Illusory contours

As with many brain computations, we can understand segmentation better by observing its action when it deals with an exceptionally difficult task. Usually segmentation is done so efficiently by the brain that we (as observers) are unaware that it is happening. But for certain special visual images, the segmentation process becomes evident. This is the reason for the fascination with these special images, the so-called illusory contours (ICs). An example of such a visual image is Figure 74.1, an image referred to as a Kanizsa triangle, named after the Italian Gestalt psychologist Gaetano Kanizsa, who made this image famous (Kanizsa, 1979). In this figure, the perception of a bright white triangle is very strong, but if one scrutinizes the boundaries of the triangle, it becomes evident that there is no difference in the amount of light coming to the eye from the regions inside and outside the perceived triangle. Yet we see the inside as a bright surface segmented from its background by sharp contours along the boundary of the triangle. In this sense, the boundary between the inside and outside of the triangle is an IC. This image is a classical example in favor of the basic concept of the Gestalt psychologists, also echoed in the work of Donald Hebb, that the brain is “searching” for meaningful patterns. In this case, the brain manufactures a perceptual triangle from fragmentary information because a meaningful pattern, an occluding triangle, is consistent with the available image information even though other perceptions are possible. It is reasonable to believe that the segmentation computations the visual system performs on these exceptional Kanizsa images are the same as for more typical images.

Figure 74.1..  

Kanizsa triangle. The occluding triangle that appears in front of the three circles and the three line segments has the same physical brightness as the surroundings. But it appears somewhat brighter, and appears to be a solid surface in front, because of perceptual processes.

One of the main points of scientific investigation of ICs is the nature and location of the brain area that performs the segmentation of the illusory figure from its background. Some psychologists have favored an explanation in terms of perceptual problem solving and think of ICs as cognitive contours (e.g., Gregory, 1987). Such cognitive approaches do not usually specify or even speculate about the brain areas involved in the perception. However, we could speculate that such a cognitive explanation would involve both visual cortical areas in the posterior cerebral cortex, as well as frontal and temporal cortex. In opposition to the top-down cognitive approach, more bottom-up, stimulus-driven approaches have been proposed (e.g., Grossberg, 1997; Heitger and von der Heydt, 1993). The bottom-up explanation would seem to imply the involvement in IC perception of early visual areas in which visual signals are still arranged retinotopically. There are psychophysical as well as neurophysiological and brain imaging studies of the nature of IC processing and also of localization of IC-evoked signals. The results of these different studies provide a fairly compelling case for the concept that IC perception is the result of the combined and cooperative action of early and later, or more retinotopic and more abstract, visual cortical areas. In this chapter, we begin by discussing psychophysical studies we have done on these problems, followed by a consideration of neurophysiological and brain imaging results on IC perception and segmentation.