Embodied Cognition by Lawrence Shapiro (2011) Routledge, Oxon

 

The question vision theory addresses is this: how does stimulation on the retina become a perception of the world? Cognitive scientists also emphasize the apparent fact that the patterns of stimulation on the retina do not carry enough information on their own to determine a unique visual perception. The visual system must therefore embellish visual inputs, make assumptions, and draw inferences in order to yield a correct solution, i.e. an accurate description of the visual scene.

The retina is a concave surface, like the inside of a contact lens, and so the pattern of light projecting onto it is roughly two-dimensional.

Page 20

Marr and Poggio’s work on stereovision is paradigm….When a pair of eyes fixate on a nearby object, the object will reflect light to a point on each retina. However, often the positions of these points on the two retinas are not the same. If it were possible to lay the left retina atop the right, you would find that the elements of the image on the left retina do not all rest on top of the same elements of the image on the right retina. This fact is easy enough to confirm.

Page 21

If you hold a pencil at arm’s length between you and a more distant object, perhaps a doorknob, and alternatively close one eye and then the other, you will notice that if the pencil appears to the left of the doorknob when viewing it through the right eye, it may appear to the right of the doorknob when viewing it through the left eye. This shows that the location of the image of the pencil relative to the image of the doorknob is not in the same position on each retina.

The distance between corresponding points on the two images is called the degree of disparity.

That disparity information can be used to compute relative depth is well known. The calculation is actually quite simple, relying on some basic trigonometry and requiring nothing more than information about the degree of disparity (measured either in radians or arc seconds), the distance between the eyes, and the angle of the lines of sight. However, recovering the necessary information for the calculation is not at all simple. The problem is that the visual system must be able to match the points of an image on the retina with points of the image on the other retina in order to identify the precise degree of disparity. Thus, imagining the pattern of light on each retina to be like a picture, the visual system must be able to determine for each point on picture 1, which point on picture 2 is its match.

Page 22

Gibson’s Ecological Theory of Perception

In the discussion of stereo vision, we saw that the computation of relative depth from disparity information required assumptions about the uniqueness of the location of objects and the tendency of objects to be cohesive and their surfaces smooth. Without the additions of these assumptions, information on the retinal image is insufficient to determine facts about relative depth. In these cases and others, perception is conceived as an inferential process – reconstructing the whole body from a bare skeleton.   

Page 29

What comes to the eye does not, on its own, specify what is in the world, and so the visual system must rely on various assumptions in order to “get things right;” in order to derive correct conclusions about distal causes of the retinal image.

Page 33

As one moves forward, the point dead ahead remains stable, but the world surrounding this point appears to smear, with points along the edge of vision smearing at the highest rate.

Page 34

The Brain’s Role in Vision

Visual processes are not exclusively located in the nervous system. A complete characterization of the processes are going on in the brain when light hits a subject’s eyes would not, for Gibson, suffice to explain vision. Vision is an extended process, involving bodies, motions and interactions; thus, the methodological solipsist who finds nothing disagreeable in the idea of partitioning the subject’s brain from the rest of the world would, if Gibson’s view is accepted, be focused on just a small piece of a much larger visual system.

If information arrived to an observer like a parcel in the mail, then perhaps there would be something to the idea that vision could begin with the retinal image – observers need just sit patiently, waiting for the world to come to them. However, on Gibson’s view, information does not typically come to you, but instead must be actively hunted. Perceptual systems include the various organs and actions that do this hunting:

Each perceptual system orients itself in appropriate ways for the pickup of environmental information, and depends on the general orienting system of the whole body. Head movements, ear movements, hand movements, nose and mouth movements, and eye movements are part and parcel of the perceptual systems they serve… They serve to explore the information available in sound, mechanical contact, chemical contact, and light.

(Gibson 1966: 58)

We saw in the previous section that the invariants appearing in the optic array – the relationship between the corners of a surface, the relative depth of surfaces, the point of zero flow in the direction one is headed – emerge as invariant only in the context of change. Invariants such as these are what the visual system is designed to detect, and it can do so only through activity. If the components of the visual system are those parts and actions that are necessary for collecting visual inputs, then the visual system must include things like the head, the body, and the activity of these parts.

Page 36

Summary

Standard cognitive scientists see the brain as the complete organ of cognition because they take cognition to consist in the algorithmic processes that mediate between a symbolically encoded stimulus and a symbolically encoded response. But the possibility that an organism’s interactions with the environment can suffice to extract invariants from stimuli suggests an alternative conception of the locus of cognition. The assumption that the brain functions as an inference engine begins to look misguided. Still correct is the idea that cognition involves “intelligent” responses to stimuli, but because the detection of stimuli requires active exploration of the environment, the organs and processes of cognition extend beyond the brain to include those parts of the body that are involved in the activities by which information about the environment is collected. The brain, on this view, is better conceived as a controller and organizer of activities that result in the extraction of information than as a computer for processing this information.

Page 49

How might the body contribute or constrain our psychological capacities? Might the body be a constituent in psychological processes? What might be retained from standard cognitive science, and what would be jettisoned in favor of new ideas?

Page 50