On the coexistence of stereopsis and binocular rivalry.

Dichoptically viewed complex texture stereograms with correlated spatial frequency information can yield stable depth perception, implying cooperative interaction between the two eyes. Dichoptically viewed dissimilar texture pairs may yield competition in the form of binocular rivalry. To study whether stereopsis and rivalry can spatially coexist when stimulus conditions for both are present, we had observers dichoptically view spatial frequency filtered random-dot patterns. The left eye viewed one half-image of an RDS; the right eye viewed the superimposition of the other RDS half-image (which when paired alone with the left-eye RDS yielded stereoscopic depth) and a noise target (which on its own engaged in rivalry with the right eye target). Observers judged the quality of depth and the rate of rivalry for these stereo-pairs. When the contrast of the noise component was low, observers experienced stereopsis and stable single vision that included the noise. At intermediate noise contrasts, local regions were seen either in rivalry or in stereoscopic depth, but rivalry and depth were not experienced at the same spatial location simultaneously. At high noise contrasts, the right eye target dominated almost exclusively, with little hint of stereopsis. Essentially the same pattern of results was obtained in forced-choice experiments in which observers judged the direction of stereoscopic tilt from vertical cosine gratings differing slightly in spatial frequency. Considered together, these results are inconsistent with theories positing that rivalry and stereopsis coexist at the same spatial location because they occur within independent, parallel pathways.