FMRI study of binocular rivalry in human visual cortex

During binocular rivalry two incompatible monocular images compete for perceptual dominance, with one pattern temporarily suppressed from awareness. Because of this dissociation between visual stimulation and perception, binocular rivalry is useful for studying the neural basis of visual awareness. We measured fMRI response in early visual cortex while subjects viewed rival dichoptic images of two different contrasts; the contrast difference served as a “tag” for the representations of the two monocular images. Activity in the primary visual cortex (V1) increased when subjects perceived the higher contrast pattern and decreased when subjects perceived the lower contrast pattern. These fluctuations in V1 activity during rivalry were about 55% as large as those evoked by alternately presenting the two monocular images without rivalry. The rivalry-related fluctuations in V1 activity were roughly equal to those observed in extrastriate visual areas (V2, V3, and V4v). These results challenge the view that binocular rivalry primarily takes place later in the cortical visual pathways.