Abstract
Binocular combination is the first step towards stereoscopic vision. Recent models of binocular combination have posited the existence of two channels: a binocular summing (S+) channel and a binocular differencing (S -) channel. We tested for the existence of S+ and S- channels in binocular summation in the context of surround masking. Stimuli were horizontally oriented 0.5 cpd gratings. During a session, there were 4 interleaved target conditions: monocular left(l), monocular right (r), binocular correlated (c) and binocular anticorrelated (a) and one of 5 high contrast surround mask conditions: no surround(n), monocular left (l), monocular right(r), binocular correlated (c) and binocular anticorrelated(a), giving a total of 20 conditions. Binocular summation was defined as the ratio of best eye to binocular threshold. We observed that the opposite eye surround masks notably elevated thresholds more than the same eye surround masks. For the binocular surround masks and tests we found that thresholds for ‘c’ targets were elevated more by ‘c’ surrounds than by ‘a’ surrounds and thresholds for ‘a’ targets were elevated more by ‘a’ surrounds than by ‘c’ surrounds. Binocular summation for ‘c’ targets was reduced by ‘c’ surrounds and in some cases increased by ‘a’ surrounds. Binocular summation for ‘a’ target was reduced by ‘a ’surrounds and in some cases increased by ‘c’ surrounds. These results reveal that surround suppression is inherently an interocular mechanism. Moreover, our results are hard to reconcile with the traditional single-channel S+ model of binocular summation. Such a channel would be unresponsive to the ‘a‘ targets and hence also unresponsive to the differential effects of the ‘c’ and ‘a’ surround masks on the ‘a’ target. Rather, the results support the existence of 2 binocular summation channels, one S+ the other S-, their relative channel gains modifiable by the ‘c’ and ‘a’ surround mask.