Sensation and perception

1. 1
Sensation and Perception

2. • Identify basic principles governing human
perception
• Compare theoretical perspectives based on
knowledge from research on perception
• Based on their different underlying
assumptions about the psychological process
(function) of perceiving

3. Sensation vs. Perception
• Activity 1: Read Madeleine J’s case presented
by O. Sacks and infer the difference between
sensation and perception from her impaired
and normal functions.
• Spasm: involuntary muscular contraction,
consisting of a continued muscular contraction
(tonic spasm)

4. 1. According to Sacks, her sensory capacities are
intact. List them.
2. In contrast, she had the profoundest
impairment of perception.
How was that expressed?
What was the evidence for that?
3. Based on this case, write a sentence explaining
the difference between sensation and perception.

5. Conventional Definition
• Sensation is the process by which our sensory
receptors receive sensory stimulation from the
environment and transform that into neural
impulses and finally deliver the neural
information to the brain.

6. Conventional Definition
• Perception: The process of selecting,
organizing, and interpreting sensory
information, which enables us to recognize
meaningful objects and events.

7. Principle # 1
Activity 2
Watch a video clip and identify principles of
perceptual functioning

8. 8
Inattentional Blindness
Inattentional blindness refers to the inability to
see an object or a person in our midst. Simmons
& Chabris (1999) showed that half of the
observers failed to see the gorilla-suited
assistant in a ball passing game.
DanielSimons,UniversityofIllinois

9. 9
Change Blindness
Change blindness is a form of inattentional blindness in
which two-thirds of individuals giving directions failed
to notice a change in the individual asking for
directions.
© 1998 Psychonomic Society Inc. Image provided courtesy of Daniel J. Simmons.

10. • What conclusions can you draw about human
perception based on these examples?
• What principle of perceptual functioning can
you infer from these cases on inattentional
blindness?
• Write all possible statements about perceiving
that you can draw from these cases

11. • Attention is a selective process
• Perceiving is grounded in one’s activity
• Perception and Action are inextricably linked,
part of a unified system

12. Principle # 2

13. Necker Cube

14. 14

15. Phi Phenomenon
http://upload.wikimedia.org/wikipedia/commo
ns/8/8b/Phi_Phenomenon.gif

16. 16
Tall Arch
RickFriedman/BlackStar

17. 17
Illusion of a Worm
The figure on the right gives the illusion of a blue hazy
“worm” when it is nothing else but blue lines identical
to the figure on the left.
©1981,bypermissionofChristophRediesand
LotharSpillmannandPionLimited,London

18. 18
3-D Illusion
It takes a great deal of effort to perceive this figure in
two dimensions.
ReprintedwithkindpermissionofElsevierScience-NL.Adaptedfrom
Hoffman,D.&Richards,W.Partsofrecognition.Cognition,63,29-78

19. • What can you infer about human perception
based on these examples?
• What can you infer about how humans relate
to the environment (external reality) based on
these perceptual phenomena?

20. Conceptualizing Perception
• Perception is not a copy of reality
• External objects do not determine (at least not
entirely) what we perceive
• Perception is not merely registering external
objects
• Not a passive process
• Perception is a constructive process

21. 21
Perceptual Organization
How do we form meaningful perceptions from sensory
information?
We organize it. Gestalt psychologists showed that a
figure formed a “whole” different than its
surroundings.

22. 22

23. Principles of Perceptual Organization
• What principle of perceptual organization
does the figure in the previous slide illustrate?

24. 24
Grouping
After distinguishing the figure from the ground, our
perception needs to organize the figure into a
meaningful form using grouping rules.

25. Children's schemas represent reality as well as their abilities to represent what
they see.
Schemas
Schemas are concepts that organize and interpret
unfamiliar information.

26. 26
Cognitive Approach to Perception
• The process of selecting, organizing, and interpreting
sensory information, which enables us to recognize
meaningful objects and events.
• Transitory and meaningless sense data are
transmitted to the brain, which then orders these
data into commonly held and enduring conceptual
categories.

27. The Retinal Image
• Kepler’s discovery of the retinal image
• Johannes Kepler (1571 –1630) was a German astronomer
and mathematician. A key figure in the 17th century
scientific revolution, best known for his laws of planetary
motion.
• He also extended his study of optics to the human eye, and
is generally considered by neuroscientists to be the first to
recognize that images are projected inverted and reversed
by the eye’s lens onto the retina.
• Casting light through the dissected eye of a bull,
experiment showed the 'retinal image' was indeed
'inverted.'

29. The Vision Problem
• If the 'image in my eye' is inverted, why does one see
the world 'right-side up'?
• Why does one see the world in 3-D , even though the
retina is itself flat and 2-D?
• “For centuries, it has been assumed that in vision the
problem is to explain how the difference between the
retinal image and its visual result (percept)can be
overcome” (Reed, 1988, parenthesis added).

30. • “Traditional perceptual theory has long maintained that a
profound gulf exists between the perceiver and the world to be
known.” (Costall, 2002)
• Stimulation available to the perceiver seems to be profoundly
incomplete and ambiguous.
• “It is generally assumed that the information encoded in sense
data is highly impoverished and, in itself, quite insufficient to
specify the objects and events that subjects claim to perceive in
their environment”
• All knowledge of the environment has to be reconstructed from
these inadequate and fragmentary data, and this is achieved
through processing the ‘raw’ sensory input according to cognitive
schemata located in the head of the perceiver, not ‘out there’ in
the world. (Ingold, 1992, p. 45)

31. Snapshot Conception
• “When we try to understand the nature of sensory
perception, we tend to think in terms of vision, and
when we think of vision, we tend to suppose that the
eye is like a camera and that vision is quasi-
photographic process. To see, we suppose, is to
undergo snapshot-like experiences of the scene before
us.
• Visual experience as sharply focused, uniformly
detailed, and high-resolution.

32. Mach’s Visual Field (1886)

33. Philosophical Skepticism
• Traditional:
• We can’t trust our senses.
• We see much more than is given to us.
• What we experience is an internal picture
constructed by the brain, not the world itself (cf.
Noë, 2009).

34. New Skepticism
• The world is a grand illusion
• The experience of detail is an illusion
• “It seems to us that we enjoy a visual impression of the
environment in sharp focus and detail. But we don’t!”
(Noë, 2004).
• We don’t see more than is given to us. It isn’t really true
that the brain builds up an internal model of the world.
(Noë, 2009)
• “Perceptual consciousness is a kind of false
consciousness (Noë, 2004).

35. Philosophical Issues
• Epistemology: can we know the world or
reality? How can we determine whether
knowledge is valid? Can we separate
knowledge from belief?
• Ontology: what is reality?
• Perception as contemplative detachment

36. Passive vs. Active Touch
• Passive touch involves only the excitation of receptors
in the skin and its underlying tissue.
• Active touch involves the concomitant excitation of
receptors in the joints and tendons along with new and
changing patterns in the skin.
• Active touch is an exploratory sense (not merely
receptive sense), i.e. when a person touches something
variations in skin stimulation are caused by variation in
her motor activity.

37. Comparison Active vs. Passive touch
Experiment:
Each participant was shown the set of stimulators and the
equivalent set of numbered drawing hung on the curtain in
front of him. He then put palm up on the table behind the
curtain and was either touched by the object or was allowed
to touch it for several seconds.
1) In the passive condition, the form was pressed down on
the palm by the experimenter so that a certain amount
of unsteadiness was inevitable.
2) In the active condition, the subject was permitted to
explore the edges in any manner they chose.

38. Act of touching
• When an object of any sort is placed in the hand of the
observer or when his hand is placed on it, the observer
tends to do the following:
- Trace movements with one or several fingertips,
opposition of the thumb and other fingers,
- Rubbing, grasping, or
- Pressing movements of the fingers
- Name the object or compare to any familiar one
- Trying to obtain mechanical events at the skin at various
places in various combinations
- If the hand is conceived as a sense organ, the observer
seems to be adjusting it. He appears to be searching for
stimulus information.

39. Comparison Active vs. Passive touch
Conditions of the experiment:
• The six forms were presented five times each (but the
subject was told not to expect equal frequencies) and
under two conditions, making a total of 60 trials.
• The order was random. No preliminary practice or
knowledge of results was given. Twenty subjects were
tested.
Results of the experiment:
• For passive touch the mean frequency of correct matches
was 49%.
• For active touch the mean frequency was 95%.

40. Comparison Active vs. Passive touch
Conclusions from the experiment:
• The real point of the experiment is that tactual form
perception does not depend on the pattern of local signs on
the skin.
• With active touch no forms existed on the skin, but only a
changing pattern of pressures.
• Why does the perception correspond to the form of the
object instead of to the form of the stimulus?
• The paradox is even more striking, for tactual perception
corresponds well to the form of the object when the stimulus
is almost formless.
• A clear unchanging perception arises when the flow of sense
impressions changes most.
• It might be that the skin does not have as its primary function
the registering of form as this has usually been conceived.

41. Active Touch
• Act of touching or feeling is a search for stimulation or
an effort to obtain the kind of stimulation which yields
a perception of what is being touched.
• The purpose of the exploratory movements of the hand
is to isolate and enhance the component of stimulation
which specifies the shape and other characteristics of
the object being touched.

42. • The old question of what do agents do when they
perceive? was replaced by how did they detect the
invariants in the available information?
• observers merely have to differentiate the information
until it is most clear and stable in relation to the co-
occurring context of action
• Invariants over the five kinds of sensory experiences
resolve the issue of how modal particulars become
relevant and make an experience organically whole;
invariants over adjacently and successively ordered
experiences resolve their multiplicity into coherent
objects and continuous events, respectively.

43. “The World Is Its Own Model”
• Gibson’s first important point is that the world is itself
structured or constrained in highly specific ways. (Costall, 2004)
• The world does not show up for us as present all at once in
our minds.
• We don’t represent all the detail at once, but we do have
access to all the detail.
• “When I look at a tomato on the counter before me, in what
does my sense that the tomato has a back side consist? Just
in the fact that I understand, in a practical, bodily way, that
moving my eyes and head in relation to the tomato brings
the tomato’s reverse side into view.

44. • The visual field- our visual world- is not the field
available to the fixed gaze, as in the snapshot
conception of visual experience- Mach’s picture.
• The visual field, rather, is made available by looking
around.
• “Our ability to sustain perceptual contact with the
world is not a matter of a picture of the scene in our
brains; rather it is a matter of access. And this, in
turn, is a matter of skill. For example, seeing
requires a practical understanding of the ways that
moving one’s eyes and one’s head and one’s body
changes one’s relations to what is going on around
one.” (Noë, 2009).

45. What is a stone?
• A crab hides beneath it-- shelter
• A bird uses it to break open snail shells-- tool
• An angry person picks it up and hurls it at an
adversary– weapon

46. Perception as Engagement with the World
• It was the very involvement of the person in
her environment, in the practical context of
throwing, that led her to attend to the
‘throwability’ of the object, by virtue of which
it was perceive as a missile.
• As we move around in and explore the
environment, we actively seek and pick up
information that specifies invariant properties
and qualities of the objects we encounter.

47. Affordances
• There is a tight perceptual attunement between animal
and environment.
• Animals are directly sensitive to the features of the
world that afford the animal opportunities for action
• Affordances
• Our immediate perception of the environment is in
terms of what it affords for the pursuit of the action in
which the person or animal is currently engaged.
• For the active animal, the ground is directly perceived
as walk-uponable, and the tree stump as sit-uponable.

48. Affordances
• “The affordances of the environment are what
it offers the animal, what it provides and
furnishes, either for good or ill… something
that refers to both the environment and the
animal in a way that no existing term does. It
implies the complementarity of the animal
and the environment.” (Gibson, 1979, emphasis
added)

49. Vision Through Touch
• Engineer and physiologist Paul Bach-y-Rita
• Device to enable blind people to see
• Theoretical assumptions:
- Eyes are a channel for getting information to the nervous system
- Provide the same visual information through different channel (e.g.
touch)
• Device: camera (on person’s head or shoulder) attached to
an array of vibrators (placed on the thigh or abdomen)
transfers a range of tactile stimuli on the person’s skin
• Device represents a tactile-visual substitution system
enabling blind person to make judgments about size, shape,
& number of objects placed on the other side of the room

50. Looking beyond the brain
• What does Paul Bach-y-Rita’s TVSS actually do?
• It sets up a relation between the perceiver and objects in
the scene around the person where there was no relation
before
• The TVSS establishes a new way of being connected to the
environment
• What governs the character of our experience?
• It is not the neural activity in our brains on its own; it is our
ongoing dynamic relation to objects, a relation that clearly
depends on our neural responsiveness to changes in our
relation to things.
• What causes the effects for consciousness of neural activity
in the touch-dedicated parts of the brain to change?
The world and our relation to it.

51. Tactile-Visual Substitution System
(TVSS)
• Stimulation of the skin gives rise to neural activity in touch areas
of the brain (somatosensory cortex)
• Person adapted to the TVSS, activation in somatosensory touch
areas gives rise not to the experience of being touched (at least not
only to the feeling of touch) but to a visual experience of the scene in
front of the person.
• Bach-y-Rita’s sensory substitution system is perceptual plasticity
without neural plasticity.
• We need to look beyond the brain if we want to get a handle on
what is bringing about the dramatic changes in the character of
experience that we witness.
• What explains the change in the qualitative character of
experiences associated with somatosensory cortex, if in fact
there is no rewiring, or corresponding change in
neurophysiology?

52. Action in perception
• Traditional approach: vision happens in us (to us in our
brains)
• Alva Noë & Kevin O’Regan: Seeing is a bodily activity
(moving eyes, head and body): how things look depends on
what you do
• Movement of your eyes, head, body actively produce
changes in sensory stimulation to your eyes.
• Central task for any perceiving organism is to master the
dynamic patterns of sensory stimulation and movement
• Seeing is an activity - something we do – activity of
exploring the world making use of our practical familiarity
with the ways in which our own movement drives and
modulates our sensory encounter with the world.

53. Brain and world
• Consciousness of the world around us is something that we do:
we enact it, with the world’s help, in our dynamic living activities.
• It is not something that happens to us
• The brain does not generate consciousness (like a stove
generates heat)
• The brain’s function:
- is to coordinate our dealings with the environment
- can only be understood in the context of an animal’s embodied existence,
situated in an environment, dynamically interacting with objects and
situations.
• The body gives structure and shape to the kinds of relations we
can have to the world around us.
• We look in the wrong place if we look for consciousness in the
brain.
• Our consciousness includes not only brain but also our active
lives in the context of our worlds.

54. END

55. Somatosensory cortex

56. Sensory substitution
• Is Bach-y-Rita’s Tactile-Visual Substitution System (TVSS)
visual?
• The ways in which sensory stimulation depends on
movement in TVSS is similar to the ways in which it
depends on movement during vision – they share a
style ( e.g. in TVSS things get bigger as you approach them).
• The visual (or quasi-visual) character of the sensory
substitution system is not fixed by the nature of the
neural activity in the somatosensory cortex; rather, it is
fixed by the ways in which that activity varies as a
function of movement. The way that activity varies as a
function of movement is precisely the visual way.

57. J. J. Gibson
• “When the senses are considered as channels of
sensation (and this is how the physiologist, the
psychologist, and the philosopher have considered
them), one is thinking of the passive receptors and the
energies that stimulate them, the sensitive elements in
the eyes, ears, nose, mouth, and skin.”
• “It can be shown that easily measured variables of
stimulus energy, the intensity of light, sound, touch
…vary from place to place and from time to time as the
individual goes about his business in the environment.”
(Gibson, 1966, emphasis added)

58. • “The stimulation of the receptors and the presumed
sensations, therefore, are variable and changing in the
extreme, unless they are experimentally controlled in a
laboratory.
• The unanswered question of sense perception is how an
observer, animal or human, can obtain constant
perceptions in everyday life on the basis of these
continually changing sensations. For the fact is that
animals and [humans] do perceive and respond to the
permanent properties of the environment as well as to
the changes in it.
• The active observer gets invariant perceptions despite
varying sensations.

59. Higher-Order Variables of Stimulation
• The data in the light to the eye could not explain
perception but only elementary sensations.
• “[N]o reason to believe that perceptual experiences
could not be supported by any of a wide variety of
other patterns of variation in light. (Mace, 1977)
• Gradients of texture and texture flow.
• The fundamental gradient is the one extending from
the ground to the horizon: retinal gradient of texture.

60. • The retinal gradient of texture is deformed and
complicated by the movement of an observer.
• It is the regularity of this change, not the particular
stimulus elements, that constitute the gradient.
• Retinal motion gradient:
• When an observer moves forward, the stimulation at
her eye due to the projection of the sight line at the
horizon is unchanged,
• Whereas the stimulation coming from surfaces close
to the self undergoes the most rapid change in the
field.
• Locomotion imposes a gradient of motion over a static
gradient of texture.

62. Self-Movement vs. Object Motion
• With object motion there is an isolated region
of coherent retinal motion, not a gradient
across the field.
• With self-movement the retinal image is
deformed as a whole.
• The direction of all retinal motion is radially
outward from the point toward which one is
moving: center of expansion.

64. Perceiving as Exploratory Activity
“Heretofore we have been talking about visual
perception as if the observer stood motionless in
the environment and kept his head fixed in one
position. The normal human being, however, is
active…If he is not walking or driving a car or
looking from a train or airplane, his ordinary
adjustments of posture will produce some change in
the position of his eyes in space. Such changes will
modify the retinal images in a quite specific way.”
(Gibson, 1950).

65. Conceptual Skills
• Inductive Reasoning:
Draw general conclusion about perception from
specific cases of percepts (examples); identifying
common, invariant, stable features, attributes, or
properties among objects or phenomena thus
grouping or classifying them together.
• Deductive Reasoning:
Apply a general principle to specific cases (particular
manifestation, instantiation of the class or
phenomenon)

66. Apparent Movement
• It can’t be explained in terms of the summation of
individual elements.
• Stationary individual circles flashing on and off
• Whole is greater than the sum of its parts
• E.g., melodies or tunes played in different keys
• Gestalt: form, shape or configuration

67. Individual-Environment Relation
• Behaviorism:
• Environmental Determinism
• Stimulus determines (elicits, reinforces) responses
• Cognitivist Approach:
• Constructivist, indirect
• Reality is constructed according to cognitive
structures (‘in the head’); innate of developed
(e.g., schema)

68. Critique
• J.J. Gibson, E. Gibson, Tim Ingold
• Overemphasis on cognition
• “Perceptual activity consists in the operations
of the mind upon the deliverances of the senses
(Gibson, 1976; quoted in Ingold, 1992).
• The only activity in perception in mental activity
• Reject Cartesian dualism between sensation
and intellection (cognition).

69. Prior Knowledge
• The standard accounts of perception keep ending
up by having to account for the possibility of
perception in terms of prior knowledge (e.g.,
schemas)
• Where does this prior knowledge come from?
• “To appeal to past experience will hardly do, given
that traditional theory gives us no grounds for
supposing that the gulf between perceiver and
world could have been any less profound in the past
than it is supposed to be now.” (Costall, 2002)

70. “reconstructionist” conception of
vision
• Vision is a process whereby the brain
constructs an elaborate representation of the
visible world on the basis of information
encoded on the retina.

71. Active Touch
• Active touch – touching (impression on the skin is brought
about by the perceiver)
• Passive touch – being touched (by outside agency)
• Active touch is an exploratory sense (not merely
receptive sense), i.e. when a person touches something
variations in skin stimulation are caused by variation in
her motor activity.
• What happens at her fingers depends on the
movements that she makes and the touched object.
• Touching movements do not modify the environment
only the stimuli coming from it

72. Comparison Active vs. Passive touch
Conclusions from the experiment:
• Continuous change in the proximal stimulation is
accompanied by nonchange, that is, the set of invariant
relations. The former is not noticed; the latter is separated
out and attended to.
• The role of exploratory finger movements in active touch
would then be to isolate the invariants, that is, to discover
the particular external component in the flux of
stimulation.
• To apply a stimulus to an observer is not the same as for an
observer to obtain a stimulus.

73. “the visual world is an illusion”
• Not only visual scientists but philosophers
have proposed that the change blindness
studies support this “grand illusion
hypothesis” (Blackmore et al. 1995; Dennett,
1991, 1992, 1998; O’Regan 1992; Rensink et
al. 1997).

74. Active touching experiences
• Unity of the phenomenal object
- when feeling a single object with two fingers, only one object is perceived
although there are two separated cutaneous pressures
• Stability of the phenomenal object
- when sliding the skin over a corner or protuberance of an object, the
displacement of the cutaneous pressure, the "tactile motion," cannot usually
be noticed. The object seems to remain stationary even though the impression
moves relative to the skin.
• Rigidity or plasticity of the phenomenal object
- when pressing a finger on a rigid surface or squeezing an object with the hand,
it is difficult to notice the increase of intensity of cutaneous sensation; instead
the observer is primarily aware of the substance and its resistance.
• Shape of the phenomenal object
- When the corners, edges, or other protuberances of a strange object are being
felt, one can distinguish the pattern which these make to one another but one
cannot distinguish the pattern which the various cutaneous pressures make to
one another. One perceives the object-form but not the skin-form.

75. Relation between touch and vision
• Active touch is an excellent channel of spatial information in
that the arrangement of surfaces is readily picked up.
• Succession enters into the operation of both senses.
• The eyes normally fixate in succession just as the fingers
explore in succession. With two eyes, and by changing one's
standpoint, more of an object than its front surface is
perceived.
• Vision and touch have nothing in common only when they
are conceived as channels for pure and meaningless sensory
data. When they are conceived instead as channels for
information-pickup, having active and exploratory sense
organs, they have much in common. In some respects they
seem to register the same information and to yield the the
same phenomenal experiences.

76. Data for Vision
• “[W]hy should we suppose that data for vision is the
content of the retinal image? If we think of the
perceiver not as the brain-photoreceptor system, but
rather as the whole animal, situated in the
environment, free to move around and explore, then
we can take seriously the possibility that data for
vision … are not the content of a static snapshot-like
retinal image. At the very least, the animal or brain has
access to the ‘dynamic flow’ of continuously varying
retinal information.
• Optic flow contains information that is not available in
single retinal images

77. Detecting Invariant Properties of the
Environment
• The expanding optic field flow indicates that the
observer is approaching a fixed point;
• Contracting optic field flow indicates that she is
moving away from a fixed point.
• The animal has access not only to information
contained in optic flow, but also to information
about the way optic flow varies as a function of
movement.
• When we move through a cluttered environment,
one object may come to occlude another.

78. • Occlusion is reversible. By tracing movements back, you can
bring an occluded surface back into view. (Gibson, 1979).
• The perceiver can differentiate mere occlusion from
obliteration.
• The animal or person can explore the structure of the flow of
sensory changes and to discern in this structure invariant
properties of the environment.
• “The Eyes have feet”: the eyes are under muscular control,
are part of a moving head, which, in turn, is set on top of a
body that gets around in the world.
• Perception is an embodied activity

79. Relational Reality (ontology)
• “With the concept of affordances, Gibson challenged the
deeply entrenched notion that meaning is purely internal, by
questioning the dualism of the subjective and objective.
(Costall, 2004)
• “[The notion of affordances] is meant to capture the fact that
what animals see can be partitioned relative to the scale of
the animal. A supporting surface that is about knee high is
something that a person can sit on… The fact that they are
scaled to an animal means that they are not strictly physical
either. Yet the relation exists in the world. This kind of
relational entity, reflecting environmental properties and,
simultaneously, an animal’s point of view, is a hallmark of
Gibson’s emphases from at least 1950. (Mace, 2005)

80. Perception as Engagement with the World
• We perceive the world as, and because, we act
in it.
• The socially shared meanings known as
affordances, as invariants of invariants, offer all
creatures with the proper scale attunement
and motives, a democracy of opportunity.

81. • Widespread adherence to the view that seeing is
a process whereby the brain builds up detailed
internal models has obscured the fact that vision
is a capacity of the whole situated animal.
• Perception is primary in that it needs no
mediation by memory, expectations, inferences,
or any other cognitive process
• the human or animal does not have to construct
an awareness of the world from meaningless
energy distributions but need only detect the
meaning in the invariant information conveyed by
the environmental energy distributions.
• the information does not have to be processed,
it needs only to be detected!