IS THE SITE OF NONLINEAR FILTERING IN STEREOPSIS BEFORE OR AFTER BINOCULAR COMBINATION

There is recent evidence that both linear and non-linear filtering ope rations subserve stereoscopic localization. For example, for spatially band-pass stimuli, the overall Gaussian envelope, which is not explic itly represented by the output of linear filters, can provide coarse d isparity information, Here we ask three questions about the nature of this non-linear processing in stereopsis. First, is the site of the no n-linearity before or after binocular combination? Second, is the stim ulus envelope extracted by orientation or non-orientation selective sp atial filters? Finally, we ask whether the envelope-based 3-D localiza tion performance is similar to that for monocular 2-D localization as would be the case if the localization of tbe monocular contrast envelo pe was common to both operations, Our results suggest that envelope ex traction occurs before binocular combination and that the filters invo lved are orientation selective, Finally, we provide preliminary eviden ce that is compatible with the proposal that 3-D and 2-D localization use the same envelope extraction operations.