The time course of 2-D shape discrimination in random dot stereograms.

Predictions of a coarse-to-fine and co-operative stereo matching algorithm were compared using a 2-D shape discrimination task for disparity-defined targets in 50% random dot stereograms. Uniform disparity targets, square wave modulated targets with a different mean disparity to the background, or uncorrelated dots could be seen at much briefer exposures (down to 33 msec) than square wave modulated targets with the same mean disparity as the background. In the latter case, performance at brief exposures was improved by using coarser disparity corrugations. The results are readily explained by a coarse-to-fine matching scheme such as that proposed by Marr & Poggio (1979) [Proceedings of the Royal Society of London (B), 204, 301-328] and suggest that the correspondence problem is not the limiting step in the perception of simple cyclopean forms.