Abstract
Psychophysical experiments have shown that it is possible to simultaneously perceive binocular depth and rivalry in plaids. Depth may be perceived from the near-vertical components, while the diagonal components undergo rivalry, as long as the components are in different spatial frequency or orientation bands (Buckthought & Wilson, 2007).
Here we used fMRI at 3T to image activity in visual cortex while human subjects perceived depth and rivalry simultaneously from plaids. This was compared to the activation obtained with near-vertical or diagonal gratings in which depth or rivalry alone was perceived. In conditions with depth, an interocular orientation disparity (5 degrees) was used to produce tilt, and this cue alternated every three seconds between top tilted forward or backward. Hence, the perceived depth alternated dynamically, analogous to the rivalry percept. Six subjects performed either a rivalry or depth task with the plaid or grating stimuli by indicating when alternations occurred. In three experimental conditions the spatial frequencies of the near-vertical and diagonal components were, respectively: (a) 2.5, 6.4 cpd; (b) 6.4, 2.5 cpd; (c) 6.4, 6.4 cpd.
We found that the network of activated cortical areas was remarkably similar for the rivalry and depth task. When subjects viewed plaids, some areas were activated more during the depth task than the rivalry task independent of spatial frequency (e.g. posterior superior temporal sulcus), while in other regions the bias was for the higher (e.g., calcarine cortex) or lower spatial frequency component (e.g., MT+). Placing depth and rivalry components in different spatial frequency bands allowed us to find greater differences between the two. Activation when depth and rivalry were perceived in isolation (gratings) was higher than when depth and rivalry were perceived simultaneously (plaids), consistent with inhibitory interactions between the mechanisms for depth and rivalry.
We conclude that depth and rivalry are subserved by similar mechanisms, and are perceived simultaneously by coexisting in different spatial frequency or orientation channels. Binocular vision models must be revised to incorporate these findings.
Supported by NIH R01 EY015219.