Monocular gain control explains dichoptic benefit in binocular global motion perception

摘要

When viewing random-dot kinematograms (RDKs), percent coherence thresholds for global motion perception are lower if signal and noise dots are presented to different eyes (Cai, Yuan, and Backus, VSS 2015). This dichoptic benefit might come about either from monocular gain control (MGC) -- more dots in the noise eye causing each noise dot to be less effective -- or from monocular bias in global motion integrators (MBGM). The roles of MGC and MBGM can be tested by comparing performance in two conditions: a "binocular" condition that has same number of signal and noise dots in each eye (no overlapping), and a "biased-binocular" condition that has same number of dots in each eye but signal all in one eye. If MBGM occurs, then putting all signal in one eye should be beneficial. But if MGC alone was responsible for the previously observed dichoptic benefit, then this benefit should disappear in both conditions. We therefore measured percent coherence thresholds in four conditions: "monocular" (all dots in one eye), "dichoptic" (signal in one eye, noise in the other), "biased-binocular", and "binocular". We tested 6 subjects with normal vision in a forced-choice net motion discrimination task with 100 dots using a stereoscope. Threshold was estimated using a 3-down-1-up staircase procedure. A brief display (300 ms) of two-frame dot motion at 30 Hz was used to minimize selective tracking. Trials from different conditions were intermixed. All subjects performed similarly in the "monocular" and "binocular" conditions but significantly better in the "dichoptic" condition, which replicated the previous finding. Performance in the "biased-binocular" condition was significantly worse than "dichoptic", although still better than "binocular" with marginal significance. We conclude that the dichoptic benefit during binocular RDKs motion tasks appears to be explained mostly by MGC. More evidence is required to definitively confirm or rule out MBGM.