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.