When the two eyes view dissimilar images, we experience binocular rivalry, in which one eye's view dominates for several seconds and is then replaced by that of the other eye. What causes these perceptual changes in the absence of any change in the stimulus? We showed previously that some neurons in monkey cortical area MT show changes in activity during motion rivalry that reflect the perceived direction of motion. To determine whether perception-related modulation of activity occurs in other visual cortical areas, we recorded from individual neurons in V1, V2 and V4 while monkeys reported the perceived orientation of rival gratings of two orthogonal orientations. Many cells, particularly in V4, showed patterns of activity that correlated with the perceptual dominance and suppression of one stimulus. The majority were orientation-selective and could be driven equally well from either eye. It has been previously suggested that binocular rivalry involves reciprocal inhibition between monocular neurons within V1 (for example, see ref. 4), but our results do not support this view; rather, we propose that binocular rivalry arises through interactions between binocular neurons at several levels in the visual pathways, and that similar mechanisms may underlie other multistable perceptual states that occur when viewing ambiguous images.