Neuronal activity in human primary visual cortex correlates with perception during binocular rivalry

During binocular rivalry, two incompatible monocular images compete for per ceptual dominance, with one pattern temporarily suppressed from conscious a wareness. We measured fMRI signals in early visual cortex while subjects vi ewed rival dichoptic images of two different contrasts; the contrast differ ence served as a 'tag' for the neuronal representations of the two monocula r images. Activity in primary visual cortex (V1) increased when subjects pe rceived the higher contrast pattern and decreased when subjects perceived t he 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 ac tivity were roughly equal to those observed in other visual areas (V2, V3, V3a and V4v). These results challenge the view that the neuronal mechanisms responsible for binocular rivalry occur primarily in later visual areas.