Stereodeficient subjects demonstrate non-linear stereopsis

There appear to be two modes of stereoscopic processing: a conventional lin ens operation that is dependent on correspondence between local luminance c omponents in the two eyes' views, and a non-linens or second-order processi ng mode. This second mode may use disparity information provided by particu lar 'non-Fourier' features of the stimulus such as the contrast envelope. P reliminary results suggest that people who fail standard clinical stereotes ts are able to extract non-linear disparity information from Gabor stimuli [McColl & Mitchell, 1998. Vision Research, 38, 1889-1900]. Here we evaluate the status of the non-linear mechanism in such individuals by using two ty pes of contrast enveloped stimuli, namely random line and Gabor micropatter ns, in a task that requires near/far depth judgements [Ziegler & Hess, 1999 . Vision Research, 39, 1491-1507]. Although our sample was small, three of our four subjects who had performed poorly on at least one standard clinica l test of stereopsis could perform the task, as well as one 'stereoblind' s ubject who had failed all four standard clinical tests. The overall results suggest that individuals with stereoanomalies show a diversity of deficits , but some nevertheless can see depth using 'non-linear' mechanisms. (C) 20 00 Elsevier Science Ltd. All rights reserved.