Dichoptic color saturation mixture: Binocular luminance contrast promotes perceptual averaging

We demonstrate a new type of interaction between suprathreshold color (chromatic) and luminance contrast in the context of binocular vision. When two isoluminant colored disks of identical hue but different saturations are presented to different eyes, the apparent saturation of the resulting "dichoptic" mix is close to that of the more saturated patch if presented binocularly. This result is commensurate with previous findings using luminance contrast and is close to the scenario termed "winner-take-all." However, when binocularly matched luminance contrast is added to the dichoptic saturation mixture, the apparent saturation of the mixture shifts away from winner-take-all towards the average of the two dichoptic saturations. The likely cause of this effect is that the matched luminance contrasts reduce the interocular suppression between the unmatched color saturations. We suggest that the presence of binocularly matched luminance contrast promotes the interpretation that the dichoptic color saturations, even though unmatched, nevertheless originate from the same object. We term this idea the "object commonality" hypothesis.