Further consideration of size illusions in random dot stereograms

It has been suggested that many application of depth or size-constancy rule s to an image which does not have sufficient cues to establish chat the ele ments fie in a flat plane. Thus, converging lines are taken as depth cues, and the attributed depth provides the basis for adjusting the perceived siz e of stimulus elements. If this is the case, one should not see a distortio n of relative size if the disparity cues provide for strong stratification, i.e., localization in depth, of the linear perspective cues. This expectat ion 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 tor theories of depth. Here we redemonstra te these distortions of length and size in autostereograms which contain th e Ponzo and Corridor configurations, The illusory distortions can be seen i n the cyclopean view even though the linear perspective elements are well s tratified. We suggest that the processing of binocular disparity cues, as r equired for judgments of absolute distance, may involve the dorsal stream o f vision, i.e., activity passing into and including the parietal lobe. Pict orial 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.