Visual grouping on binocular rivalry in a split-brain observer.

We studied the effects of visual grouping on binocular rivalry in the left and right hemispheres of a split-brain observer, JW. In Experiments 1 and 2, we compared responses to traditional rivalry stimuli (e.g., a red vertical grating presented to the left eye and a green horizontal grating presented to the right eye) with responses to Diaz-Caneja stimuli (i.e., half of each grating was presented to one eye and the other half to the other eye). As found for intact-brain observers, JW reported episodes of exclusive visibility of coherent stimuli (e.g., of a red vertical grating alternating with a green horizontal grating) with Diaz-Caneja stimuli that were fewer and briefer than with traditional stimuli. This occurred in both hemispheres, demonstrating that during binocular rivalry, contours from one eye can be grouped with those of the opposite eye to create a coherent percept, even in the isolated hemispheres of the split-brain observer. In Experiment 3, we studied the tendency of rivalry in adjacent patches to synchronize. When both patches were in one of JW's hemifields, rivalry synchronized for similarly oriented stimuli, the same as happened for intact-brain observers. When the patches were in JW's opposite hemifields, there was no synchronizing of rivalry, unlike what happened for intact-brain observers. This suggests that rivalry processed in JW's two hemispheres is independent. We conclude that rivalry is processed fully within each hemisphere.