As you review this chapter, you may find it useful to think about the following three themes.
1. The survival functions of vision Our visual system, like all the basic mechanisms underlying behavior, evolved through natural selection to promote our ancestors’ survival and reproduction. Our visual system is not an unbiased recorder of physical patterns of light. Rather, it is a biological tool designed to pick out the information in patterns of light that is potentially most useful. As you review, think about the survival advantage of each of the following: (a) the processes of light and dark adaptation, (b) the distinctions between cone vision and rod vision, (c) color vision, (d) the enhancement of contrast at borders, (e) feature detection by neurons, (f) Treisman’s postulated two stages of feature pickup and integration, (g) the complementary roles of parallel and serial processing, (h) the Gestalt laws of perceptual grouping and of figure and ground, (i) illusory contours, (j) illusory lightness differences, (k) Biederman’s idea of geons serving as a basic alphabet of perception, (l) the distinction between the “what” and “where-and-how” visual pathways, (m) the multiple cues that allow us to assess depth, (n) the role of depth cues in perception of size, and (o) multisensory integration. In most cases the “illusions” that our visual system produces are useful in helping us to recognize and interact effectively with objects in the real world in which we live.
2. The interaction of bottom-up and top-down processes in vision The neural processes that bring sensory information into the brain and move it along to higher centers of analysis are referred to as bottom-up processes. Those that bring information down from higher centers to contribute to the analysis occurring at lower centers are referred to as top-down processes. Bottom-up processes discussed in this chapter include: (a) the coding of variations in light by rods and cones; (b) the organization of that information to register elementary sensory features such as color, lightness, contour orientations, and degree of binocular disparity; and (c), in Treisman’s theory, the integration of sensory features that occupy a given spatial location. Top-down processes are assumed to be involved whenever perceptions of the whole, or expectations generated by the context, are brought to bear in the analysis of the sensory information. The phenomena discussed in this chapter that are assumed to involve top-down control include illusory contours, illusory lightness differences that cannot be explained by lateral inhibition, and size judgments that are based on unconscious assessments of depth.
3. The integration of behavioral and neurophysiological discoveries Hermann von Helmholtz realized long ago that all our perceptions result somehow from the interaction between physical properties of the external world and physiological processes occurring in our sensory organs and brain. His trichromatic theory of color vision is an example of his attempts to explain perceptual phenomena—in this case, the three-primaries law of color mixing—in physiological terms. In recent times, great strides have been made in carrying out the general program of research pioneered by Helmholtz.
Examples of modern explanations of perceptual phenomena, discussed in this chapter, that are in the direct tradition of Helmholtz include: (a) the explanation of differences between bright-light vision and dim-light vision in terms of differences between cones and rods in their photochemicals; (b) the explanations of the two laws of additive color mixing in terms of three varieties of cones and the pattern of neural wiring of those cones to opponent-process neurons; (c) the explanation of rapid stimulus feature detection in terms of the properties of individual neurons in the primary visual cortex; (d) the explanation of distinctions between conscious visual perception and unconscious use of visual input to guide movement in terms of two separate pathways of visual processing; (e) the explanation of the role of components in object perception in terms of processes occurring in the “what” pathway of visual processing; and (f) the explanation of binocular disparity as a depth cue in terms of neurons that receive input from slightly different locations on the retinas of the two eyes.