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Sensation and Perception

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Sensation and Perception

  1. 1. 1 Sensation and Perception
  2. 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. 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. 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. 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. 6. Conventional Definition • Perception: The process of selecting, organizing, and interpreting sensory information, which enables us to recognize meaningful objects and events.
  7. 7. Principle # 1 Activity 2 Watch a video clip and identify principles of perceptual functioning
  8. 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. 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. 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. 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. 12. Principle # 2
  13. 13. Necker Cube
  14. 14. 14
  15. 15. Phi Phenomenon http://upload.wikimedia.org/wikipedia/commo ns/8/8b/Phi_Phenomenon.gif
  16. 16. 16 Tall Arch RickFriedman/BlackStar
  17. 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. 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. 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. 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. 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. 22
  23. 23. Principles of Perceptual Organization • What principle of perceptual organization does the figure in the previous slide illustrate?
  24. 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. 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. 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. 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.'
  28. 28. 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).
  29. 29. • “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)
  30. 30. 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.
  31. 31. Mach’s Visual Field (1886)
  32. 32. 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).
  33. 33. 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).
  34. 34. 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
  35. 35. 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.
  36. 36. 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.
  37. 37. 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.
  38. 38. 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%.
  39. 39. 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.
  40. 40. 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.
  41. 41. • 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.
  42. 42. “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.
  43. 43. • 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).
  44. 44. 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
  45. 45. 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.
  46. 46. 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.
  47. 47. 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)
  48. 48. 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
  49. 49. 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.
  50. 50. 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?
  51. 51. 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.
  52. 52. 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.
  53. 53. END
  54. 54. Somatosensory cortex
  55. 55. 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.
  56. 56. 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)
  57. 57. • “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.
  58. 58. 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.
  59. 59. • 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.
  60. 60. 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.
  61. 61. 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).
  62. 62. 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)
  63. 63. 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
  64. 64. 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)
  65. 65. 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).
  66. 66. 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)
  67. 67. “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.
  68. 68. 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
  69. 69. 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.
  70. 70. “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).
  71. 71. 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.
  72. 72. 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.
  73. 73. 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
  74. 74. 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.
  75. 75. • 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
  76. 76. 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)
  77. 77. 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.
  78. 78. • 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!