Stimulus manipulations that reduce the square-wave illusion

When observers view a triangle-wave luminance profile, they often report a square-wave illusion with a depth component. Alternate bars appear to be in different depth planes and the surface appears corrugated; illuminated from either the right or the left. These perspectives alternate with continuous viewing. One explanation for this illusion stems from a local energy model of feature detection proposed by Morrone and Burr (Proc. Roy. Soc. Lond. B235, 221-245, 1988). This model assumes two phase-sensitive mechanisms that process lines and edges in the visual image. It is suggested that the square-wave illusion derives from rivalry between these two mechanisms. In experiment 1, the aim was to determine whether phase alternation of the triangle wave would lead to differences in the duration and number of perceptual reversals of the illusion. The results indicate a decline in illusion duration and frequency of reversal rate with increased alternation rate. With the addition of some assumptions about the temporal resolution of the line and edge detectors, the results support the proposed explanation. In Experiment 2, the effects of high spatial-frequency contrast increments and decrements were explored. Increments did not lead to significant increases in the duration of the square-wave illusion or reversal rate, but decrements resulted in a substantial reduction in the illusion duration and reversal rate. The results indicate that manipulations which alter the phase relationships of the triangle wave decrease the illusion, but manipulations which maintain them do not.