The effect of retinal stabilization on anorthoscopic percepts under free-viewing conditions

Outline shapes were moved back and forth at .5 Hz behind a simulated narrow (10′ or 15′) slit without visible borders. Under free-viewing conditions observers reported spontaneous transitions between periods in which they saw a horizontally moving shape and periods in which they saw only vertically moving contour segments. Eye movements were monitored with a dual-Purkinje image eye-tracker. On half the trials we used retinal stabilization to eliminate any pursuit dependent retinal painting. With no stabilization, we observed low amplitude spontaneous pursuit of the horizontal figure motion when a shape was seen, and occasionally lower amplitude pursuit even when no shape was seen. Pursuit amplitudes increased when the display was stabilized. However, stabilization did not alter either the duration or frequency of the episodes of shape perception. Our findings indicate that under the free-viewing conditions we employed, spontaneous smooth pursuit and pursuit dependent painting did not contribute to the formation or maintenance of anorthoscopic percepts.