touch,taste, and smell.

1. SENSATION AND
PERCEPTION
The Auditory, Somatosensory, Olfactory and
Gustatory Systems

3. Introduction
• Sensation refers to the detection of stimuli: the sensory
organ in question must be able to pick up stimuli which it
must then successfully transmit to the brain in order to
evoke sensation.
• Perception refers to the neural pathways in the brain that
integrate, recognise and interpret these sensory signals
and give the sensation meaning.
• Example: a bell is struck behind a patient. If the subject
can hear the sound but not recognise it as coming from a
bell then this indicates good sensation but implicates a
lesion in the neural pathways involved in perception

4. Introduction
• Sensory systems are orgainsed in a hierarchial manner
such that stimuli detected by primary receptors
(specialised cells in the eyes, ears, skin, nose and
tongue) such as light, vibrations/pressure changes and
chemicals are converted into action potentials which
almost always travel from the sensory organ to the
thalamus/hypothalamus in the diencephalon and from
there to the primary sensory cortex before the
information is relayed to the secondary sensory cortex
and then association cortex all of 3 which are in the
brain.

5. Introduction
• Above the level of the receptors, it has been discovered
that there is intercommunication within and between the
different levels of the hierarchy: parallel processing.
• In addition to this, sensory cortices have been to shown to
be functionally segragated in that different parts sense
the different characteristics of a given stimulus.
• Example: different neurons in the auditory cortex are
responsible for pitch and amplitude sensation, damage to
one population of these will mean that overall sensation is
kept but perception would suffer.
• Thus it can be concluded that sensation is “low-order” and
involves the lower levels of the hierarchy while perception
is “high-order” and involves the higher levels.

6. The Hierarchical Organisation of Sensation
Figure 1 J. Pinel Biopsychology 2011

7. The Auditory System: Intro
• Sounds are vibrations of air molecules.
• Human ears can sense sounds of frequencies from 20Hz -
20KHz.
• The amplitude, frequency and characteristics of sound are
perceived as the loudness, pitch and timbre, respectively.
• Sounds made up of more
than one frequency are
perceived at that sound’s
fundamental frequency,
the highest common
multiple of a sound’s
constituent frequencies,
even if that frequency is not
a constituent of that sound.
Figure 2 J. Pinel Biopsychology 2011

8. The Auditory System: Anatomy
Figure 3. Outer and Middle ear

9. The Auditory System: Anatomy
• Sound waves are funneled through the auditory canal
until they impact on and displace the tympanic
membrane.
• The vibrations then are propagated through the 3 solid
ossicles: malleus to incus to stapes.
• The stapes displaces the oval window (a membrane) of
the coiled, fluid-filled cochlea where the auditory receptor
is (organ of corti).
• Vibrations travel through the cochlea and are eventually
dissipated at the round window thus re-equilibrating the
pressure within the cochlea.

10. The Auditory System: Anatomy
• Sound waves travel through the cochlea, their wavefronts
eventually reach and displace the basilar membrane of
the organ of corti.
• Hair cells are lifted up and
tectorial membrane shears
through their cilia.
• Results in depolarisation
and an action potential being sent via CN VIII.
Figure 4. Uncoiled view of cochlea
• Hair cells arranged along basilar membrane: different
frequencies activate corresponding populations of cells.

11. The Auditory System: Anatomy
Figure 5. Transverse view of cochlea
Figure 6. Differentiation of pitch

12. Auditory Pathways
• Auditory nerve of CNVIII carries
action potential to cochlear
nucleus of ipsilateral ear.
• Cochlear nuclei to ipsi and
contra superior olives, and to
contra inferior colliculus.
• Superior olives to ipsi and
contra inf. and sup. Colliculi.
• Inf. Colliculi to ipsi and contra
medial geniculate nuclei of
thalamus
• Thalamus to ipsi primary
auditory cortex.
Figure 6. J. Pinel Biopsychology 2011

13. Auditory Pathways
• Lateral and medial superior olives responsible for the
localisation of sound.
• Medial responds to difference in time of arrival of signal,
lateral to amplitude differences.
• Superior colliculi also believed to serve same purpose.

14. Organisation of Auditory Cortex
• Organised tonotopically (according to different
frequencies) and in functional columns.
• 2 streams:
1. Anterior auditory pathway transmits to pre-frontal
association cortex involved in identification of sounds
2. Posterior auditory pathway transmits to parietal
association cortex involved in localisation of sounds.
• There is intercommunication between the visual and
auditory systems.
• Pitch is perceived by a small population of neurons
anterior to the primary auditory cortex.

15. Damage to the Auditory System
• Total deafness rare
• Even a complete unilateral lesion of auditory cortex only
causes loss of perception for the affected side and not
complete sensation loss due to the complex parallel
pathways and intercommunication involved which can
compensate for the affected region/pathway.
• 2 types of deafness: Conductive and Neural
• The former implicates there is a problem in the
transmitting of the sound wave to the hair cells whereas
the latter implicates a lesion in the neural pathways or
damage to the hair cells (as occurs with ageing).

16. Somatosensory System: Intro
• Comprised of 3 separate systems:
1. Exteroceptive – senses external stimuli.
2. Proprioceptive – monitors position of body in space.
3. Interoceptive – senses the body’s internal environment
(e.g. blood pressure, blood pH, O2 saturation etc.)
• The exteroceptive itself has 3 divisions responsible for the
sensation of pressure/vibration changes (touch),
temperature and one specifically for nociceptive (painful)
stimuli.

17. Receptors
• Many different types to reflect the various types of “touch”
• Broadly can be divided into slow and fast adapting
• Most nociceptors and thermoceptors are slow adapting
and high threshold meaning the continue to create action
potentials for the duration of the stimulus and are
generally not very sensitive (require an intense stimulus to
elicit an AP).
• This is unlike mechanoceptors which are usually fast
adapting and low threshold meaning a continually applied
stimulus will stop being registered (stop eliciting APs) but
even the slightest of touches can depolarise them.

18. Somatosensory Neural Pathways
• Neurones innervating given parts of skin enter the spinal
cord at a given level: the area of skin innervated by all of
those neurones that all enter the spinal cord at that given
level is know as the dermatome.
• 3 neurons are involved in the transmission of touch/pain
information from skin to cortex with cell body loaction,
axon tract location and synapses being different for these
2 modalities.

19. Somatosensory Neural Pathways
• Touch:
• From receptor terminal, axon enters spinal cord via dorsal
root at a given level (the lower the area of skin innervated,
the lower the level). Axon goes up in the dorsal column
region of the spinal cord via either the cuneate (for
hands/arms) or gracile (for legs/feet) fasciculus to
synapse with the cuneate/gracile nuclei respectively, in
the medulla. Cell body of receptor is in periphery.
• This 2nd neuron’s axon now decussates and goes up and
synapses with a neuron in the ventral posterior nucleus of
the thalamus (contralateral to the dermatome).
• This 3rd and final neuron then synapses with a nerve cell
body in the primary/secondary somatosensory cortex.

20. Somatosensory Neural Pathways
• Pain and temperature:
• From the receptor, the axon of the 1st neuron synapses
immediately in the dorsal gray matter of the spinal cord as
soon as it enters it.
• The 2nd neuron’s axon then immediately decussates and
travels up the antero-lateral tracts until it again synapses
with VPN on the contralateral thalamus.
• The 3rd neuron then again synapses with the contralateral
somatosensory cortex like in the touch pathway.
• VPN involved in touch, acute pain and temperature.
• Parafascicular and intralaminar nuclei involved in chronic
pain.

21. Somatosensory Neural Pathways
Figure 8. J. Pinel Biopsychology 2011
Figure 7. J. Pinel Biopsychology 2011

22. Somatosensory Sensation
• Postcentral gyrus is SI.
• Organised somatotopically (somatosensory/motor
homunculus) with each small area being further divided
into even smaller ones corresponding to the type of
stimulus that area received (pain, temperature, touch etc).
• SII is just slightly ventral to this and each receives inputs
from both primary SSCs.
• Information is then relayed to parietal association cortex.
• 2 streams:
1. Dorsal stream to parietal cortex involved in multisensory
integration and direction of attention.
2. Ventral stream to SII involved in interpretation of
objects’ shapes.

23. Somatosensory Lesions
• As in the auditory system, full damage to SI only affects
some of the touch perception of the affected side
(contralateral side of the body) as the patient can still feel
when stimuli are presented on the affected side. Again
this is most likely due to neural intercommunication and
the parallel pathways.
• Damage to the association cortex through, for example,
stroke among other lesions can result in various agnosias
and anopias and these illustrate how connected the
somatosensory system is to the visual system.

24. Pain Perception and Control
• Pain is beneficial –important to know when to seek
help/what to avoid.
• Failure to discover a single “pain centre” in the brain
though the anterior cingulate gyrus is most implicated.
• Pain can be suppressed through the analgesic effect of
the periaqueductal grey matter synapsing with the raphe
nuclei whose axons extend down into the spinal cord,
inhibiting excitatory and painful transmission and through
the endogenous release of endorphins acting on opiate
receptors. Recent research has shown that pathways
exist to increase pain perception as well.

25. Neuropathic Pain
• Neuropathic pain is the sensation of pain in the absence
of a stimulus.
• Thought originate with ectopic stimulation of the CNS at
higher levels than the receptor.
• Research points to aberrant glial cell inappropriately
stimulating CNS neurones involved in pain perception.

26. Olfactory System Receptors
• Receptors located in olfactory mucosa.
• Axons pass through cribiform plate and synapse with cells
in olfactory bulb of brain (this collection of synapses onto
cell bodies is known as a glomerulus) which themselves
project via olfactory tract.
• Several thousand olfactory receptor cells per glomerulus
• Humans have almost 1000 receptor proteins.
• 2 glomeruli in each bulb for each receptor protein.
• 1:1 olfactory cell to receptor protein ratio.

27. Olfactory System Receptors
• Receptors scattered across mucosa however receptors
with same receptor protein all generally synapse in the
same glomerulus in the olfactory bulb.
• Glomeruli seem to be topographically organised as in
other sensory systems but principle governing this
organisation still undiscovered.
• Olfactory receptor cells are replaceable and each has a
lifespan of a few weeks.

28. Olfactory System Pathways
• Each tract projects to medial temporal lobe including
amygdala and piriform cortex.
• Do NOT synapse in thalamus.
• 2 pathways from amygdala-piriform:
1. To limbic system
2. To orbitofrontal cortex via medial dorsal nuclei of
thalamus
• Former involved in emotional response to smells, latter in
conscious perception of odours.

29. Gustatory System
• Taste sensation picked up by taste buds.
• Approx. 50 taste cells per taste bud.
• Each taste bud has its own neuron, therefore ratio of 50
taste receptors to each 2ndary neuron.
• Generally accepted that there are 5 receptor sub-types
corresponding to a given taste: sweet, salty, bitter, sour,
unami (meaty).
• Receptors for sweet, umami and bitter have been
identified with other flavours thought to arise from foods
with those flavours having a direct effect on the ion
channels of the taste cell (not receptor).

30. Taste Sensation
• sensation for anterior 2/3 is trigeminal nerve (V3), taste is
facial (VII).
• Both taste and touch is glossopharyngeal nerve for
posterior 1/3 (IX).
• And finally just a few taste buds furthest back and on
epiglottis which are supplied by vagus nerve (X).
• Taste fibres go through facial nerve to nucleus solitarius
(via chorda tympani) which is key taste centre.
• From medullar solitary nucleus to thalamic VPN.
• From VPN to gustatory cortex on lateral fissure.

31. Taste Sensation

32. Chemical Sense Loss
• Anosmia – inability to smell.
• Ageusia – inability to taste.
• Complete loss of this extremely uncommon though
trauma causing shearing of olfactory bulb can be a
reason for the former.
• Many disease result in partial anosmia of which:
Parkinson’s, Down’s, Kallman’s, epilepsy, Alzheimer’s,
Korsakoff.
• Damage to CN VII can result in ageusia depending on
where along the nerve the damage has occurred.

33. Selective Attention
• Definition: The ability to focus on and divert attention to a specific
subset of stimuli in the presence of a large number of competing
stimuli.
• Works by improving perception of stimuli attention is diverted towards
and reducing perception of competing stimuli.
• Evolutionarily important in that it allows individual to focus on
important matters and discard trivial ones.
• Can occur because of endogenous or exogenous attention.
• The former refers to the individual his/herself having a reason to
divert attention to given stimuli whereas the latter refers to an external
factor having diverted the individual’s attention to a given stimulus.
• Endogenous attention is top-down whereas exogenous attention is
bottom-up.
• Neurally, this corellates to increased or decreased activity in the
dorsal or ventral streams depending on what was being focused on.