Distortions in perceptual judgments have been attributed to the axes of bilateral symmetry of surrounding figures. In the figures used in prior investigations, the axes of bilateral symmetry have been confounded with loci where heightened neural interaction might be expected to occur. In the present experiment, subjects reproduced dots surrounded by figures for which the axes of symmetry and loci of hypothesized neural interactions differed. The observed displacements supported the hypothesis that distortions were due to spreading neural effects of the surrounding figures rather than to the effects of axes of bilateral symmetry. need not be due to lateral inhibition, nor need they be due to cortical rather than retinal interaction. It is only necessary to assume that real lines give rise to neural effects that spread away from their borders.The bilateral symmetry hypothesis predicts that the apparent location of a point within the boundaries of the figures should be displaced toward a real line, a figural contour, or an implicit line, an axis of bilateral symmetry, as shown in Figure 2, whichever is closer to the point. The neural interaction hypothesis predicts that points should only be affected by surrounding physically present lines. Whether the locations are displaced toward the lines or away from them depends on whether excitatory or inhibitory effects are posited. The bilateral symmetry hypothesis predicts that points falling on a virtual contour will show no displacement whatever. The neural interaction hypothesis predicts that, even after displacement, points lying on a line of heightened neural interaction will still lie on that line. For example, points along the diagonal of the square may appear closer to or farther from the corner of the square, but they will remain equidistant from the borders of the square.
