Writer or Philosopher? 1.2 Was Alcmaeon a Pythagorean? 1.3 Date 1.4 Alcmaeon's Book and the Evidence for Its Contents 2. Epistemology and Psychology 2.1 The Limits of Knowledge 2.2 Understanding and Empiricism 2.3 Did Alcmaeon Practice Dissection? 2.4 Sense Perception 2.5 The Immortality of the Soul 3. Medical Doctrines 3.1 Health and Disease 3.2 Sleep, Embryology and the Use of Analogy 4. Cosmology 4.1 Astronomy 4.2 Opposites 5. Importance and Influence Bibliography Other Internet Resources Related Entries Life and Works 1.1 Medical Writer or Philosopher? Alcmaeon, son of Peirithous (otherwise unknown), lived in the Greek city of Croton on the instep of the boot of Italy. Diogenes Laertius, in his brief life of Alcmaeon (VIII. 83), asserts that he wrote mostly on medical matters. There is, however, little direct evidence for his work as a practicing physician. Later writers in the medical tradition, such as Galen (DK A2), treat him as a philosopher-scientist rather than as a physician, so that some scholars (Mansfeld 1975) have concluded that he was not a doctor at all but rather a typical Presocratic physiologos (writer on nature). The majority of scholars, however, because of Diogenes' remark and because of the focus on the functioning of the human body in the testimonia and fragments, refer to Alcmaeon as a physician-philosopher. The historian Herodotus tells us that, in the second half of the sixth century, the physicians of Croton were the best in the Greek world (III. 131) and recounts in some detail the activities of the most prominent Crotoniate physician of the time, Democedes (III. 125-138). Thus, whether a practicing physician or not, Alcmaeon undoubtedly owes some of his interest in human physiology and psychology to the medical tradition in Croton. At this point in the history of Greek thought, it is difficult to draw clear lines between the work of a medical writer/physician and a philosopher/scientist. Most Presocratic cosmologies devoted some attention to questions of human physiology and medicine, and conversely the early treatises in the Hippocratic corpus often paid some attention to cosmology (see Aristotle, Resp. 480b23 ff.). 1.2 Was Alcmaeon a Pythagorean? Croton is also famous as the center of Pythagoras' activity from ca. 530, when he left Samos, until his death ca. 490. Alcmaeon addressed his book to three men who may have been Pythagoreans: Alcmaeon of Croton, son of Peirithous, said the following to Brotinus, Leon and Bathyllus… (DK, B1)We know nothing of Leon and Bathyllus, except that Iamblichus, in On the Pythagorean Way of Life (=VP), lists a Pythagorean named Leon from Metapontum and a Pythagorean Bathylaus from Poseidonia (Paestum), both Greek cities of southern Italy (VP 267). Bro(n)tinus is identified as a Pythagorean from Croton in some places (D.L. VIII. 42; Iamb. VP 132) and from Metapontum in others (VP 194, 267). He is either the father or the husband of Theano, who is in turn either the wife or student of Pythagoras. Does this "dedication" of his book to Pythagoreans indicate that Alcmaeon himself was a Pythagorean? Even if this is a dedication, it does not follow that Alcmaeon agreed with the views of his addressees. Comparison with Empedocles' address to Pausanias suggests, moreover, that what we have is not a dedication but an exhortation or an attempt to instruct (Vlastos 1953, 344, n. 25). Alcmaeon might be quite independent of these Pythagoreans and trying to persuade them of his distinct point of view. Diogenes Laertius, in his Lives of the Philosophers (3rd century AD), includes Alcmaeon among the Pythagoreans and says that he studied with Pythagoras (VIII. 83). Later authors such as Iamblichus (VP 104, 267), Philoponus (De An. p. 88) and the scholiast on Plato (Alc. 121e) also call Alcmaeon a Pythagorean. A majority of scholars up to the middle of the twentieth century followed this tradition. On the other hand, no one earlier than Diogenes calls Alcmaeon a Pythagorean. Aristotle wrote two books on the Pythagoreans but wrote a separate book on Alcmaeon. Aristotle and Theophrastus refer to him a number of times but never identify him as a Pythagorean, and this is the practice of the doxographical tradition (see A4, 6, 8-10, 13-14, 17-18, in DK 24). Simplicius (6th AD) reports that some have handed down the view that Alcmaeon is a Pythagorean but notes that Aristotle denies it (De An. 32.3). Most telling is Aristotle's discussion of Alcmaeon in Metaphysics book I (986a22 ff.). He notes a similarity between Alcmaeon and a group of Pythagoreans in positing opposites as the principles of things but expresses uncertainty as to who influenced whom. Earlier scholars took this comparison of Alcmaeon to the Pythagoreans as confirmation that he was a Pythagorean. Most scholars of the last fifty years, however, have come to recognize that Aristotle's treatment of Alcmaeon here suggests the exact opposite; it only makes sense to compare him with the Pythagoreans and wonder who influenced whom, if he is not a Pythagorean (e.g., Lloyd 1991, 167). Certainly most of the opposites which are mentioned as crucial to Alcmaeon do not appear in the Pythagorean table of opposites, and there is no trace of the crucial Pythagorean opposition between limit and unlimited in Alcmaeon. The overwhelming majority of scholars since 1950 have accordingly regarded Alcmaeon as a figure independent of the Pythagoreans (e.g., Guthrie 1962 and Lloyd 1991, 167; Zhmud 1997, 70-71, is one of the few exceptions), although, as a fellow citizen of Croton, he will have been familiar with their thought. 1.3 Date No issue concerning Alcmaeon has been more controversial than his date. Some have sought to date him on the basis of his address to Brotinus. Brotinus' dates are too uncertain to be of much help, however. If he is Theano's father and Theano was Pythagoras' wife, he could be a contemporary of Pythagoras (570-490) or even older. If he is Theano's husband and she was a student of Pythagoras in his old age and thus twenty in 490, Brotinus could have been born as late as 520. This suggests that Brotinus could have been the addressee of the book any time between 550 and 450 BC. The center of controversy, however, has been a sentence in the passage of Aristotle's Metaphysics discussed above: Alcmaeon of Croton also seems to have thought along similar lines, and either he took this theory over from them [i.e. the Pythagoreans] or they from him. For in age Alcmaeon was in the old age of Pythagoras, and his views were similar to theirs (986a27-31).The sentence in bold above is missing in one of the major manuscripts and Alexander makes no mention of it in his commentary on the Metaphysics. It does appear in the other two major manuscripts and in Asclepius' commentary. A number of scholars have regarded it as a remark written in the margin by a later commentator, which has crept into the text (e.g., Ross 1924, 152; Burkert 1972, 29, n.60). It is a surprising remark for Aristotle to make, since he only refers to Pythagoras once elsewhere in all his extant writings and throughout this passage of the Metaphysics refers to the Pythagoreans or Italians in the plural. An almost equal number of scholars regard the remark as genuine (e.g., Wachtler 1896; Guthrie 1962, 341-3). Even if we accept the remark as genuine, the assertion that Alcmaeon "was" (egeneto) in the old age of Pythagoras is ambiguous. Does it mean that Alcmaeon was born in the old age of Pythagoras, or that he lived in the old age of Pythagoras? Diels emended the sentence to say that Alcmaeon was "young" in the old age of Pythagoras and a similar remark can be found in Iamblichus (VP 104). Although Diels accepted the text as Aristotelian, others have seen the parallel with Iamblichus as evidence that it is a remark by a later commentator and have pointed out that the report in Iamblichus involves several chronological impossibilities (e.g., that Philolaus was young in the old age of Pythagoras). Whatever the provenance of the remark, scholars have been influenced by it, because there is little else to go on. One group of scholars dates the publication of Alcmaeon's book to around 500 (Burkert 1972, 292; Kirk, Raven, Schofield 1983, 339 [early 5th]) so that he would have been born around 540. Another group has him born around 510 so that his book would have been published in 470 or later (Guthrie 1962, 358 [480-440 BC]; Lloyd 1991, 168 [490-430 BC]). In either case Alcmaeon probably wrote before Empedocles, Anaxagoras and Philolaus. He is either the contemporary or the predecessor of Parmenides. Attempts to date him on the basis of internal evidence alone, i.e. comparison of his doctrines with those of other thinkers, have led to the widest divergence of dates. Edelstein says that he may have lived in the late fifth century (1942, 372), while Lebedev makes him active in the late 6th (1993). 1.4 Alcmaeon's Book and the Evidence for Its Contents The ancient tradition assigns one book to Alcmaeon, which came to bear the traditional title of Presocratic treatises, On Nature (DK, A2), although this title probably does not go back to Alcmaeon himself. Favorinus' report that Alcmaeon was the first to write such a treatise (DK, A1) is almost certainly wrong, since Anaximander wrote before Alcmaeon. Theophrastus' detailed report of Alcmaeon's account of the senses (DK, A5) and the fact that Aristotle wrote a treatise in response to Alcmaeon (D.L. V. 25) suggest that the book was available in the fourth century BC. It is unclear whether Alcmaeon wrote in the Doric dialect of Croton or in the Ionic Greek of the first Presocratics (Burkert, 1972, 222, n. 21). The report that an Alcimon of Croton was the first to write animal fables might be a reference to a poet of a similar name. Fragment 5 in DK ("It is easier to be on one's guard against an enemy than a friend.") sounds very much like the moral of such a fable (Gomperz 1953, 64-5). Diels and Kranz (=DK) identify five other fragments of Alcmaeon (Frs. 1, 1a, 2, 3, 4) and arrange the testimonia under 18 headings. Fragments 1a, 3 and 4, however, are really testimonia which use language of a later date, although some of Alcmaeon's terminology is embedded in them. The three lines of Fragment 1, which probably began the book, and the half line in Fragment 2 are the only continuous texts of Alcmaeon. Another brief fragment/testimonium should be added to the material in DK: "the earth is the mother of plants and the sun their father" (Nicolaus Damascenus, De plantis I 2.44; see Kirk 1956 and Lebedev 1993). There is also the possibility that Fr. 125 of the Spartan poet Alcman ("Experience is the beginning of learning.") should, in fact, be assigned to Alcmaeon (Lanza 1965; Barnes 1982, 610). 2. Epistemology and Psychology 2.1 The Limits of Knowledge Alcmaeon began his book by defining the limits of human knowledge: Concerning things that are not perceptible [and concerning mortal things] the gods have clarity, but insofar as it is possible for human beings to judge … (DK, B1)Such skepticism about human knowledge is characteristic of one strand of early Greek thought. Both Alcmaeon's predecessors (e.g., Xenophanes B34) and his successors (e.g., Philolaus B6) made similar contrasts between divine and human knowledge, but in Alcmaeon's case, as in these other cases, we do not have enough evidence to be sure what he intended. Most of the subjects that Alcmaeon went on to discuss in his book could not be settled by a direct appeal to sense perception (e.g., the functioning of the senses, the balance of opposites in the healthy body, the immortality of the soul). In this sense he was dealing largely with what is "not perceptible" (aphanes). Alcmaeon is decidedly not an extreme skeptic, however, in that he is willing to assign clear understanding about such things to the gods and by implication admits that even humans have clear understanding of what is directly perceptible. Moreover, while humans cannot attain clarity about what cannot be perceived, Alcmaeon thinks that they can make reasonable judgments from the signs that are presented to them by sensation (tekmairesthai). He thus takes the stance of the scientist who draws inferences from what can be perceived, and he implicitly rejects the claims of those who base their account of the world on the certainty of a divine revelation (e.g., Pythagoras, Parmenides B1). 2.2 Understanding and Empiricism According to Theophrastus, Alcmaeon was the first Greek thinker to distinguish between sense perception and understanding and to use this distinction to separate animals, which only have sense perception, from humans, who have both sense perception and understanding (DK, B1a, A5). Alcmaeon is also the first to argue that the brain is the central organ of sensation and thought (DK, A5, A8, A10). There is no explicit evidence, however, as to what Alcmaeon meant by understanding. The word translated as understanding here is suniêmi, which in its earliest uses means "to bring together," so that it is possible that Alcmaeon simply meant that humans are able to bring the information provided by the senses together in a way that animals cannot (Solmsen 1961, 151). Animals have brains too, however, and thus might appear to be able to carry out the simple correlation of the evidence from the various senses, whereas the human ability to make inferences and judgments (DK, B1) appears to be a more plausible candidate for the distinctive activity of human intelligence. It is possible that we should use a passage in Plato's Phaedo (96a-b = A11) to explicate further Alcmaeon's epistemology. The passage is part of Socrates' report of his early infatuation with natural science and with questions such as whether it is the blood, or air, or fire with which we think. He also reports the view that it is the brain that furnishes the sensations of hearing, sight and smell. This corresponds very well with Alcmaeon's view of the brain as the central sensory organ and, although Alcmaeon is not mentioned by name, many scholars think that Plato must be referring to him here. Socrates connects this view of the brain with an empiricist epistemology, which Aristotle will later adopt (Posterior Analytics 100a3 ff.). This epistemology involves three steps: first, the brain provides the sensations of hearing, sight and smell, then, memory and opinion arise from these, and finally, when memory and opinion achieve fixity, knowledge arises. Some scholars suppose that this entire epistemology is Alcmaeon's (e.g., Barnes 1982, 149 ff.), while others more cautiously note that we only have explicit evidence that Alcmaeon took the first step (e.g., Vlastos 1970, 47, n.8). 2.3 Did Alcmaeon Practice Dissection? Alcmaeon's empiricism has sometimes been thought to have arisen from his experience as a practicing physician (Guthrie 1962). He has also been hailed as the first to use dissection, but this is based on a hasty reading of the evidence. Calcidius, in his Latin commentary on Plato's Timaeus, praises Alcmaeon, along with Callisthenes and Herophilus, for having brought many things to light about the nature of the eye (DK, A10). Most of what Calcidius goes on to describe, however, are the discoveries of Herophilus some two centuries after Alcmaeon (Lloyd 1975, Mansfeld 1975, Solmsen 1961). The only conclusions we can reasonably draw about Alcmaeon from the passage are that he excised the eyeball of an animal and observed poroi (channels, i.e. the optic nerve) leading from the eye in the direction of the brain (Lloyd 1975). Theophrastus' account of Alcmaeon's theory of sensation implies that he thought that there were such channels leading from each of the senses to the brain: All the senses are connected in some way with the brain. As a result, they are incapacitated when it is disturbed or changes its place, for it then stops the channels, through which the senses operate. (DK, A5)There is no evidence, however, that Alcmaeon dissected the eye itself or that he dissected the skull in order to trace the optic nerve all the way to the brain. Alcmaeon's account of the other senses, far from suggesting that he carried out dissections in order to explain their function, implies that he did not (Lloyd 1975). Alcmaeon's conclusion that all of the senses are connected to the brain may have been drawn from nothing more that the excision of the eye and the general observation that the sense organs for sight, hearing, smell and taste are located on the head and appear connected to passages which lead inward towards the brain (Gomperz 1953, 69). It is striking in this regard that Alcmaeon gave no account of touch (DK, A5), which is the only sense not specifically tied to the head. It would be a serious mistake then to say that Alcmaeon discovered dissection or that he was the father of anatomy, since there is no evidence that he used dissection systematically or even that he did more than excise a single eyeball. 2.4 Sense Preception Theophrastus says that Alcmaeon did not explain sensation by the principle of like to like (i.e. by the likeness between the sense organ and what is perceived), a principle which was used by many early Greek thinkers (e.g., Empedocles). Unfortunately he gives no general account of how Alcmaeon did think sensation worked (DK, A5). Alcmaeon explained each of the individual senses with the exception of touch, but these accounts are fairly rudimentary. He regarded the eye as composed of water and fire and vision as taking place when what is seen is reflected in the gleaming and translucent part of the eye. Hearing arises when an external sound is first transmitted to the outer ear and then picked up by the empty space (kenon) in the inner ear, which transmits it to the brain. Taste occurs through the tongue, which being warm and soft dissolves things with its heat and, because of its loose texture, receives and transmits the sensation. Smell is the simplest of all. It occurs "at the same time as we breathe in, thus bringing the breath to the brain" (DK, A5). 2.5 The Immortality of the Soul Alcmaeon developed the first argument for the immortality of the soul, but the testimonia concerning it differ slightly from one another, and it appears to have been taken over and developed by Plato, so that it is very hard to determine exactly how to reconstruct Alcmaeon's own argument. Barnes (1982, 116-120) and Hankinson (1998, 30-3) provide the most insightful analysis. Alcmaeon appears to have started from the assumption that the soul is always in motion. At one extreme we might suppose that Alcmaeon only developed the crude argument from analogy, which Aristotle assigns to him (De An. 405a29). The soul is like the heavenly bodies, which Alcmaeon regarded as divine and immortal (DK, A1, A12), in being always in motion, so it is also like them in being immortal. This is clearly fallacious, since it assumes that things that are alike in one respect will be alike in all others. The version in the doxographical tradition is somewhat better (DK, A12). Alcmaeon thought that the soul moved itself in continual motion and was therefore immortal and like to the divine. The similarity to the divine is not part of the inference here but simply an illustrative comparison. The core of the argument is the necessary truth that what is always in motion must be immortal. This is the assumption from which Plato starts his argument for the immortality of the soul at Phaedrus 245c, but how much of Plato's analysis of what is always in motion can be assigned to Alcmaeon? Plato makes no mention of Alcmaeon in the passage. There are still serious questions for Alcmaeon, even on the more sophisticated version of the argument. We might well recognize that things with souls, i.e. things that are alive, are able to move themselves, and conclude that it is souls that bring this motion about. We might also conclude that the soul, as what moves something else, must be in motion itself (the synonymy principle of causation). But why did Alcmaeon suppose that the soul must be always in motion? (Hankinson [1998, 32] provides two possible answers and discusses the difficulties with them.) Finally, what sort of motion is being ascribed to souls? The natural assumption might be that the soul's motion is thinking, but, at this early point in Greek thought, Alcmaeon was more likely to have thought that, if the soul is going to cause motion in space, it too must be in locomotion. Plato describes the soul as composed of two circles with contrary motions, which imitate the contrary motions of the fixed stars and the planets, so that the soul becomes a sort of orrery in the head (Timaeus 44d). It has been suggested that this image is borrowed from Alcmaeon (Barnes 1982, 118; Skemp 1942, 36 ff.). In Fragment 2, Alcmaeon is reported to have said that: Human beings perish because they are not able to join their beginning to their end.At first sight, this assertion might appear to conflict with Alcmaeon's belief that the soul is immortal. It seems likely, however, that Alcmaeon distinguished between human beings as individual bodies, which do perish, and as souls, which do not (Barnes 1982, 115). There is no evidence about what Alcmaeon thought happened to the soul, when the body perished, however. He might have believed in reincarnation (along with the Pythagoreans), although his sharp distinction between animals and human beings may argue against this, or he might have thought that the soul joined other divine beings in the heavens. The point of Fragment 2 may be that, whereas the heavenly bodies do join their beginnings to their ends in circular motion, humans are not able to join their end in old age to their beginning in childhood, i.e. human life does not have a cyclical structure. We might even suppose that the soul tries to impose such a structure on the body from its own circular motion but ultimately fails (Guthrie 1962, 353). 3. Medical Doctrines 3.1 Health and Disease Fragment 4 presents Alcmaeon's account of health and disease: Alcmaeon said that the equality (isonomia) of the powers (wet, dry, cold, hot, bitter, sweet, etc.) maintains health but that monarchy among them produces disease.This is, in fact, not a fragment but a testimonium and most of the language comes from the doxographical tradition rather than Alcmaeon. The report goes on to say that Alcmaeon thought that disease arose because of an excess of heat or cold, which in turn arose because of an excess or deficiency in nutrition. Disease is said to arise in the blood, the marrow or the brain. It can also be caused by external factors such as the water, the locality, toil, or violence. The idea that health depends on a balance of opposed factors in the body is a commonplace in Greek medical writers. Although Alcmaeon is the earliest figure to whom such a conception of health is attributed, it may well be that he is not presenting an original thesis but rather drawing on the earlier medical tradition in Croton. Perhaps what is distinctive to Alcmaeon is the use of the specific political metaphor and terminology (isonomia, monarchia). Just as Anaximander explained the order of the cosmos in terms of justice in the city-state, so Alcmaeon used a political metaphor to explain the order of the human body. Although some have tried to find an application for isonomia in aristocratic politics, it is usually associated with the radical democracy which emerged in Athens in the late sixth century (Vlastos 1973, 175-7). Is Alcmaeon's use of the term simply descriptive of the equality of powers that is necessary for the healthy body, or does his use of the term to describe health suggest that he was in sympathy with radical democracy (Vlastos 1953, 363)? 3.2 Sleep, Embryology, and the Use of Analogy Alcmaeon is the first to raise a series of questions in human and animal physiology that later become stock problems, which every thinker tries to address. He thus sets the initial agenda for Greek physiology (Longrigg 1993, 54-7; Lloyd 1966, 322 ff.). He said that sleep is produced by the withdrawal of the blood away from the surface of the body to the larger ("blood-flowing") vessels and that we awake when the blood diffuses throughout the body again (DK, A18). Death occurs when the blood withdraws entirely. Hippocratic writers (Epid. VI 5.15) and Aristotle (H.A. 521a15) both seem to have adopted Alcmaeon's account of sleep. It is very unlikely, however, that Alcmaeon distinguished between veins (the "blood-flowing vessels") and arteries, as some have claimed. It is more likely that he simply distinguished between larger more interior blood vessels as opposed to smaller ones close to the surface (Lloyd 1991, 177). He probably argued that human seed was drawn from the brain (DK, A13). Contrary to a popular Greek view, which regarded the father alone as providing seed, a view that would be followed by Aristotle (Lloyd 1983, 86 ff.), Alcmaeon argued that both parents contribute seed (DK, A13) and that the child takes the sex of the parent who contributes the most seed (DK, A14). According to one report, Alcmaeon thought that the head was the first part of the embryo to develop, although another report has him confessing that he did not have definite knowledge in this area, because no one is able to perceive what is formed first in the infant (DK, A13). These reports are not inconsistent and conform to the epistemology with which Alcmaeon began his book. It is plausible to suppose that he regarded the development of the embryo as one of the imperceptibles about which we can have no certain knowledge. On the other hand, he may have regarded it as a reasonable inference that the part of the body which controls it in life, i.e. the brain, developed first in the womb. Dissection is of obvious relevance to the debate about the development of the embryo, and Alcmaeon's failure to appeal to dissection of animals in this case is further evidence that he did not employ it as a regular method (Lloyd 1979, 163). Alcmaeon studied not just humans but also animals and plants. He gave an explanation of the sterility of mules (DK, B3) and, if we can believe Aristotle, thought that goats breathed through their ears (DK, A7). More significantly, he used analogies with animals and plants in developing his accounts of human physiology. Thus, the pubic hair that develops when human males are about to produce seed for the first time at age fourteen is analogous to the flowering of plants before they produce seed (DK, A15); milk in mammals is analogous to egg white in birds (DK, A16). The infant in the womb absorbs nutrients through its entire body, like a sponge, although another report suggests that the embryo already feeds through its mouth (DK, A17). However, the text of the latter report should perhaps be amended from stomati [mouth] to sômati [body], thus removing the contradiction (Olivieri 1919, 34). Analogies such as these will become a staple item in later Greek biological treatises, but Alcmaeon is one of the earliest figures in this tradition. 4. Cosmology 4.1 Astronomy As indicated in section 1 above, there is considerable controversy as to whether and to what extent Alcmaeon was a typical Presocratic cosmologist. Certainly the evidence for his cosmology is meager. There are three references to his astronomical theory (DK, A4). He is repor
- Hippocrates and Plato linked brain with thinking and emotion while Aristotle diverged from their view and acknowledged heart as seat of intelligence and brain as radiator to cool the passion of heart. Aristotle (384-322 B.C.) diverged from his contemporaries and acknowledged the heart as the organ of thinking, of perception and feelings, whereas the brain was important to keep the body’s temperature, acting as a radiator. According to him, nutrients would go up through the blood vessels and part of them, a kind of garbage, would be cooled in the brain, being changed into a liquid, in a way that could be compared with what happens to water in nature, when rain is formed Aristotle was not the first to erroneously generalize a very old notion among all kinds of antique civilizations, that the seat of emotions was the heart. Even today we are still influenced by this, such as when we refer to a heart as the symbol of love, or when we say that we got a "broken heart", or we have a "heavy heart", or still when we say that we love something with our very hearts. "Knowing by heart", such as when we memorize something, has the same origin. All this probably comes from the fact that the sympathetic branch of the autonomous nervous system is activated during strong emotions, causing a perceptible increase in heart rate and force of contractions. The temporal association of effect to its cause in its peripheral expression has led to the erroneous interpretation, which the natural philosophers tried to "explain" in scientific terms.
- Greek physician Roman empire (130-200) rejected Aristotles’ ideas, arguing that there were no sense in believing that the brain could cool the passions of heart. Galen dissected a lot (the animal of choice was the ox) and paid more attention to the meninges and cerebral ventricles than to the brain itself. In those days, working with unfixed material, it is only natural that the ventricles would call more attention than the brain, that would resemble an amorphous paste. For Galen, the nutrients absorbed in the guts went to the liver, where the natural spirit was formed. This spirit was taken to the heart and, in the left ventricle, was changed into a vital spirit. The vital spirit, going through the carotid arteries, would reach the rete mirabile , a net of vessels localized in the base of the skull. There, it was blended with the inspired air, forming the animal spirit, that was stored in the cerebral ventricles, from where it could reach the rest of the brain. The animal spirit, a product of the mixture of a liquid and the air, was considered as the essence of life and the source of intellectual skills. When necessary, it could travel along the hollow nerves, eliciting movements or mediating sensations. According to Galen, the substance refined in the rete mirabile would produce a certain amount of refuse, part of which was gaseous, the other part being liquid. The gaseous part escaped through the bone sutures and air sinuses of the skull, and its passage was not perceived by the senses. The liquid part leaked from the anterior ventricles to the openings of the cribiform plate of the ethmoid bones or, yet, from the third ventricle through the pituitary fossa. From there, it could reach the nasal cavity to be discharged as plhegm, or mucus. Many of Galen's conclusions were wrong because he used mostly animals after an early career using humans at a gladiatorial school outside Rome Human dissection was forbidden in ancient Rome. Galen was considered "the authority" until the Renaissance During the Middle Ages, questioning Galen was questioning authority and because authority came from God, it was heresy and punishable by death --so NOBODY questioned Galen's conclusions
- Brain Ventricles and the Concept of Mind Nemesius (circa 320), bishop of Emesa, a city in Syria, embraced Galen’s ideas and based in the cerebral ventricles the intellectual faculties. In his book “ On the Nature of Man ”, a treatise of physiology modeled on Greek medicine, it is said that the soul could not be localized, but the functions of the mind could. The cerebral ventricles were supposed to be responsible for mental operations, from sensation to memorization. The first pair of ventricles were the seat of the “common senses”. They would make the analysis of the information originated in the sense organs. The resultant images were carried to the middle ventricle, the seat of reason, thinking and wisdom. Then came into action the last ventricle, the seat of memory. Up to the Middle Age, the figures depicting the brain would show the ventricles with great detail. The idea that spirits wandered in the ventricles, favored by the Church, prevailed up to the Renaissance. In a book published in the thirteenth century, named “ On the Properties of Things ”, a compilation made by Bartholomew the Englishman, it is stated that “the anterior cavity is soft and moist in order to facilitate association of sensual perceptions and imagination. The middle cell must also be warm, since thinking is a process of separation of pure from impure, comparable to digestion, and heat is known to be the main factor in digestion. The posterior cell, however, is a place for cold storage in which a cool and dry atmosphere must allow for the stocking of goods. That is why the cerebellum is harder, i.e. less medullary and airy, than the rest of the brain”.
- Leonardo da Vinci (1452-1519) first became interested in anatomic art when he was asked by a Veronese anatomist named Marc Antonia Della Torre to do the illustrations for a text of anatomy. Della Torre was to do the dissecting and Leonardo the drawings. But Della Torre died unexpectedly. Not to be deterred by Della Torre's demise, Leonardo assumed both tasks. He dissected and drew more than 10 human bodies in the cathedral cellar of the mortuary of Santa Spirito under the secrecy of candlelight, necessitated by the Church's belief in the sanctity of the human body and a papal decree that forbade human dissection. Leonardo recognized that a scientific knowledge of human anatomy could only be gained by dissecting the human body. This was in striking contrast to the pronouncements of Galen and other anatomists. Da Vinci injected the blood vessels and cerebral ventricles with wax for preservation, an anatomical technique still used today. His drawings of the human anatomy have long been considered as unrivaled.
- Andreas Vesalius (1514-1564 CE) of Brussels (later Padua, Italy) Flemish anatomist willing to "use his own eyes" in human dissections correcting over 200 mistakes of Galen During the middle ages, everyone had simply trusted Galen, but Vesalius and others during the Renaissance began to question and correct Galen Vesalius was one of the first to dissect cadavers himself (rather than rely on others or on animal dissections); even Leonardo da Vinci and other "artist-anatomists" didn't do their own dissection (of course, they lived nearly twice as long because they weren't exposed to infections in the dead bodies) Vesalius was obsessed with dissections, even stacking up cadavers in his bedroom as a medical student in Paris. Later in his life, he told his students to keep a list of their really sick patients so he'd know where to go to get a freshly dead body (yikes) Published Concerning the Structure of the Human Body - De Corporis Humani Fabrica in 1543 (first "modern" human anatomy text) will detailed illustrations by Jan Stephan van Calcar (student of Titian) The Illustrations from the Works of Andreas Vesalius of Brussels ACT This book was published the same year as Copernicus's text, marking this year as the beginning of the "Scientific Revolution" "It is perfectly clear to me that my attempt will have all too little authority because I have not yet passed the twenty-eighth year of my life; it is equally clear that, because of the numerous indications of the false dogmas of Galen, it will be exceedingly unsafe from the attacks of conservatives, who, as with us in the Italian schools, have constantly avoided anatomy, and who, being old men, will be consumed with envy because of the correct discoveries of the young, and will be ashamed at having been blind thus far, along with other followers of Galen."
- The first roots of cognitive neuroscience lie in phrenology, which was a pseudoscientific theory that claimed that behavior could be determined by the shape of the scalp . In the early 19th century, Franz Joseph Gall and J. G. Spurzheim believed that the human brain was localized into approximately 35 different sections. In his book, The Anatomy and Physiology of the Nervous System in General, and of the Brain in Particular , Gall claimed that a larger bump in one of these areas meant that that area of the brain was used more frequently by that person. This theory gained significant public attention, leading to the publication of phrenology journals and the creation of phrenometers, which measured the bumps on a human subject's head. Figure 1-1 According to the nineteenth-century doctrine of phrenology, complex traits such as combativeness, spirituality, hope, and conscientiousness are controlled by specific areas in the brain, which expand as the traits develop. This enlargement of local areas of the brain was thought to produce characteristic bumps and ridges on the overlying skull, from which an individual's character could be determined. This map, taken from a drawing of the early 1800s, purports to show 35 intellectual and emotional faculties in distinct areas of the skull and the cerebral cortex underneath. In fact, by as early as the end of the eighteenth century the first attempts had been made to bring together biological and psychological concepts in the study of behavior. Franz Joseph Gall, a German physician and neuroanatomist, proposed three radical new ideas. First, he advocated that all behavior emanated from the brain. Second, he argued that particular regions of the cerebral cortex controlled specific functions. Gall asserted that the cerebral cortex did not act as a single organ but was divided into at least 35 organs (others were added later), each corresponding to a specific mental faculty. Even the most abstract of human behaviors, such as generosity, secretiveness, and religiosity were assigned their spot in the brain. Third, Gall proposed that the center for each mental function grew with use, much as a muscle bulks up with exercise. As each center grew, it purportedly caused the overlying skull to bulge, creating a pattern of bumps and ridges on the skull that indicated which brain regions were most developed (Figure 1-1). Rather than looking within the brain, Gall sought to establish an anatomical basis for describing character traits by correlating the personality of individuals with the bumps on their skulls. His psychology, based on the distribution of bumps on the outside of the head, became known as phrenology . In the late 1820s Gall's ideas were subjected to experimental analysis by the French physiologist Pierre Flourens. By systematically removing Gall's functional centers from the brains of experimental animals, Flourens attempted to isolate the contributions of each “cerebral organ” to behavior. From these experiments he concluded that specific brain regions were not responsible for specific behaviors, but that all brain regions, especially the cerebral hemispheres of the forebrain, participated in every mental operation. Any part of the cerebral hemisphere, he proposed, was able to perform all the functions of the hemisphere. Injury to a specific area of the cerebral hemisphere would therefore affect all higher functions equally. In 1823 Flourens wrote: “All perceptions, all volitions occupy the same seat in these cerebral) organs; the faculty of perceiving, of conceiving, of willing merely constitutes therefore a faculty which is essentially one.” The rapid acceptance of this belief (later called the aggregate-field view of the brain) was based only partly on Flourens's experimental work. It also represented a cultural reaction against the reductionist view that the human mind has a biological basis, the notion that there was no soul, that all mental processes could be reduced to actions within different regions in the brain!
- Advocated functional localization by cerebral convolution. In 1862, showing brain lesions in a stroke patient who could understand language but could not speak (could only say "tan"), he demonstrated that the left frontal lobe was responsible for articular speech. Demonstrated this in several patients. This region has since been named Broca’s area . The French neurologist Pierre Paul Broca was much influenced by Gall and by the idea that functions could be localized. But he extended Gall's thinking in an important way. He argued that phrenology, the attempt to localize the functions of the mind, should be based on examining damage to the brain produced by clinical lesions rather than by examining the distribution of bumps on the outside of the head. Thus he wrote in 1861: “I had thought that if there were ever a phrenological science, it would be the phrenology of convolutions (in the cortex), and not the phrenology of bumps (on the head).” Based on this insight Broca founded neuropsychology , a new science of mental processes that he was to distinguish from the phrenology of Gall. In 1861 Broca described a patient named Leborgne, who could understand language but could not speak. The patient had none of the conventional motor deficits (of the tongue, mouth, or vocal cords) that would affect speech. In fact, he could utter isolated words, whistle, and sing a melody without difficulty. But he could not speak grammatically or create complete sentences, nor could he express ideas in writing. Postmortem examination of this patient's brain showed a lesion in the posterior region of the frontal lobe (now called Broca's area ; Figure 1-4B). Broca studied eight similar patients, all with lesions in this region, and in each case found that the lesion was located in the left cerebral hemisphere. This discovery led Broca to announce in 1864 one of the most famous principles of brain function: “Nous parlons avec l'hémisphère gauche!” (“We speak with the left hemisphere!”) The aggregate-field view was first seriously challenged in the mid-nineteenth century by the British neurologist J. Hughlings Jackson. In his studies of focal epilepsy, a disease characterized by convulsions that begin in a particular part of the body, Jackson showed that different motor and sensory functions can be traced to different parts of the cerebral cortex. These studies were later refined by the German neurologist Karl Wernicke, the English physiologist Charles Sherrington, and Ramón y Cajal into a view of brain function called cellular connectionism . According to this view, individual neurons are the signaling units of the brain; they are generally arranged in functional groups and connect to one another in a precise fashion. Wernicke's work in particular showed that different behaviors are produced by different brain regions interconnected by specific neural pathways. Broca's work stimulated a search for the cortical sites of other specific behavioral functions—a search soon rewarded. In 1870 Gustav Fritsch and Eduard Hitzig galvanized the scientific community by showing that characteristic and discrete limb movements in dogs, such as extending a paw, can be produced by electrically stimulating a localized region of the precentral gyrus of the brain. These discrete regions were invariably located in the contralateral motor cortex. Thus, the right hand, the one most humans use for writing and skilled movements, is controlled by the left hemisphere, the same hemisphere that controls speech. In most people, therefore, the left hemisphere is regarded as dominant . Symptom-Complex of Aphasia: APsychological Study on an Anatomical Basis.” In it he described another type of aphasia, one involving a failure to comprehend language rather than to speak (a receptive as opposed to an expressive malfunction). Whereas Broca's patients could understand language but not speak, Wernicke's patient could speak but could not understand language. Moreover, the locus of this new type of aphasia was different from that described by Broca: the critical cortical lesion was located in the posterior part of the temporal lobe where it joins the parietal and occipital lobes (Figure 1-4B). On the basis of this discovery, and the work of Broca, Fritsch, and Hitzig, Wernicke formulated a theory of language that attempted to reconcile and extend the two theories of brain function holding sway at that time. Phrenologists argued that the cortex was a mosaic of functionally specific areas, whereas the aggregate-field school argued that mental functions were distributed homogeneously throughout the cerebral cortex. Wernicke proposed that only the most basic mental functions, those concerned with simple perceptual and motor activities, are localized to single areas of the cortex. More complex cognitive functions, he argued, result from interconnections between several functional sites. In placing the principle of localized function within a connectionist framework, Wernicke appreciated that different components of a single behavior are processed in different regions of the brain. He was thus the first to advance the idea of distributed processing , now central to our understanding of brain function. Wernicke postulated that language involves separate motor and sensory programs, each governed by separate cortical regions. He proposed that the motor program, which governs the mouth movements for speech, is located in Broca's area, suitably situated in front of the motor area that controls the mouth, tongue, palate, and vocal cords (Figure 1-4B). And he assigned the sensory program, which governs word perception, to the temporal lobe area he discovered (now called Wernicke's area). This area is conveniently surrounded by the auditory cortex as well as by areas collectively known as association cortex , areas that integrate auditory, visual, and somatic sensation into complex perceptions. Thus Wernicke formulated the first coherent model for language organization that (with modifications and elaborations we shall soon learn about) is still of some use today. According to this model, the initial steps in the processing of spoken or written words by the brain occur in separate sensory areas of the cortex specialized for auditory or visual information. This information is then conveyed to a cortical association area specialized for both visual and auditory information, the angular gyrus. Here, according to Wernicke, spoken or written words are transformed into a common neural representation shared by both speech and writing. From the angular gyrus this representation is conveyed to Wernicke's area, where it is recognized as language and associated with meaning. Without that association, the ability to comprehend language is lost. The common neural representation is then relayed from Wernicke's to Broca's area, where it is transformed from a sensory (auditory or visual) representation into a motor representation that can potentially lead to spoken or written language. When the last-stage transformation from sensory to motor representation cannot take place, the ability to express language (either as spoken words or in writing) is lost. Based on this premise, Wernicke correctly predicted the existence of a third type of aphasia, one that results from disconnection. Here the receptive and motor speech zones themselves are spared but the neuronal fiber pathways that connect them are destroyed. This conduction aphasia , as it is now called, is characterized by an incorrect use of words ( paraphasia ). Patients with conduction aphasia understand words that they hear and read and have no motor difficulties when they speak. Yet they cannot speak coherently; they omit parts of words or substitute incorrect sounds. Painfully aware of their own errors, they are unable to put them right.
- Figure 1-5 In the early part of the twentieth century Korbinian Brodmann divided the human cerebral cortex into 52 discrete areas on the basis of distinctive nerve cell structures and characteristic arrangements of cell layers . Brodmann's scheme of the cortex is still widely used today and is continually updated. In this drawing each area is represented by its own symbol and is assigned a unique number. Several areas defined by Brodmann have been found to control specific brain functions. For instance, area 4, the motor cortex, is responsible for voluntary movement. Areas 1, 2, and 3 comprise the primary somatosensory cortex, which receives information on bodily sensation. Area 17 is the primary visual cortex, which receives signals from the eyes and relays them to other areas for further deciphering. Areas 41 and 42 comprise the primary auditory cortex. Areas not visible from the outer surface of the cortex are not shown in this drawing. Inspired in part by Wernicke, a new school of cortical localization arose in Germany at the beginning of the twentieth century led by the anatomist Korbinian Brodmann. This school sought to distinguish different functional areas of the cortex based on variations in the structure of cells and in the characteristic arrangement of these cells into layers. Using this cytoarchitectonic method, Brodmann distinguished 52 anatomically and functionally distinct areas in the human cerebral cortex (Figure 1-5). Thus, by the beginning of the twentieth century there was compelling biological evidence for many discrete areas in the cortex, some with specialized roles in behavior. Yet during the first half of this century the aggregate-field view of the brain, not cellular connectionism, continued to dominate experimental thinking and clinical practice. This surprising state of affairs owed much to the arguments of several prominent neural scientists, among them the British neurologist Henry Head, the German neuropsychologist Kurt Goldstein, the Russian behavioral physiologist Ivan Pavlov, and the American psychologist Karl Lashley, all advocates of the aggregate-field view. The most influential of this group was Lashley, who was deeply skeptical of the cytoarchitectonic approach to functional delineation of the cortex. “The ‘ideal’ architectonic map is nearly worthless,” Lashley wrote. “ The area subdivisions are in large part anatomically meaningless, and misleading as to the presumptive functional divisions of the cortex.” Lashley's skepticism was reinforced by his attempts, in the tradition of Flourens's work, to find a specific seat of learning by studying the effects of various brain lesions on the ability of rats to learn to run a maze. But Lashley found that the severity of the learning defect seemed to depend on the size of the lesions, not on their precise site. Disillusioned, Lashley—and, after him, many other psychologists — concluded that learning and other mental functions have no special locus in the brain and consequently cannot be pinned down to specific collections of neurons. On the basis of his observations, Lashley reformulated the aggregate-field view into a theory of brain function called mass action , which further belittled the importance of individual neurons, specific neuronal connections, and brain regions dedicated to particular tasks. According to this view, it was brain mass, not its neuronal components, that was crucial to its function. Applying this logic to aphasia, Head and Goldstein asserted that language disorders could result from injury to almost any cortical area. Cortical damage, regardless of site, caused patients to regress from a rich, abstract language to the impoverished utterances of aphasia. Lashley's experiments with rats, and Head's observations on human patients, have gradually been reinterpreted. A variety of studies have demonstrated that the maze-learning task used by Lashley is unsuited to the study of local cortical function because the task involves so many motor and sensory capabilities. Deprived of one sensory capability (such as vision), a rat can still learn to run a maze using another (by following tactile or olfactory cues). Besides, as we shall see, many mental functions are handled by more than one region or neuronal pathway, and a single lesion may not eliminate them all. In addition, the evidence for the localization of function soon became overwhelming. Beginning in the late 1930s, Edgar Adrian in England and Wade Marshall and Philip Bard in the United States discovered that applying a tactile stimulus to different parts of a cat's body elicits electrical activity in distinctly different subregions of the cortex, allowing for the establishment of a precise map of the body surface in specific areas of the cerebral cortex described by Brodmann. These studies established that cytoarchitectonic areas of cortex can be defined unambiguously according to several independent criteria, such as cell type and cell layering, connections, and—most important—physiological function. As we shall see in later chapters, local functional specialization has emerged as a key principle of cortical organization, extending even to individual columns of cells within a functional area. Indeed, More refined methods have made it possible to learn even more about the function of different brain regions involved in language. In the late 1950s Wilder Penfield, and more recently George Ojemann used small electrodes to stimulate the cortex of awake patients during brain surgery for epilepsy (carried out under local anesthesia), in search of areas that produce language. Patients were asked to name objects or use language in other ways while different areas of the cortex were stimulated. If the area of the cortex was critical for language, application of the electrical stimulus blocked the patient's ability to name objects. In this way Penfield and Ojemann were able to confirm—in the living conscious brain—the language areas of the cortex described by Broca and Wernicke. In addition, Ojemann discovered other sites essential for language, indicating that the neural networks for language are larger than those delineated by Broca and Wernicke. Our understanding of the neural basis of language has also advanced through brain localization studies that combine linguistic and cognitive psychological approaches. From these studies we have learned that a brain area dedicated to even a specific component of language, such as Wernicke's area for language comprehension, is further subdivided functionally. These modular subdivisions of what had previously appeared to be fairly elementary operations were first discovered in the mid 1970s by Alfonso Caramazza and Edgar Zurif. They found that different lesions within Wernicke's area give rise to different failures to comprehend. Lesions of the frontal-temporal region of Wernicke's area result in failures in lexical processing , an inability to understand the meaning of words. By contrast, lesions in the parietal-temporal region of Wernicke's area result in failures in syntactical processing , the ability to understand the relationship between the words of a sentence. (Thus syntactical knowledge allows one to appreciate that the sentence “Jim is in love with Harriet” has a different meaning from “Harriet is in love with Jim.”)
- Affective Traits and Aspects of Personality Are Also Anatomically Localized Despite the persuasive evidence for localized languagerelated functions in the cortex, the idea nevertheless persisted that affective (emotional) functions are not localized. Emotion, it was believed, must be an expression of whole-brain activity. Only recently has this view been modified. Although the emotional aspects of behavior have not been as precisely mapped as sensory, motor, and cognitive functions, distinct emotions can be elicited by stimulating specific parts of the brain in humans or experimental animals. The localization of affect has been dramatically demonstrated in patients with certain language disorders and those with a particular type of epilepsy. Aphasia patients not only manifest cognitive defects in language, but also have trouble with the affective aspects of language, such as intonation (or prosody ). These affective aspects are represented in the right hemisphere and, rather strikingly, the neural organization of the affective elements of language mirrors the organization of the logical content of language in the left hemisphere. Damage to the right temporal area corresponding to Wernicke's area in the left temporal region leads to disturbances in comprehending the emotional quality of language, for example, appreciating from a person's tone of voice whether he is describing a sad or happy event. In contrast, damage to the right frontal area corresponding to Broca's area leads to difficulty in expressing emotional aspects of language. Thus some linguistic functions also exist in the right hemisphere. Indeed, there is now considerable evidence that an intact right hemisphere may be necessary to an appreciation of subtleties of language, such as irony, metaphor, and wit, as well as the emotional content of speech. Certain disorders of affective language that are localized to the right hemisphere, called aprosodias , are classified as sensory, motor, or conduction aprosodias, following the classification used for aphasias. This pattern of localization appears to be inborn, but it is by no means completely determined until the age of about seven or eight. Young children in whom the left cerebral hemisphere is severely damaged early in life can still develop an essentially normal grasp of language. Further clues to the localization of affect come from patients with chronic temporal lobe epilepsy. These patients manifest characteristic emotional changes, some of which occur only fleetingly during the seizure itself and are called ictal phenomena (Latin ictus , a blow or a strike). Common ictal phenomena include feelings of unreality and déjàvu (the sensation of having been in a place before or of having had a particular experience before); transient visual or auditory hallucinations; feelings of depersonalization, fear, or anger; delusions; sexual feelings; and paranoia. More enduring emotional changes, however, are evident when patients are not having seizures. These interictal phenomena are interesting because they represent a true psychiatric syndrome. A detailed study of such patients indicates they lose all interest in sex, and the decline in sexual interest is often paralleled by a rise in social aggressiveness. Most exhibit one or more distinctive personality traits: They can be intensely emotional, ardently religious, extremely moralistic, and totally lacking in humor. In striking contrast, patients with epileptic foci outside the temporal lobe show no abnormal emotion and behavior. One important structure for the expression and perception of emotion is the amygdala, which lies deep within the cerebral hemispheres. The role of this structure in emotion was discovered through studies of the effects of the irritative lesions of epilepsy within the temporal lobe. The consequences of such irritative lesions are exactly the opposite of those of destructive lesions resulting from a stroke or injury. Whereas destructive lesions bring about loss of function, often through the disconnection of specialized areas, the electrical storm of epilepsy can increase activity in the regions affected, leading to excessive expression of emotion or over-elaboration of ideas. We consider the neurobiology of emotion in Part VIII of this book.
- In computerized tomography, the X-ray source and detectors are moved around the patient's head. This approach generates a matrix of intersecting points that have been obtained from several directions. The signal at each point can then be computed, allowing reconstruction of a "slice" through the brain that preserves three-dimensional relationships. (B) This CT scan shows a horizontal section of a normal adult brain. Until recently, almost everything we knew about the anatomical organization of language came from studies of patients who had suffered brain lesions. Positron emission tomography (PET) and functional magnetic resonance imaging (MRI) have extended this approach to normal people (Chapter 20). PET is a noninvasive imaging technique for visualizing the local changes in cerebral blood flow and metabolism that accompany mental activities, such as reading, speaking, and thinking. In 1988, using this new imaging form, Michael Posner, Marcus Raichle, and their colleagues made an interesting discovery. They found that the incoming sensory information that leads to language production and understanding is processed in more than one pathway. Recall that Wernicke believed that both written and spoken words are transformed into a representation of language by both auditory and visual inputs. This information, he thought, is then conveyed to Wernicke's area, where it becomes associated with meaning before being transformed in Broca's area into output as spoken language. Posner and his colleagues asked: Must the neural code for a word that is read be translated into an auditory representation before it can be associated with a meaning? Or can visual information be sent directly to Broca's area with no involvement of the auditory system? Using PET, they determined how individual words are coded in the brain of normal subjects when the words are read on a screen or heard through earphones. Thus, when words are heard Wernicke's area becomes active, but when words are seen but not heard or spoken Wernicke's area is not activated. The visual information from the occipital cortex appears to be conveyed directly to Broca's area without first being transformed into an auditory representation in the posterior temporal cortex. Posner and his colleagues concluded that the brain pathways and sensory codes used to see words are different from those used to hear words. They proposed, therefore, that these pathways have independent access to higher-order regions of the cortex concerned with the meaning of words and with the ability to express language (Figure 1-6). Not only are reading and listening processed separately, but the act of thinking about a word's meaning (in the absence of sensory inputs) activates a still different area in the left frontal cortex. Thus language processing is parallel as well as serial; as we shall learn in Chapter 59, it is considerably more complex than initially envisaged by Wernicke. Indeed, similar conclusions have been reached from studies of behavior other than language. These studies demonstrate that information processing requires many individual cortical areas that are appropriately interconnected—each of them responding to, and therefore coding for, only some aspects of specific sensory stimuli or motor movement, and not for others. Studies of aphasia afford unusual insight into how the brain is organized for language. One of the most impressive insights comes from a study of deaf people who lost their ability to speak American Sign Language after suffering cerebral damage. Unlike spoken language, American signing is accomplished with hand gestures rather than by sound and is perceived by visual rather than auditory pathways. Nonetheless, signing, which has the same structural complexities characteristic of spoken languages, is also localized to the left hemisphere. Thus, deaf people can become aphasic for sign language as a result of lesions in the left hemisphere. Lesions in the right hemisphere do not produce these defects. Moreover, damage to the left hemisphere can have quite specific consequences, affecting either sign comprehension (following damage in Wernicke's area) or grammar (following damage in Broca's area) or signing fluency. These observations illustrate three points. First, the cognitive processing for language occurs in the left hemisphere and is independent of pathways that process the sensory or motor modalities used in language. Second, speech and hearing are not necessary conditions for the emergence of language capabilities in the left hemisphere. Third, spoken language represents only one of a family of cognitive skills mediated by the left hemisphere.
- (C) Diagram of the machine used to obtain clinical MR images. Additional magnetic coils (not shown) produce linearly varying magnetic field gradients oriented along three orthogonal axes. (D) An MR image taken in the midsagittal plane. Note the extraordinary clarity with which all major brain components can be seen (compare with Figure 1.10 ).
- Functional Brain Imaging: PET, SPECT, and f MRI The most informative brain imaging techniques for monitoring brain function now rely on detecting small changes in blood flow to visualize active areas of the brain. The brain utilizes a remarkably large fraction of the body's energy resources (about 20% of circulating glucose is consumed by the brain). Not surprisingly, at any given moment the most active nerve cells use more glucose and oxygen than relatively quiescent neurons. To meet the increased metabolic demands of particularly active neurons, the local flow of blood to the relevant brain area increases. Detecting and mapping these local changes in cerebral blood flow form the basis for three widely used functional brain imaging techniques: positron emission tomography (PET), single-photon emission computerized tomography (SPECT), and functional magnetic resonance imaging ( f MRI). Because these techniques reveal patterns of activity in the intact brain, they have greatly enhanced the ability to understand both normal brain function and abnormal brain states associated with a variety of pathologies. In PET scanning, unstable positron-emitting isotopes are synthesized in a cyclotron by bombarding nitrogen, carbon, oxygen, or fluorine with protons. Examples of the isotopes used include 15O (half-life, 2 min), 18F (110 min), and 11C (20 min). These probes can be incorporated into many different reagents (including water, precursor molecules of specific neurotransmitters, or glucose) and used to analyze specific aspects of brain function. When the radiolabeled compounds are injected into the bloodstream, they distribute according to the physiological state of the brain. Thus, labeled oxygen and glucose accumulate in more metabolically active areas, and labeled transmitter probes are taken up selectively by appropriate regions. As the unstable isotope decays, the extra proton breaks down into a neutron and a positron. The emitted positrons travel several millimeters, on average, until they collide with an electron. The collision of a positron with an electron destroys both particles, emitting two gamma rays from the site of the collision in directions that are exactly 180° apart. Gamma ray detectors placed around the head are therefore arranged to register a "hit" only when two detectors 180° apart react simultaneously. By reconstructing the sites of the positron-electron collisions, the location of active regions can be imaged. The mean free path of the positrons in brain tissue limits the resolution of PET scanning to about 4 mm. Nonetheless, PET images can be superimposed on MRI images from the same subject (see Box B ), providing detailed information about specific brain areas involved in a wide variety of functions. The elegance and power of this technique are evident in Figures 24.6 and 25.6 in Unit V of this book. SPECT imaging is an outgrowth of older techniques for measuring regional cerebral blood flow. A radiolabeled compound with a relatively short half-life (for example, 133Xe) is inhaled or injected into the circulation (in the latter case, 123I-labeled iodoamphetamine is used); the probes bind to red blood cells and are carried throughout the body. As the label undergoes radioactive decay, it emits high-energy photons. The rate of clearance of the probes was initially detected using an array of sodium iodide photon detectors placed around the head. More recent approaches have used a gamma camera that can be rapidly moved around the head to collect photons from many different angles, thus permitting a more accurate three-dimensional image. The information gathered using SPECT can also be combined with structural information from other imaging techniques, such as CT scans and MRI scans, to provide better localization of the active areas. A limitation of SPECT scanning is its relatively low resolution (about 8 mm). Although this level is not sufficient to resolve the finer features of the brain, it reveals the major areas involved in normal processing or disease. SPECT imaging is neither as flexible nor as accurate as PET imaging, but it is much simpler, primarily because the radiolabeled probes are commercially available and do not require an on-site cyclotron (as does the synthesis of PET probes). A variant of MRI, called functional MRI ( f MRI), now offers the best approach to analyzing the brain at work (see figure). f MRI is based on the fact that oxyhemoglobin (the oxygen-carrying form of hemoglobin) has a different magnetic resonance signal than deoxyhemoglobin (the oxygen-depleted form of hemoglobin) or the surrounding brain tissue. Brain areas activated by a specific task (e.g., the occipital cortex during visual behavior; see figure) utilize more oxygen. Initially, this activity decreases the levels of oxyhemoglobin and increases levels of deoxyhemoglobin. Within seconds, the brain microvasculature responds to this local oxygen depletion by increasing the flow of oxygen-rich blood to the active area. These changes in the concentration of oxyhemoglobin lead to localized blood oxygenation level dependent (BOLD) changes in the magnetic resonance signal, which form the basis for the f MRI signal. Thus, unlike PET or SPECT, f MRI uses signals intrinsic to the brain rather than signals originating from exogenous, radioactive probes; consequently, repeated observations can be made on the same individual, providing a major advantage over other imaging methods. f MRI also offers superior spatial localization (currently a few millimeters), as well as good temporal resolution (on the order of seconds or less under optimal circumstances, compared to minutes for other functional imaging techniques). As a result of these advantages, f MRI has emerged as the technology of choice for probing both the normal and abnormal functional architecture of the human brain. Example of functional magnetic resonance imaging. Regional changes in cerebral blood flow were measured during visual stimulation; the area of activated visual cortex (color) was then mapped onto the brain, a section of which is shown at the appropriate level in the head. (From Belliveau et al., 1991.)
- Descartes, Brain and Mind During the seventeenth century, spirits still commanded behavior. At that time Rene Descartes (1596-1650) had chosen the pineal body, not properly as the seat of the soul, but as the place of its activity. The pineal was picked because it is a single organ, unlike the other brain structures, that come in pairs. Descartes’ neurophysiology was independent of neuroanatomy, which he deliberately ignored. It was based in the animal spirits , pores and ways by means of which they flew to exert their actions. According to him, the “most active and quickest particles of the blood” were taken by the arteries from the heart to the brain, where they were transformed in a very subtle air or wind, a very pure and active flame: the “animal spirits” [ 5 ]. The arteries were supposed to come together around the gland localized at the center of the brain: the pineal. Descartes presumed that filaments in the nerves (supposed to be tubes) could move little valvules, opening pores that would allow the flowing or the animal spirits. A stimulus in the skin, for example, would move those filaments, inducing a contraction as a reflex response. Starting in the brain, the animal spirits would travel along the nerves up to the muscles, inflating them to cause movements. That would be the mechanism for voluntary acts. A reflex response according to Descartes’ physiology. The fire elicits movement of animal spirits in hollow nerves. The movement opens pores in the ventricle (F), letting flow spirits that will inflate the muscles of the leg, that moves away from the heat. External stimuli should open pores in the brain and the spirits would be carried to the pineal gland, which had in its surface a complete sensorial and motor map. The will was under pineal’s control, which could manage the flow of the animal spirits into different nerves. A drawing from the book by Rene Descartes, De Homine, published in 1662. Visual information is taken to the brain by hollow optic nerves. From there, it reaches the Pineal body (H), which regulates the flowing of animal spirits into the nerves. The spirits will go to the muscles of the arm, to produce motion.
- Bioelectricity and the Neuronal Dogma The belief in animal spirits travelling along the nerves, born among the Greeks, remained current up to the eighteen century, when the electric nature of nerve conduction was verified. For that, it was important the work of Luigi Galvani (1737-1798) and, in the following century, the work of Emil du Bois-Reymond (1818-1896). Du Bois-Reymond made his studies on nervous transmission in the 1840 decade and in the 1870 decade he proposed that the effector organs were excited by the nerves via currents or by means of chemical substances liberated by the nerve endings.
- Before the neuron doctrine was accepted, it was widely believed that the nervous system was a reticulum, or a connected meshwork, rather than a system made up of discrete cells . [3] This theory, the reticular theory , held that neurons' somata mainly provided nourishment for the system. [4] The nineteenth century brought the acknowledgment that brain’s tissue was important for nervous functions. Theodor Schwann (1810-1882), who described the myelin sheath, was the first to propose that the body is made of individual cells. His cellular theory was accepted for the whole body, with the exception of the nervous system, that was supposed to be made of cells whose branches formed a continuous net. Only after the discovery of the staining method of silver impregnation of nervous elements (Golgi method) an accurate analysis was possible, introducing the works of Santiago Ramon y Cajal (1852-1934) who affirmed, in 1889, that the nervous cells were isolated units. Wilhelm von Waldeyer (1836-1921), in 1891, coined the term “neuron” to designate the anatomical and functional unit of the nervous tissue. Finally, spaces in the junctions between nervous cells or between nervous and muscular cells were described by Charles Scott Sherrington (1857-1952). Sherrington gave these structures the name of “synapses”. Drawing by Ramón y Cajal from "Structure of the Mammalian Retina " Madrid, 1900. The initial failure to accept the doctrine was due in part to inadequate ability to visualize cells using microscopes , which were not developed enough to provide clear pictures of nerves. With the cell staining techniques of the day, a slice of neural tissue appeared under a microscope as a complex web and individual cells were difficult to make out. Since neurons have a large number of neural processes an individual cell can be quite long and complex, and it can be difficult to find an individual cell when it is closely associated with many other cells. Thus, a major breakthrough for the neuron doctrine occurred in the late 1800s when Ramón y Cajal used a technique developed by Camillo Golgi to visualize neurons. The staining technique, which uses a silver solution, only stains one in about a hundred cells, effectively isolating the cell visually and showing that cells are separate and do not form a continuous web. Further, the cells that are stained are not stained partially, but rather all their processes are stained as well. Ramón y Cajal altered the staining technique and used it on samples from younger, less myelinated brains, because the technique did not work on myelinated cells. [1] He was able to see neurons clearly and produce drawings like the one at right. For their technique and discovery respectively, Golgi and Ramón y Cajal shared the 1906 Nobel Prize in Physiology or Medicine . Golgi could not tell for certain that neurons were not connected, and in his acceptance speech he defended the reticular theory. Ramón y Cajal , in his speech, contradicted that of Golgi and defended the now accepted neuron doctrine. A paper written in 1891 by Wilhelm von Waldeyer , a supporter of Ramón y Cajal , debunked the reticular theory and outlined the Neuron Doctrine. Updating the neuron doctrine Wiki While the neuron doctrine is a central tenet of modern neuroscience , recent studies suggest that there are notable exceptions and important additions to our knowledge about how neurons function. First, electrical synapses are more common in the central nervous system than previously thought. Thus, rather than functioning as individual units, in some parts of the brain large ensembles of neurons may be active simultaneously to process neural information. [5] Electrical synapses are formed by gap junctions that allow molecules to directly pass between neurons, creating a cytoplasm-to-cytoplasm connection. Second, dendrites, like axons, also have voltage-gated ion channels and can generate electrical potentials that carry information to and from the soma. This challenges the view that dendrites are simply passive recipients of information and axons the sole transmitters. It also suggests that the neuron is not simply active as a single element, but that complex computations can occur within a single neuron. [6] Third, the role of glia in processing neural information has begun to be appreciated. Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons: glia outnumber neurons by as many as 10:1. Recent experimental results have suggested that glia play a vital role in information processing. [7] Finally, recent research has challenged the historical view that neurogenesis , or the generation of new neurons, does not occur in adult mammalian brains. It is now known that the adult brain continuously creates new neurons in the hippocampus and in an area contributing to the olfactory bulb . This research has shown that neurogenesis is environment-dependent (eg. exercise, diet, interactive surroundings), age-related, upregulated by a number of growth factors, and halted by survival-type stress factors. [8] [9] Of particularly compelling interest, Charles Gross and Elizabeth Gould provided evidence suggesting that neurogenesis occurred in neocortex after birth, in areas of the brain known to be important for cognitive function. [10] Strong challenges to this work have come from more well-controlled studies by Pasko Rakic and others which support Rakic's original hypothesis that neurogenesis after birth is restricted to the olfactory bulb and hippocampus. [11] [12] [13] Rakic argues that the Princeton group's work has not been substantiated by multiple other groups. [14]
- Figure 2-2 Structure of a neuron. Most neurons in the vertebrate nervous system have several main features in common. The cell body contains the nucleus, the storehouse of genetic information, and gives rise to two types of cell processes, axons and dendrites. Axons, the transmitting element of neurons, can vary greatly in length; some can extend more than 3 m within the body. Most axons in the central nervous system are very thin (between 0.2 and 20 μm in diameter) compared with the diameter of the cell body (50 μm or more). Many axons are insulated by a fatty sheath of myelin that is interrupted at regular intervals by the nodes of Ranvier. The action potential, the cell's conducting signal, is initiated either at the axon hillock, the initial segment of the axon, or in some cases slightly farther down the axon at the first node of Ranvier. Branches of the axon of one neuron (the presynaptic neuron) transmit signals to another neuron (the postsynaptic cell) at a site called the synapse. The branches of a single axon may form synapses with as many as 1000 other neurons. Whereas the axon is the output element of the neuron, the dendrites (apical and basal) are input elements of the neuron. Together with the cell body, they receive synaptic contacts from other neurons. Figure 2-4 Neurons can be classified as unipolar, bipolar, or multipolar according to the number of processes that originate from the cell body. A. Unipolar cells have a single process, with different segments serving as receptive surfaces or releasing terminals. Unipolar cells are characteristic of the invertebrate nervous system. B. Bipolar cells have two processes that are functionally specialized: the dendrite carries information to the cell, and the axon transmits information to other cells. C. Certain neurons that carry sensory information, such as information about touch or stretch, to the spinal cord belong to a subclass of bipolar cells designated as pseudo-unipolar. As such cells develop, the two processes of the embryonic bipolar cell become fused and emerge from the cell body as a single process. This outgrowth then splits into two processes, both of which function as axons, one going to peripheral skin or muscle, the other going to the central spinal cord. D. Multipolar cells have an axon and many dendrites. They are the most common type of neuron in the mammalian nervous system. Three examples illustrate the large diversity of these cells. Spinal motor neurons (left) innervate skeletal muscle fibers. Pyramidal cells (middle) have a roughly triangular cell body; dendrites emerge from both the apex (the apical dendrite) and the base (the basal dendrites). Pyramidal cells are found in the hippocampus and throughout the cerebral cortex. Purkinje cells of the cerebellum (right) are characterized by the rich and extensive dendritic tree in one plane. Such a structure permits enormous synaptic input. (Adapted from Ramón y Cajal 1933.) Nerve Cells Differ Most at the Molecular Level The model of neuronal signaling we have outlined is a simplification that applies to most neurons, but there are some important variations. For example, some neurons do not generate action potentials. These are typically local interneurons without a conductile component — they have no axon, or such a short one that a conducted signal is not required. In these neurons the input signals are summed and spread passively to the nearby terminal region, where transmitter is released. There are also neurons that lack a steady resting potential and are spontaneously active. Even cells with similar organization can differ in important molecular details, expressing different combinations of ion channels, for example. As we shall learn in Chapters 6 and 9, different ion channels provide neurons with various thresholds, excitability properties, and firing patterns. Thus, neurons with different ion channels can encode the same class of synaptic potential into different firing patterns and thereby convey different signals. Neurons also differ in the chemical transmitters they use to transmit information to other neurons, and in the receptors they have to receive information from other neurons. Indeed, many drugs that act on the brain do so by modifying the actions of specific chemical transmitters or a particular subtype of receptor for a given transmitter. These differences not only have physiological importance for day-to-day functioning of the brain, but account for the fact that a disease may affect one class of neurons but not others. Certain diseases, such as amyotrophic lateral sclerosis and poliomyelitis, strike only motor neurons, while others, such as tabes dorsalis, a late stage of syphilis, affect primarily sensory neurons. Parkinson's disease, a disorder of voluntary movement, damages a small population of interneurons that use dopamine as a chemical transmitter. Some diseases are selective even within the neuron, affecting only the receptive elements, the cell body, or the axon. In Chapter 16 we shall see how research into myasthenia gravis, caused by a faulty transmitter receptor in the muscle membrane, has provided important insights into synaptic transmission. Indeed, because the nervous system has so many cell types and variations at the molecular level, it is susceptible to more diseases (psychiatric as well as neurological) than any other organ of the body. Despite the differences among nerve cells, the basic mechanisms of electrical signaling are surprisingly similar. This simplicity is fortunate for those who study the brain. By understanding the molecular mechanisms that produce signaling in one kind of nerve cell, we are well on the way to understanding these mechanisms in many other nerve cells.
- Nerve Cells Are Able to Convey Unique Information Because They Form Specific Networks The stretch reflex illustrates how just a few types of nerve cells can interact to produce a simple behavior. But even the stretch reflex involves populations of neurons— perhaps a few hundred sensory neurons and a hundred motor neurons. Can the individual neurons implicated in a complex behavior be identified with the same precision? In invertebrate animals, and in some lower vertebrates, a single cell (the so-called command cell ) can initiate a complex behavioral sequence. But, as far as we know, no complex human behavior is initiated by a single neuron. Rather, each behavior is generated by the actions of many cells. Broadly speaking, as we have seen, there are three neural components of behavior: sensory input, intermediate (interneuronal) processing, and motor output. Each of these components is mediated by a single group or several distinct groups of neurons. As discussed in Chapter 1, one of the key strategies of the nervous system is localization of function: specific types of information are processed in particular brain regions. Thus, information for each of our senses is processed in a distinct brain region where the afferent connections typically form a precise map of the pertinent receptor sheet on the body surface—the skin (touch), the retina (sight), the basilar membrane of the cochlea (hearing), or the olfactory epithelium (smell). These maps are the first stage in creating a representation in the brain of the outside world in which we live. Similarly, areas of the brain concerned with movement contain an orderly arrangement of neural connections representing the musculature and specific movements. The brain, therefore, contains at least two types of neural maps: one for sensory perceptions and another for motor commands. The two maps are interconnected in ways we do not yet fully understand. The neurons that make up these maps—motor, sensory, and interneuronal—do not differ greatly in their electrical properties. They have different functions because of the connections they make. These connections, established as the brain develops, determine the behavioral function of individual cells. Although our understanding of how sensory and motor information is processed and represented in the brain is based on the detailed studies of only a few regions, in those regions in which our understanding is particularly well advanced it is clear that the logical operations of a mental representation can be understood only by defining the flow of information through the connections that make up the various maps. A single component of behavior sometimes recruits a number of groups of neurons that simultaneously provide the same or similar information. The deployment of several neuron groups or several pathways to convey similar information is called parallel processing. Parallel processing also occurs in a single pathway when different neurons in the pathway perform similar computations simultaneously. Parallel processing makes enormous sense as an evolutionary strategy for building a more powerful brain: it increases both the speed and reliability of function within the central nervous system. The importance of abundant, highly specific parallel connections is now also being recognized by scientists attempting to construct computer models of the brain. Scientists working in this field, a branch of computer science known as artificial intelligence , first used serial processing to simulate the brain's higher-level cognitive processes—processes such as pattern recognition, learning, memory, and motor performance. They soon realized that although these serial models solved many problems rather well, including the challenge of playing chess, they performed poorly with other computations that the brain does almost instantaneously, such as recognizing faces or comprehending speech. As a result, most computational neurobiologists have turned to systems with both serial and parallel (distributed) components, which they call connectionist models. In these models elements distributed throughout the system process related information simultaneously. Preliminary insights from this work are often consistent with physiological studies. Connectionist models show that individual elements of a system do not transmit large amounts of information. Thus, what makes the brain a remarkable information processing machine is not the complexity of its neurons, but rather its many elements and, in particular, the complexity of connections between them. Individual stereotyped neurons are able to convey unique information because they are wired together and organized in different ways. The Modifiability of Specific Connections Contributes to the Adaptability of Behavior That neurons make specific connections with one another simple reflexes can undergo modification that lasts minutes, and much learning results in behavioral change that can endure for years. How can neural activity produce such long-term changes in the function of a set of prewired connections? A number of solutions for these dilemmas have been proposed. The proposal that has proven most farsighted is the plasticity hypothesis , first put forward at the turn of the century by Ramón y Cajal. A modern form of this hypothesis was advanced by the Polish psychologist Jerzy Konorski in 1948: The application of a stimulus leads to changes of a twofold kind in the nervous system. … [T]he first property, by virtue of which the nerve cells react to the incoming impulse … we call excitability , and … changes arising … because of this property we shall call changes due to excitability. The second property, by virtue of which certain permanent functional transformations arise in particular systems of neurons as a result of appropriate stimuli or their combination, we shall call plasticity and the corresponding changes plastic changes. There is now considerable evidence for plasticity at chemical synapses. Chemical synapses often have a remarkable capacity for short-term physiological changes (lasting hours) that increase or decrease the effectiveness of the synapse. Long-term changes (lasting days) can give rise to further physiological changes that lead to anatomical changes, including pruning of preexisting connections, and even growth of new connections. As we shall see in later chapters, chemical synapses can be modified functionally and anatomically during development and regeneration, and, most importantly, through experience and learning. Functional alterations are typically short term and involve changes in the effectiveness of existing synaptic connections. Anatomical alterations are typically long-term and consist of the growth of new synaptic connections between neurons. It is this potential for plasticity of the relatively stereotyped units of the nervous system that endows each of us with our individuality.
- Cognitive Neural Science Integrates Five Major Approaches to the Study of Cognitive Function Cognitive neural science is an integrative approach to the study of mental activity that emerged from five major technical and conceptual developments. First, in the 1960s and 1970s techniques for studying the activity of single cells in the brains of intact and behaving animals, including primates, were developed by Ed Evarts and by Vernon Mountcastle. Soon these techniques were used to correlate the actions of individual cells under controlled behavioral conditions. It then also became possible to stimulate small groups of cells and to increase their activity or to lesion them so as to reduce their activity. By correlating individual cells with behavior, seeing the effects of introducing activity (stimulation) and reducing activity (lesion), these studies made it possible to examine perceptual and motor processes at the cellular level while animals were engaged in typical sensory or motor behaviors. As a result, we know that the mechanisms of perception are much the same in humans and monkeys and other simpler animals. Second, cellular studies in monkeys also led to the ability to correlate the patterns of firing of individual cells in specific brain regions with higher cognitive processes, such as attention and decision making. This changed the way behavior is studied in both experimental animals and humans. Unlike the behaviorists, we no longer focus only on the stimulus response properties of behavior; instead, we focus on the information processing in the brain that leads to a behavior. Third, developments in systems neural science and cognitive psychology stimulated a renewed interest in the behavioral analysis of patients with lesions of the brain that interfere with mental functioning. This field had remained strong in Europe but was neglected in the United States. Patients with lesions of specific regions of the brain exhibit quite specific cognitive deficits. The behavioral consequences of brain lesions thus tell us much about the function of specific areas and pathways in the brain. Lesion studies have shown that cognition is not a unitary process but that there are several cognitive systems, each with many independent information-processing modules. For example, the visual system, a prototype of a cognitive system concerned with sensory perception, has specialized pathways for processing information about color, form, and movement. Fourth, new radiological imaging techniques—positron emission tomography (PET), magnetic resonance imaging (MRI), magnetoencephalography, and voltage-sensitive dyes have made it possible to relate changes in activity in entire populations of neurons to specific mental acts in living humans Finally, computer science has made a distinctive contribution to cognitive neural science. Computers have made it possible to model the activity of large populations of neurons and to begin to test ideas about the role of specific components of the brain in particular behaviors. To understand the neural organization of a complex behavior such as speech, we must understand not only the properties of individual cells and pathways but also the network properties of functional circuits in the brain. Network properties, although dependent on the properties of individual neurons in the network, need not be identical or even similar to the properties of individual cells in the network. Computational approaches, especially when combined with psychophysics , the analysis of the relationship between the physical attributes of a stimulus and perception, are helpful in characterizing the system as whole, in specifying what the system is capable of doing, and in determining how the properties of the constituent cells account for system properties.
- The central nervous system (defined as the brain and spinal cord) is usually considered to have seven basic parts: the spinal cord , the medulla , the pons , the cerebellum , the midbrain , the diencephalon , and the cerebral hemispheres ( Figure 1.10 ; see also Figure 1.8 ). The medulla, pons, and midbrain are collectively called the brainstem ( Box A ); the diencephalon and cerebral hemispheres are collectively called the forebrain . Within the brainstem are found cranial nerve nuclei that either receive input from cranial sensory ganglia via their respective cranial sensory nerves or give rise to axons that constitute cranial motor nerves ( Table 1.1 ). In addition, the brainstem is the conduit for several major tracts in the central nervous system. These tracts either relay sensory information from the spinal cord and brainstem to the midbrain and forebrain, or relay motor commands from the midbrain and forebrain back to motor neurons in the brainstem and spinal cord. Figure 1.10. The subdivisions and components of the central nervous system. (A) A lateral view indicating the seven major components of the central nervous system. (Note that the position of the brackets on the left side of the figure refers to the vertebrae, not the spinal segments.) (B) The central nervous system in ventral view, indicating the emergence of the segmental nerves and the cervical and lumbar enlargements. (C) Diagram of several spinal cord segments, showing the relationship of the spinal cord to the bony canal in which it lies.
- Figure 3.13. Saltatory action potential conduction along a myelinated axon. (A) Diagram of a myelinated axon. (B) Local current in response to action potential initiation at a particular site flows locally, as described in Figure 3.12 . However, the presence of myelin prevents the local current from leaking across the internodal membrane; it therefore flows farther along the axon than it would in the absence of myelin. Moreover, voltage-gated Na+ channels are present only at the nodes of Ranvier. This arrangement means that the generation of active, voltage-gated currents need only occur at these unmyelinated regions. The result is a greatly enhanced velocity of action potential conduction. Panel to the left of the figure legend shows the changing membrane potential as a function of time at the points indicated. Figure 3.14. Comparison of speed of action potential conduction in unmyelinated (upper) and myelinated (lower) axons. Increased Conduction Velocity as a Result of Myelination The rate of action potential conduction limits the flow of information within the nervous system. It is not surprising, then, that various mechanisms have developed to optimize the propagation of action potentials along axons. Because action potential conduction requires passive and active flow of current (see Figure 3.12 ), the rate of action potential propagation is determined by both of these phenomena. One way of improving passive current flow is to increase the diameter of an axon, which effectively decreases the internal resistance to passive current flow (see Box C ). The consequent increase in action potential conduction velocity presumably explains why giant axons evolved in invertebrates such as squid, and why rapidly conducting axons in all animals tend to be larger than slowly conducting ones. Another strategy to improve the passive flow of electrical current is to insulate the axonal membrane, reducing the ability of current to leak out of the axon and thus increasing the distance along the axon that a given local current can flow passively (see Box C ). This strategy is evident in the myelination of axons, a process by which oligodendrocytes in the central nervous system (and Schwann cells in the peripheral nervous system) wrap the axon in myelin , which consists of multiple layers of closely opposed glial membranes ( Figure 3.13 ; see also Chapter 1 ). By acting as an electrical insulator, myelin greatly speeds up action potential conduction ( Figure 3.14 ). For example, whereas unmyelinated axon conduction velocities range from about 0.5 to 10 m/s, myelinated axons can conduct at velocities up to 150 m/s. The major reason underlying this marked increase in speed is that the time-consuming process of action potential generation occurs only at specific points along the axon, called nodes of Ranvier , where there is a gap in the myelin wrapping (see Figure 1.4F ). If the entire surface of an axon were insulated, there would be no place for current to flow out of the axon and action potentials could not be generated. As it happens, an action potential generated at one node of Ranvier elicits current that flows passively within the myelinated segment until the next node is reached. This local current flow then generates an action potential in the neighboring segment, and the cycle is repeated along the length of the axon. Because current flows across the neuronal membrane only at the nodes (see Figure 3.13 ), this type of propagation is called saltatory , meaning that the action potential jumps from node to node. Not surprisingly, loss of myelin, as occurs in diseases such as multiple sclerosis, causes a variety of serious neurological problems ( Box D ).
- Sensory Modality Is Determined by the Stimulus Energy Since ancient times five major sensory modalities have been recognized: vision, hearing, touch, taste, and smell. In addition to these classical senses we also consider the somatic senses of pain, temperature, itch, and proprioception (posture and the movement of parts of the body) and the vestibular sense of balance (the position of the body in the gravitational field). An early insight into the neuronal basis of sensation came in 1826, when Johannes Müller advanced his “laws of specific sense energies.” Müller proposed that modality is a property of the sensory nerve fiber. Each nerve fiber is activated primarily by a certain type of stimulus and each makes specific connections to structures in the central nervous system whose activity gives rise to specific sensations. Thus Müller's laws of specific sense energies identified the most important mechanism for neural coding of stimulus modality. Here is Müller's statement of the law, from Handbuch der Physiologie des Menschen für Vorlesungen , 2nd Ed., translated by Edwin Clarke and Charles Donald O'Malley: The same cause, such as electricity, can simultaneously affect all sensory organs, since they are all sensitive to it; and yet, every sensory nerve reacts to it differently; one nerve perceives it as light, another hears its sound, another one smells it; another tastes the electricity, and another one feels it as pain and shock. One nerve perceives a luminous picture through mechanical irritation, another one hears it as buzzing, another one senses it as pain. . . He who feels compelled to consider the consequences of these facts cannot but realize that the specific sensibility of nerves for certain impressions is not enough, since all nerves are sensitive to the same cause but react to the same cause in different ways. . . (S)ensation is not the conduction of a quality or state of external bodies to consciousness, but the conduction of a quality or state of our nerves to consciousness, excited by an external cause. [edit] Clarification As the above quotation shows, Müller's law seems to differ from the modern statement of the law in one key way. Müller attributed the quality of an experience to some specific quality of the energy in the nerves. For example, the visual experience from light shining into the eye, or from a poke in the eye, arises from some special quality of the energy carried by optic nerve, and the auditory experience from sound coming into the ear, or from electrical stimulation of the cochlea, arises from some different, special quality of the energy carried by the auditory nerve. In 1912, Lord Edgar Douglas Adrian showed that all neurons carry the same energy, electrical energy in the form of action potentials. That means that the quality of an experience depends on the part of the brain to which nerves deliver their action potentials (e.g., light from nerves arriving at the visual cortex and sound from nerves arriving at the auditory cortex). In 1945, Roger Sperry showed that it is the location in the brain to which nerves attach that determines experience. He studied amphibians whose optic nerves cross completely, so that the left eye connects to the right side of the brain and the right eye connects to the left side of the brain. He was able to cut the optic nerves and cause them to regrow on the opposite side of the brain so that the left eye now connected to the left side of the brain and the right eye connected to the right side of the brain. He then showed that these animals made the opposite movements from the ones they would have made before the operation. For example, before the operation, the animal would move to the left to get away from a large object approaching from the right. After the operation, the animal would move to the right in response to the same large object approaching from the right. Sperry showed similar results in other animals including mammals (rats), this work contributing to his Nobel Prize in 1981. The Sensory Neurons for Hearing, Taste, and Smell Are Spatially Organized According to Sensitivity For hearing and the chemical senses (taste and smell), the receptors are spatially distributed following the energy spectrum for these modalities. For example, auditory receptors are arranged according to the sound frequencies to which they respond. Receptors at a specific location vibrate most strongly when stimulated by a particular range of sounds, with high frequencies located at the base of the cochlea and low frequencies at the apex. Thus the organization of the inner ear's receptor sheet represents the spectrum of sound, not the location of the sounds in space. For taste and smell, receptors that have particular chemical sensitivities are located in different parts of the receptive surface of the tongue and inside the nose. For example, specific regions of the tongue contain receptors sensitive to salts, sugars, acids, bases, or proteins. Different foods will excite specific combinations of these receptors to evoke their characteristic tastes. The spatial distribution of activity in the chemoreceptor population allows the brain to differentiate salty from sweet or bitter tastes.
- Brain Plasticity--An Overview What is brain plasticity ? Does it mean that our brains are made of plastic? Of course not. Plasticity, or neuroplasticity, is the lifelong ability of the brain to reorganize neural pathways based on new experiences. As we learn, we acquire new knowledge and skills through instruction or experience. In order to learn or memorize a fact or skill, there must be persistent functional changes in the brain that represent the new knowledge. The ability of the brain to change with learning is what is known as neuroplasticity . To illustrate the concept of plasticity, imagine the film of a camera. Pretend that the film represents your brain. Now imagine using the camera to take a picture of a tree. When a picture is taken, the film is exposed to new information -- that of the image of a tree. In order for the image to be retained, the film must react to the light and “change” to record the image of the tree. Similarly, in order for new knowledge to be retained in memory, changes in the brain representing the new knowledge must occur. To illustrate plasticity in another way, imagine making an impression of a coin in a lump of clay. In order for the impression of the coin to appear in the clay, changes must occur in the clay -- the shape of the clay changes as the coin is pressed into the clay. Similarly, the neural circuitry in the brain must reorganize in response to experience or sensory stimulation FACT 1 : Neuroplasticity includes several different processes that take place throughout a lifetime. Neuroplasticity does not consist of a single type of morphological change, but rather includes several different processes that occur throughout an individual’s lifetime. Many types of brain cells are involved in neuroplasticity, including neurons, glia, and vascular cells. FACT 2 : Neuroplasticity has a clear age-dependent determinant. Although plasticity occurs over an individual’s lifetime, different types of plasticity dominate during certain periods of one’s life and are less prevalent during other periods. FACT 3 : Neuroplasticity occurs in the brain under two primary conditions: 1. During normal brain development when the immature brain first begins to process sensory information through adulthood (developmental plasticity and plasticity of learning and memory). 2. As an adaptive mechanism to compensate for lost function and/or to maximize remaining functions in the event of brain injury. FACT 4 : The environment plays a key role in influencing plasticity. In addition to genetic factors, the brain is shaped by the characteristics of a person's environment and by the actions of that same person. Developmental Plasticity: Synaptic Pruning Gopnick et al. (1999) describe neurons as growing telephone wires that communicate with one another. Following birth, the brain of a newborn is flooded with information from the baby’s sense organs. This sensory information must somehow make it back to the brain where it can be processed. To do so, nerve cells must make connections with one another, transmitting the impulses to the brain. Continuing with the telephone wire analogy, like the basic telephone trunk lines strung between cities, the newborn’s genes instruct the "pathway" to the correct area of the brain from a particular nerve cell. For example, nerve cells in the retina of the eye send impulses to the primary visual area in the occipital lobe of the brain and not to the area of language production (Wernicke’s area) in the left posterior temporal lobe. The basic trunk lines have been established, but the specific connections from one house to another require additional signals. Over the first few years of life, the brain grows rapidly. As each neuron matures, it sends out multiple branches (axons, which send information out, and dendrites, which take in information), increasing the number of synaptic contacts and laying the specific connections from house to house, or in the case of the brain, from neuron to neuron. At birth, each neuron in the cerebral cortex has approximately 2,500 synapses . By the time an infant is two or three years old, the number of synapses is approximately 15,000 synapses per neuron (Gopnick, et al., 1999). This amount is about twice that of the average adult brain. As we age, old connections are deleted through a process called synaptic pruning . Synaptic pruning eliminates weaker synaptic contacts while stronger connections are kept and strengthened. Experience determines which connections will be strengthened and which will be pruned; connections that have been activated most frequently are preserved. Neurons must have a purpose to survive. Without a purpose, neurons die through a process called apoptosis in which neurons that do not receive or transmit information become damaged and die. Ineffective or weak connections are "pruned" in much the same way a gardener would prune a tree or bush, giving the plant the desired shape. It is plasticity that enables the process of developing and pruning connections, allowing the brain to adapt itself to its environment. Plasticity of Learning and Memory It was once believed that as we aged, the brain’s networks became fixed. In the past two decades, however, an enormous amount of research has revealed that the brain never stops changing and adjusting. Learning, as defined by Tortora and Grabowski (1996), is “the ability to acquire new knowledge or skills through instruction or experience. Memory is the process by which that knowledge is retained over time.” The capacity of the brain to change with learning is plasticity. So how does the brain change with learning? According to Durbach (2000), there appear to be at least two types of modifications that occur in the brain with learning: A change in the internal structure of the neurons, the most notable being in the area of synapses. An increase in the number of synapses between neurons. Initially, newly learned data are "stored" in short-term memory, which is a temporary ability to recall a few pieces of information. Some evidence supports the concept that short-term memory depends upon electrical and chemical events in the brain as opposed to structural changes such as the formation of new synapses. One theory of short-term memory states that memories may be caused by “reverberating” neuronal circuits -- that is, an incoming nerve impulse stimulates the first neuron which stimulates the second, and so on, with branches from the second neuron synapsing with the first. After a period of time, information may be moved into a more permanent type of memory, long-term memory, which is the result of anatomical or biochemical changes that occur in the brain (Tortora and Grabowski, 1996). Injury-induced Plasticity: Plasticity and Brain Repair During brain repair following injury, plastic changes are geared towards maximizing function in spite of the damaged brain. In studies involving rats in which one area of the brain was damaged, brain cells surrounding the damaged area underwent changes in their function and shape that allowed them to take on the functions of the damaged cells. Although this phenomenon has not been widely studied in humans, data indicate that similar (though less effective) changes occur in human brains following injury. WHAT IS NEUROPLASTICITY, ANYWAY? The human brain is incredibly adaptive. Our mental capacity is astonishingly large, and our ability to process widely varied information and complex new experiences with relative ease can often be surprising. The brain’s ability to act and react in ever-changing ways is known, in the scientific community, as “neuroplasticity.” This special characteristic allows the brain’s estimated 100 billion nerve cells, also called neurons (aka “gray matter”), to constantly lay down new pathways for neural communication and to rearrange existing ones throughout life, thereby aiding the processes of learning, memory, and adaptation through experience. Without the ability to make such functional changes, our brains would not be able to memorize a new fact or master a new skill, form a new memory or adjust to a new environment; we, as individuals, would not be able to recover from brain injuries or overcome cognitive disabilities. Because of the brain’s neuroplasticity, old dogs, so to speak, regularly learn new tricks of every conceivable kind. WHAT PARTS OF THE BRAIN HAVE PLASTICITY? Plasticity Happens Wherever Neuro-processing Occurs Neuroplasticity is not a trait found in a single brain structure, nor does it consist of just one simple type of physical or chemical event. Rather, the brain’s ability to be molded – its plasticity – is the result of many different, complex processes that occur in our brains throughout our lifetime . A host of different structures and types of cells play some part in making neuroplasticity possible. There are even different types of plasticity that, depending on one’s age, are more or less involved in reshaping the brain as it handles new information. Plasticity works throughout the brain not just in the normal processes of learning and adaptation (most obvious in the early developmental years, though continuing throughout life), but also in response to injuries or diseases that cause loss of mental functioning. NEUROPLASTICITY AND THE NATURE/NURTURE DEBATE: WHICH IS IT? On The One Hand There’s Nature, On The Other, Nurture. While genetics certainly play a role in establishing the brain’s plasticity, the environment also exerts heavy influence in maintaining it. Take, for example, the newborn’s brain, which every day is flooded with new information. When the infant body receives input through its many different sensory organs, neurons are responsible for sending that input back to the part of the brain best equipped to handle it – and this requires each neuron to “know” something about the proper neural pathways through which to send its bits and pieces of information. To make this mental roadmap work, each neuron develops an axon to send information to other brain cells via electrical impulses, and also develops many dendrites that connect it to other neurons so that it can receive information from them. Each point of connection between two neurons is termed a “ synapse.” Our genes have, at birth, laid down the basic directions for neurons to follow along this roadmap, and have built its major “highways” between the basic functional areas of the brain. Environmental influence then plays the key role in forging a much denser, more complex network of interconnections. These smaller avenues and side roads, always under construction, can make the transfer of information between neurons more efficient and rich with situation-specific detail. This is clearly evidenced by the rapid increase in synaptic density that can be seen in a normally developing human. Genetics form a neural framework that, at birth, starts each neuron off with roughly 2,500 connections. By age two or three, however, sensory stimulation and environmental experience have taken full advantage of the brain’s plasticity; each neuron now boasts around 15,000 synapses. This number will have declined somewhat by the time we enter adulthood, as many of the more ineffective or rarely used connections – formed during the early years, when neuroplasticity is at its peak -- are done away with. HOW DOES NEUROPLASTICITY WORK? A Matter Of Neuron Networks And Connections. Complete neuron cell diagram. Neuroplasticity can work in two directions; it is responsible for deleting old connections as frequently as it enables the creation of new ones. Through this process, called “synaptic pruning,” connections that are inefficient or infrequently used are allowed to fade away, while neurons that are highly routed with information will be preserved, strengthened, made even more synaptically dense. Closely tied in with the pruning process, then, is our ability to learn and to remember. While each neuron acts independently, learning new skills may require large collections of neurons to be active simultaneously to process neural information; the more neurons activated, the better we learn. DOES NEUROPLASTICITY HELP ME LEARN AND REMEMBER? Remolding Connections Through Synaptic Transmission. Learning affects the brain in two different ways, neither of which would be possible without the special plasticity of our brains. In response to a new experience or novel information, neuroplasticity allows either an alteration to the structure of already-existing connections between neurons, or forms brand-new connections between neurons; the latter leads to an increase in overall synaptic density, while the former merely makes existing pathways more efficient or suitable. In either way, the brain is remolded to take in this new data and, if useful, retain it. While the precise mechanism that allows this process to occur is still unclear, some scientists theorize that long-term memories are formed successfully when something called “reverbration” occurs. When we are first exposed to something new, that information enters our short-term memory, which depends mostly upon chemical and electrical processes known as synaptic transmission to retain information, rather than deeper and more lasting structural changes such as those mentioned above. The electrochemical impulses of short-term memory stimulate one neuron, which then stimulates another; the key to making information last, however, occurs only when the second neuron repeats the impulse back again to the first. This is most likely to happen when we perceive the new information as especially important or when a certain experience is repeated fairly often. In these cases, the neural “echo” is sustained long enough to kick plasticity into high gear, leading to lasting structural changes that hard-wire the new information into the neural pathways of our brains. These changes result either in an alteration to an existing brain pathway, or in the formation of an all-new one. In this way, the new information or sensory experience is cemented into what seems, at its present moment, to be the most useful and efficient location within the massive neurocommunication network. Further repetition of the same information or experience may lead to more modifications in the connections that house it, or an increase in the number of connections that can access it – again, as a result of the amazing plasticity of our brains. THE DAMAGED OR DISABLED BRAIN: CAN NEUROPLASTICITY HELP? Rebuilding Connections That Rebuild Skills. Neuroplasticity is the saving grace of the damaged or disabled brain; without it, lost functions could never be regained, nor could disabled processes ever hope to be improved. Plasticity allows the brain to rebuild the connections that, because of trauma, disease, or genetic misfortune, have resulted in decreased abilities. It also allows us to compensate for irreparably damaged or dysfunctional neural pathways by strengthening or rerouting our remaining ones. While these processes are likely to occur in any number of ways, scientists have identified four major patterns of plasticity that seem to work best in different situations. Take, for example, the case in which healthy cells surrounding an injured area of the brain change their function, even their shape, so as to perform the tasks and transfer the signals previously dealt with by the now-damaged neurons at the site of injury. This process, called “functional map expansion,” results in changes to the amount of brain surface area dedicated to sending and receiving signals from some specific part of the body. Brain cells can also reorganize existing synaptic pathways; this form of plasticity, known as a “compensatory masquerade,” allows already-constructed pathways that neighbor a damaged area to respond to changes in the body’s demands caused by lost function in some other area. Yet another neuroplastic process, “homologous region adoption,” allows one entire brain area to take over functions from another distant brain area (one not immediately neighboring the compensatory area, as in functional map expansion) that has been damaged. And, finally, neuroplasticity can occur in the form of “cross model reassignment,” which allows one type of sensory input to entirely replace another damaged one. Cross-model reassignment allows the brain of a blind individual, in learning to read Braille, to rewire the sense of touch so that it replaces the responsibilities of vision in the brain areas linked with reading. One or several of these neuroplastic responses enable us to recover, sometimes with astonishing completeness, from head injury, brain disease, or cognitive disability. fMRI Images courtesy of Dr. Susan Bookheimer, Ahmanson-Lovelace Brain Mapping Center, UCLA The Brain showing plasticity with regeneration of function below, after the right hemisphere is removed. Reorganization after Hemispherectomy - 12 year old girl. NEUROPLASTICITY CAN’T LAST FOREVER . . . CAN IT? From Fresh Experiences Throughout Your Lifetime. Contrary to widespread belief, the “garden” of the brain never ceases being pruned and newly planted. Though long believed by scientists to be the case, research over the past decade or so has proven that our neural connections do not ever reach, by some age, a fixed pattern that thereafter cannot change. Rather, the ongoing process of synaptic reformation and death is what gives the brain its plasticity – its ability to learn and remember, to adapt to its environment and all the challenges brought with it, to acquire new knowledge and learn from fresh experiences – throughout an individual’s lifetime. Groundbreaking new research suggests that, beyond modifying pathways and forming new ones between existing neurons, the human brain is even able to generate entirely new brain cells. While this neural regeneration was long believed to be impossible after age three or four, research now shows that new neurons can develop late into the life span, even into the golden years of age 70 and beyond. Thus, the old adage “use it or lose it” is brought soundly home. If one’s brain is constantly challenged by and engaged with a variety of stimulations and new experiences, while also exposed regularly to that which it already knows, it is better able to retain its adaptive flexibility, regenerative capacity, and remarkable efficiency throughout life. WHAT DIRECTIONS MIGHT NEUROPLASTICITY TAKE IN THE FUTURE? Seeking A World Without Mental Health Issues. Current research suggests that neuroplasticity may be key to the development of many new and more effective treatments for brain damage, whether resulting from traumatic injury, stroke, age-related cognitive decline, or any number of degenerative diseases (Alzheimer’s, Parkinson’s, and cerebral palsy, among many others). Plasticity also offers hope to people suffering from cognitive disabilities such as ADHD, dyslexia, and Down Syndrome; it may possibly lead to breakthroughs in the treatment of depression, anorexia, and other behavioral and emotional disorders as well. Some scientists have even ventured to suggest that, one day, neuroplasticity could be used in short-circuiting the brain’s racist, sexist, or otherwise culturally unacceptable thinking patterns; even the body’s ability to perform intricate sequences of activities necessary for sports and other highly complex physical processes might eventually be perfected through the power of neuroplasticity. Whether currently in use or only the product of futuristic hopes, the theory upon which harnessing the brain’s plasticity is based is a relatively simple one. With “directed neuroplasticity,” scientists and clinicians can deliver calculated sequences of input, and/or specific repetitive patterns of stimulation, to cause desirable and specific changes in the brain. As further research reveals the best ways to create and direct these stimuli, the amazing potential of the brain’s plasticity can begin to be taken advantage of in medicine, mental health, and a wealth of as-yet-uncharted territory in human behavior and consciousness. Thus, increasing our understanding of neuroplasticity holds great promise – through its complex workings – skills lost can be relearned; the decline of abilities can be staved off, even reversed; and entirely new functions can even, perhaps, be gained.