What is contour integration?

Consider the image shown in Figure 70.1A. Because reflection and illumination vary across the different surfaces, occlusions between surfaces commonly produce a luminance discontinuity (i.e., an edge), as shown in the edge map in Figure 70.1B. However, edges in scenes do not occur only at occlusions. They may also arise from textures within surfaces, as well as from shading discontinuities.

In the 1980s, a number of modeling studies were published that proposed computational strategies that would help to identify which of the edges in a scene made up the principal boundaries of an object. Under the assumption that boundary edges were likely to extend over large regions of the visual field, the computations were designed to extract only those edges that were continuous over an extended area. The algorithms that were developed were based on the assumption that the problem could be at least partially solved by integrating over neighboring regions that had similar orientations. However, although some of these integration models included, or were derived from, known physiology (e.g., Grossberg and Mingolla, 1985; Parent and Zucker, 1989), the evidence that an integration algorithm of this kind was actually performed by the visual system was not widely accepted.

Two lines of research have recently helped to support the plausibility of a scheme such as the one described above. The first line comes from a series of anatomical and physiological studies that used both cat and primate and suggest that there exist long-range connections between neurons in primary visual cortex that link neurons with similar orientations. The second line consists of two types of psychophysical study that have provided evidence for the sorts of associations implied by the physiological and anatomical results (Field et al., 1993; Polat and Sagi, 1993, 1994). The results of these studies converge on an account that suggests that neurons in primary visual cortex integrate information from outside the classical receptive field in a way that promotes the integration of contours. Below we review some of these studies.