When two eyes are worse than one: Binocular summation for chromatic, interocular-anti-phase stimuli

Numerous studies have shown that sensitivity to binocular targets is higher than to its monocular components, a phenomenon known as binocular summation. Binocular summation has been demonstrated with luminance contrast targets that are not only interocularly in-phase, that is, identical in both eyes, but also interocularly anti-phase, that is, of opposite polarity in the two eyes. Here we show that for the detection of anti-phase targets defined along the red-cyan and violet-lime axes of cardinal color space two eyes are more often than not worse than one. We suggest this is because channels that detect interocular differences, or S- channels are relatively insensitive to chromatic stimuli. We tested this idea by measuring binocular summation for chromatic anti-phase targets in the context of a chromatic surround that itself was either interocularly in-phase or anti-phase. The anti-phase surrounds reduced even further binocular summation for the anti-phase targets whereas the in-phase surrounds increased the level of summation. We show that a model that combines via probability summation the independent activities of adding S+ and differencing S- channels gave a good account of the data, especially for the anti-phase targets. We conclude that binocular adding and differencing channels play an important role in binocular color vision.