Further consideration of size illusions in random dot stereograms.

It has been suggested that many geometric illusions are caused by the application of depth or size constancy rules to an image which does not have sufficient cues to establish that the elements lie in a flat plane. Thus, converging lines are taken as depth cues, and the attributed depth provides the basis for adjusting the perceived size of stimulus elements. It this is the case, one should not see a distortion of relative size if the disparity cues provide for strong statification, i.e., localization in depth of the linear perspective cues. This expectation is challenged by demonstrations that show distortions of relative size using random-dot stereograms. In 1971 Julesz provided such examples but did not comment on the implications for theories of depth. Here we redemonstrate these distortion of length and size in autostereograms which contain the Ponzo and Corridor configurations. The illusory distortions can be seen in the cyclopean view even though the linear perspective elements are well stratified. We suggest that the processing of binocular disparity cues, as required for judgments of absolute distance, may involve the dorsal stream of vision, i.e., activity passing into and including the parietal lobe. Pictorial cues, on the other hand, are likely passed through the ventral stream into the temporal lobe. The analysis of depth by this system provides for size constancy and, possibly, the calibration of relative motion.