Binocular unmasking with unequal interocular contrast: the case for multiple Cyclopean eyes

Under certain conditions, the detection threshold for a sinusoidal grating embedded in a noisy background may be an order of magnitude lower when binocular cues are available than when monocular cues only are present. Such binocular unmasking occurs only when the degree of interocular disparity for the target differs from that of the background. Two classes of models have been advanced to account for such unmasking. The first assumes that orientation-specific, spatial frequency channels in each eye encode the amplitude and phase of the spatial frequency component of the pattern the channel is tuned to detect. Thus, a difference in interocular disparity between target and background could result in interocular amplitude and/or phase differences in left- and right-eye spatial frequency channels. When, however, there are no disparity differences between target and background, there will be no interocular differences in amplitude and phase in the left- and right-eye channels. In this model, then, binocular unmasking reflects the binocular system's ability to respond to interocular amplitude and/or phase differences in the patterns presented to the two eyes. In the second class of models, it is assumed that the left- and right-eye patterns are first summed to form a "Cyclopean" eye. In these models, detection depends on the effect this summation process has on the power spectrum of the summated patterns. To decide between these two classes of models, we observed the occurrence of binocular unmasking when (1) the contrast of masker and signal was varied identically in both eyes and (2) the contrast of masker and signal was varied in one eye only. Consistent with our previous research, we found that the results can be accounted for in terms of a linear summation model of binocular unmasking; the alternative interocular phase detection model was disproved. The implications of these findings for binocular contrast summation in the absence of visual noise are discussed.