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WE HAVE SEEN IN Chapters 21 and 22 that the eye is not a mere camera, but instead contains sophisticated retinal circuitry that decomposes the retinal image into signals representing contrast and movement. These data are conveyed through the optic nerve to the primary visual cortex, which uses this information to analyze the shape of objects. It first identifies the boundaries of objects, represented by numerous short line segments, each with a specific orientation. The cortex then integrates this information into a representation of specific objects, a process referred to as contour integration.

These two steps, local analysis of orientation and contour integration, exemplify two distinct stages of visual processing. Computation of local orientation is an example of low-level visual processing, which is concerned with identifying local elements of the light structure of the visual field. Contour integration is an example of intermediate-level visual processing, the first step in generating a representation of the unified visual field. At the earliest stages of analysis in the cerebral cortex, these two levels of processing are accomplished together.

A visual scene comprises many thousands of line segments and surfaces. Intermediate-level visual processing is concerned with determining which boundaries and surfaces belong to specific objects and which are part of the background (see Figure 21–4). It is also involved in distinguishing the brightness and color of a surface from the intensity and wavelength of light reflected from that surface. The physical characteristics of reflected light result as much from the intensity and color balance of the light that illuminates a surface as from the color of that surface. Determining the actual surface color of a single object requires comparison of the wavelengths of light reflected from multiple surfaces in a scene.

Intermediate-level visual processing thus involves assembling local elements of an image into a unified percept of objects and background. Although determining which elements belong together in a single object is a highly complex problem with an astronomical number of potential solutions, each relay in the visual circuity of the brain has built-in logic that allows assumptions to be made about the likely spatial relationships between elements. In certain cases, these inherent rules can lead to the illusion of contours and surfaces that do not actually exist in the visual field (Figure 23–1).

Figure 23–1

Illusory contours and perceptual fill-in. The visual system uses information about local orientation and contrast to construct the contours and surfaces of objects. This constructive process can lead to the perception of contours and surfaces that do not appear in the visual field, including those seen in illusory figures. Top left: In the Kanizsa triangle illusion, one perceives continuous boundaries extending between the apices of a white triangle, even though the only real contour elements are those formed by the Pac-Man–like figures and the acute angles. Top right: The inside and outside of ...

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