Sensation and Perception

Sensation and Perception What is it? Why study it? How do you study it? What will you learn in this course?

History: The Greeks Two schools of thought about light: Emanation theory of light- light leaves the eyes in the form of a cone of rays. These rays sense the surfaces upon which they fall (Empedoclés, Plato, Euclid, Hipparchus, Ptolemy) – “solves” the problem of how the visual world is externalized and seen in proper size Éidola or simulacra- objects continuously emit corpuscular images of themselves that move in straight lines through the “diaphanous medium” (Aristotle) – note simulative assumption: what a person knows is a copy of the world, not the world itself (and not a very good copy at that…)

Euclid’s optics (~300 BC): Equated light and vision (like most lay-people). Light traveled in straight lines from the eye, forming a cone with the apex centered on the eye. Only objects within the cone of light are visible, and only a fixed number of rays emitted Al-Kindi (~800AD) Asserted that everything in the world produces rays in all directions (like a star). This radiation binds the world into a network in which everything acts on everything else. Adopted Euclid’s geometric approach, but still subscribed to an emanation theory of vision.

Alhazen (~1000 AD): Adopted Al-Kinde’s idea that radiation is emitted in all directions from each point of an object. Light enters the eye from sources that either reflect or refract light. Laid foundation for modern geometrical optics. Likened the eye to a camera obscura, or pinhole camera, creating an image in the eye. Muslim faith forbade dissection, so the placement of the optic nerve was a bit off…..

Al-hazen noted that images are: • Upside down • Metrically distorted • Same sized image produced by different objects at different distances • Two-dimensional Mapping of world properties into the energy medium is a destructive process. Thus, the transition from retinal image to percept must be a constructive process. Q: How can vision be veridical with such an impoverised starting point? Where do the needed constructive powers come from? A: God.

Homunculus Since Plato, mind and body were considered distinct.

Descartes: First to investigate the relationship between mind and body (“two distinct substances”). Mind was unextended: indivisible or nonspatial. Feels, perceives, thinks, and has sensation. Bodyis extended, divisible into parts, and can’t feel “…..amused by their cries and yelps since these were nothing but the hydraulic hisses and vibrations of machines.”

“Cartesian theater” homunculus Descartes (cont.): Rejected the proposition that mind was distributed throughout body (phantom limbs). All parts of brain double except for pineal gland (or “common sense”). We have two images, yet see one world.

Descartes “psychophysical postulate”: One-to-one correlation between sensory perceptions and corresponding brain states at pineal gland (the “seat of sensation”). Cerebral image should possess as many differences as exist in external objects, so that perception is veridical. The corrections needed for the retinal images were supplied by the functioning of the innate machinery of the body. Knowledge embodied in the physiological hardware (“wetware”)

John Locke (1690) Focused on the issue of how knowledge is acquired. - denied the innate ideas of Descartes - mind is a tablua rasa Reaction to a philosophical history of rationalism and dogmatism. In direct epistemic contact with ideas Ideas can be generated from sensation or reflection, and came in two flavors: - simple ideas are elemental or unanalyzable - complex ideas can be constructed from simple ideas via laws of association. Simple ideas (of sense) also come in two flavors: - primary qualities exist in the object whether they are perceived or not. - secondary qualities only exist in perception (color). Needed to understand the lack of correspondence between physical world and our perception of it.

George “Bishop” Berkeley (1709) Locke was a realist. But some things resemble the world (primary qualities) and some don’t (secondary qualities). World gave rise to ideas. Problem: How do you tell which are which? Berkeley: denied the primary/secondary dichotomy by denying primary qualities! Perception is the only reality of which we can be sure. Physical objects nothing more than an accumulation of sensations experienced together. Primacy of touch in “educating” perception.

Psychology as an empirical science Helmholtz, Weber, Fechner, and Wundt All Germans: deductive (rationalist) and mathematical approach favored in France and England. German “temperament’ well suited to careful, conscientious, thorough collection of observable facts. Johannes Müller (1830s) Embodiment of Descartes’ principle of psychophysical isomorphism: Law of specific nerve energies: arousal or stimulation of a given nerve always gives rise to a characteristic sensation because each sensory nerve has its own “energy” (today known as “labeled lines”) – visual phosphenes

The birth of experimental psychology Hermann von Helmholtz (1856 +) Coined the empiricism-nativism dichotomy to characterize theories of vision Helmholtz was an empiricist: what is innate are the “psychic operations” of judgment, expectation, and reasoning, regarded as “unconscious” and “obligatory.” “Nativistic theory…cuts short any investigation relative to the origin of perceptions since it considers them as original or innate.” Neural impulse was not instantaneous Worked on frog leg preparations and people, but abandoned the human work because of the variability – Precursor to modern reaction time studies. Young-Helmholtz theory of color vision

More Helmholtz... Muller’s doctrine of specific nerve energies: Each kind of experience the result of a particular kind of neural event (irrespective of what causes the nerves to fire). The scientific task: Subdivide each modality into the elementary sensations of which all perceptual experience was presumed to consist Discover the elementary receptors whose excitations initiate the physiological processes that underlie those sensations Measure the physical energies to which those receptors normally respond Vision was “unconscious inference:” we perceive those objects and events that would under normal conditions be most likely to produce the sets of effective sensory stimulation that we are receiving.

In what sense are the elements of sensation analytic units of experience? Young-Helmholtz theory of color: three basic elements. Light of any given perceived color, will mix in the same ways with other colors. …..Subjective color better predicts mixing than physical color. BUT: We cannot discern the components once the colors are mixed. Also now known that the elementary sensations cannot be defined as the peak sensitivities of the underlying receptors (cones); they do not look like “pure” colors. => the elements of experience are inaccessible to conscious experience when they are combined, and the appearance of combined sensations cannot be predicted from the appearance of the individual sensations.

Is phenomenology doomed? Is it incapable of revealing the underlying variables and dimensions that regulate experience? Edwald Hering to the rescue….

The birth of Psychophysics Ernst Weber • Two point threshold • The just noticeable difference (JND) • Weber’s law: • jnd/S = k = “Weber fraction” Gustav Fechner Coined the term psychophysics • Method of average error ( or adjustment) • Method of constant stimuli (“method of right and wrong cases”) • method of JNDs, or increment/decrement detection

The first school of “Physiological Psychology”: Structuralism Wilhelm Wundt (~1880) Experimental investigation of the structure of experience Goal: to discover the nature of elementary conscious experience, i.e, the building blocks of consciousness Since the only one that can observe experience was the person having it, the obvious method was to use introspection Principle of creative synthesis or law of psychic resultants: “Every psychic compound has characteristics which are by no means the mere sum of the characteristics of the elements” (1896).

Behaviorism Bleak period for perception research. Rejection of speculative philosophy, and embracing of logical positivism. Attempt to do a structuralist decomposition of behavior, reduce to stimulus-response pairings.

Kohler Koffka Wertheimer Gestalt psychology Reaction to “molecular,” analytic nature of structuralist approach and the S-R theory of behaviorism A refocus away from the empiricist-nativist dichotomy. What was primary was the dynamical laws that gave rise to order and organization in all natural phenomena: “If we believe that the sciences, natural and moral, are not merely a collection of independent human activities, some players playing one kind of game, others another, but that they are branches of one all-embracing science, then we must demand that the fundamental explanatory principles be the same in all (Koffka, 1935, p. 20).”

Molar versus Molecular Behavior Structuralists and behaviorists asserted that the causes and effects of behavior should be described at the level of elementary sensations (stimulation) that ultimately led to the contraction of muscles (“behavior”). Approach denies the validity of meaning. Molar behavior defines the problems, the answers are given in the molecular level descriptions of sensations and their evoked responses. Gestalt distinction of the Geographical and Behavioral Environment In which environment does behavior occur? denies that the behaviorist/structuralist enterprise can be carried out

Constancy hypothesis of structuralists: constancy of mapping of elementary sensations of the retinal mosaic. If experience does not coincide with proximal stimulation, it is the consequence of learning (similar constancy of S-R links for behaviorists). (multistability demo) Gestalt: Illusions as tools for demonstrating a failure of the constancy hypothesis (and the existence of the behavioral environment). Illusions are what they are because of dynamical laws of organization. Koffka’s question: Why do things look as they do? 1) Because they are what they are. 2) Because the proximal stimulus is what it is. 3) Because the brain processes triggered by proximal stimuli are what they are.

Gibson’s Ecological Psychology Only serious challenge to the claim that vision is underspecified. Challenged the belief that the starting point of vision is the retinal mosaic. Claimed that the “real” problem is performing an analysis of optical structure at the scale of the animal: i.e., ecological optics. Asserts that there is sufficient information in the light to specify the structure of the world if the analysis is performed at the appropriate scale. Perception is not mediated by unconscious inference, but is direct. Attempted to ontologize the behavioral environment.

Marr’s Computational Approach Vision should be studied as a kind of information processing system. Three levels of information processing tasks: Computational theory: what is the goal of the computation, why is it appropriate, and what is the logic of the strategy by which it can be carried out? Representation and algorithm: How can this computational theory be implemented? In particular, what is the representation for the input and output, and what is the algorithm for the transformation? Hardware implementation: How can the representation and algorithm be realized physically?

Sensation and Perception