Illusory contours: creative mechanisms

A prominent example of this creative process is the phenomenon of illusory contours (Fig. 76.1). In A, the system “visibly” fills in the missing contours of an overlaying triangle. Note that the illusory contours are not just interpolations between given contrast borders, as they might seem to be in A, but form also in the absence of contrast borders that could be interpolated (C). In fact, when the corners of the overlying triangle are defined by lines which could be interpolated, illusory contours do not form (B). What all illusory contour figures have in common is the presence of occlusion cues, such as terminations of lines and edges (Coren, 1972). Thus, the system seems to infer an occluding object. However, this is not an inference in abstract terms. The mere expectation of a contour does not lead to perception of illusory contours (Fig. 76.1D). Apparently, in forming the contours, the system combines evidence from occlusion cues with rules such as the Gestalt principle of good continuation.

Figure 76.1..  

Perception of illusory contours. (A and C, after Kanizsa, 1979. D, René Magritte, Paysage de Baucis, etching, 1966.)

Interestingly, illusory contours are represented in the visual cortex at a relatively early stage. In monkey area V2, many cells respond to illusory contour stimuli as if the contours were contrast borders (von der Heydt et al., 1984). Figure 76.2 shows an example of a cell that was tested with a moving illusory bar. The raster plot in B shows that the cell responds when the illusory contour traverses a small region that was determined before as the cell's minimum response field (ellipse; see legend). Figure 76.2C shows a control in which the two bars were moved exactly as in B, but the open ends were closed off with thin lines. Closing lines weaken the perceptual illusion (see the figure at the bottom), and they also reduce the responses of the neuron. Cells in V2 respond not only to figures with illusory bars, but also to other figures that produce illusory contours, such as a pattern of two abutting line gratings (Fig. 76.3B). It can be seen that the cell of Figure 76.3 responds at the same orientations for the illusory contour as for the bar stimulus; thus, it signals the orientation of an illusory contour. Using the criteria of consistent orientation tuning and response reduction by the closing lines, 30% to 40% of the cells of V2 were found to signal illusory contours of one or the other type, and the results obtained with the two types of contour were highly correlated (Peterhans and von der Heydt, 1989; von der Heydt and Peterhans, 1989).

Figure 76.2..  

Illusory contour responses in a neuron of area V2. Each line of dots in the raster plots represents a sequence of action potentials fired in response to the stimulus shown on the left. A, Responses to a moving dark bar; B, to a figure in which a moving illusory bar is perceived; C, to a modified figure in which the illusion is abolished by adding line segments. Note the reduction of responses. The figures at the bottom illustrate the perceptual effect of adding lines. D, Spontaneous activity. Ellipses indicate the minimum response field of the neuron (i.e., the minimum region outside of which a bar does not evoke a response); the cross indicates the fixation point. (From Peterhans and von der Heydt, 1989, with permission.)

Figure 76.3..  

Illusory contour responses in another neuron of V2. In A, bars, and in B, the border between two gratings were moved across the receptive field at 16 different orientations spanning 180 degrees. The neuron responds at the same orientations for bars and illusory contours. Bottom right, control: grating without a border of discontinuity. (Modified from von der Heydt and Peterhans, 1989, with permission.)

As shown in Figure 76.2, illusory contour responses can be evoked by stimuli which are devoid of contrast over the excitatory center of the receptive field. The inducing contrast features can be restricted to regions from which an optimized bar stimulus would not evoke any response. The cells seem to integrate occlusion features over a region larger than the conventional receptive field (Peterhans et al., 1986). Nevertheless, the extent of spatial integration is limited; for neurons with near-foveal receptive fields, the responses declined if the gap of the stimulus (Fig. 76.2B) was made wider than about 3 degrees visual angle.

V2 is one of the largest areas of the monkey cerebral cortex (Felleman and Van Essen, 1991), and the fact that so many cells in this area respond this way indicates that illusory contour stimuli probe a basic function of the visual cortex. V2 is an early stage of processing where responses are fast and highly reproducible. Illusory contour responses arise as early as 70 msec after stimulus onset (Lee and Nguyen, 2001; von der Heydt and Peterhans, 1989). This indicates that illusory contours are probably not the result of object recognition processes at higher levels but are generated within the visual cortex. Computational models have shown how such contours might be generated (e.g., Finkel and Sajda, 1992; Grossberg and Mingolla, 1985; Heitger et al., 1998).

Processing Stages

Representation of illusory contours has also been demonstrated in V1 of cat (Redies et al., 1986; Sheth et al., 1996) and monkey (Grosof et al., 1993; Lee and Nguyen, 2001; Ramsden et al., 2001). However, it is not clear if cells in V1 also generalize over the various types of illusory contour figures and if they signal the contour orientation. Sheth et al. (1996) and Ramsden et al. (2001) used a combination of optical imaging and single-unit recording to identify the illusory contour representation with the abutting-grating type of stimulus. Sheth et al. found cells with consistent orientation tuning for illusory contours in V1 of the cat. In the monkey, Ramsden et al. found that the representation of illusory contours in V1 is different from that of V2. Illusory contours reduced activity in columns of the corresponding orientation but increased activity in columns of the orthogonal orientation, in contrast to V2, where the same columns were activated by illusory contours and contrast borders. They conclude that V1 deemphasizes illusory contours.

Studies that compared both areas invariably found marked differences between V1 and V2 in the frequency of cells that signaled illusory contours, the signaling of orientation, and the degree of cue invariance (Bakin et al., 2000; Leventhal et al., 1995; Ramsden et al., 2001; Sheth et al., 1996; von der Heydt and Peterhans, 1989).

Correlation of Physiology and Perception

Varying the configurations and spatial parameters of the displays shows a tight correspondence between human perception and neural responses for illusory contours generated by abutting gratings (Fig. 76.3B) (Soriano et al., 1996). However, in discriminating the shape of illusory figures, the human visual system shows larger spatial integration than the neurons of monkey V2 (Ringach and Shapley, 1996). Because neurons that signal illusory contours are only a subset of the cells that signal contrast edges, orientation-dependent adaptation aftereffects should transfer from contrast-defined to illusory contours, but not in the reverse direction, and the discrimination of orientation should be less accurate for illusory contours than for contrast-defined contours. Both predictions were borne out in psychophysical experiments (Paradiso et al., 1989; Westheimer and Li, 1996). Illusory contours are usually associated with perception of overlay (Coren, 1972), and some neurons in V2 are selective for the implied direction of occlusion of illusory contours (Baumann et al., 1997). Thus, the illusory contour mechanisms may be related to the coding of border ownership, discussed below.

Illusory Contours are Universal

Perception of illusory contours has been demonstrated in a variety of nonhuman species, including the cat, owl, and bee (Bravo et al., 1988; De Weerd et al., 1990; Nieder and Wagner, 1999; Srinivasan et al., 1987; for a review, see Nieder, 2002). Most elegant is the combination of behavioral experiments with single-cell recordings (Nieder and Wagner, 1999).