Opponency versus normalization as the cause of interocular suppression in dichoptic masking

There are two models of the primary cause of interocular suppression (IOS) in binocular vision. The opponency model posits that IOS arises when imbalances in the two eyes’ signals are detected by opponency channels which then drive IOS. The normalization model posits that divisive normalization between the eyes’ signals, which occurs irrespective of whether the two eyes’ signals are imbalanced or not, drives IOS. We attempted to test between these two models in the context of a well-studied manifestation of IOS - dichoptic masking. A signature feature of dichoptic masking is that a mask in one eye increases the thresholds for detecting a test in the other eye more than it does a test in the same eye. Our masks and tests were horizontally-oriented, interocularly in-phase, 0.75 cpd gratings contained with a central 2 deg diameter test window. As expected we found a greater amount of opposite-eye compared to within-eye masking of the test stimulus. We then surrounded the mask-plus-test window with horizontally oriented, high contrast, 0.75 cpd, 6.5 deg diameter “surround masks” that were either interocularly in-phase or anti-phase. In a separate experiment we had showed that the anti-phase more than in-phase surround mask elevated thresholds for detecting an interocular difference in grating phase, in keeping with a role for the anti-phase surround mask as a desensitizer of opponency channels. We hypothesized that if IOS was driven by opponency channels the anti-phase more than in-phase surround masks would reduce the amount of signature dichoptic masking within the central mask/test window. However we found no evidence that the anti-phase surround masks reduced dichoptic masking more than the in-phase surround masks, thus providing no support for the opponency model of IOS and instead support for the normalization model.