Dichoptic colour-saturation masking is unmasked by binocular luminance contrast

We demonstrate a new type of interaction between suprathreshold colour (chromatic) and luminance contrast in the context of binocular, specifically dichoptic vision. A highly saturated isoluminant violet 'mask' disk in one eye greatly elevates detection thresholds for an isoluminant violet 'test' disk in the other eye, an example of dichoptic colour-saturation masking. However when binocular luminance contrast (i.e. luminance contrast matched in the two eyes) is added to the disks, the masking is dramatically reduced. Adding binocular luminance contrast to the test disk on its own, or to the mask and test disks presented together in both eyes had comparatively little effect on test thresholds. The likely explanation for the dichoptic unmasking effect is that the binocular luminance contrast reduced the interocular suppression between chromatic mask and test, in keeping with other recent findings from measurements of the appearance of dichoptic saturation mixtures (Kingdom & Libenson, 2015). We suggest that binocularly matched luminance contrast promotes the interpretation that the dichoptic colour saturations, even though unmatched, nevertheless originate from a single object. Under these conditions the visual system tends to blend the mask and test saturations rather than have them compete, resulting in reduced dichoptic masking. We term this idea the "object commonality" hypothesis.