Interhemispheric switching mediates perceptual rivalry

Background: Binocular rivalry refers to the alternating perceptual states t hat occur when the images seen by the two eyes are too different to be fuse d into a single percept. Logothetis and colleagues have challenged suggesti ons that this phenomenon occurs early in the visual pathway. They have show n that, in alert monkeys, neurons in the primary visual cortex continue to respond to their preferred stimulus despite the monkey reporting its absenc e. Moreover, they found that neural activity higher in the visual pathway i s highly correlated with the monkey's reported percept. These and other fin dings suggest that the neural substrate of binocular rivalry must involve h igh levels, perhaps the same levels involved in reversible figure alternati ons. Results: We present evidence that activation or disruption of a single hemi sphere in human subjects affects the perceptual alternations of binocular r ivalry. Unilateral caloric vestibular stimulation changed the ratio of time spent in each competing perceptual state. Transcranial magnetic stimulatio n applied to one hemisphere disrupted normal perceptual alternations when t he stimulation was timed to occur at one phase of the perceptual switch, bu t not at the other. Furthermore, activation of a single hemisphere by calor ic stimulation affected the perceptual alternations of a reversible figure, the Necker cube. Conclusions: Our findings suggest that interhemispheric switching mediates perceptual rivalry. Thus, competition for awareness in both binocular rival ry and reversible figures occurs between. rather than within, each hemisphe re. This interhemispheric switch hypothesis has implications for understand ing the neural mechanisms of conscious experience and also has clinical rel evance as the rate of both types of perceptual rivalry is slow in bipolar d isorder (manic depression). (C) 2000 Elsevier Science Ltd. All rights reser ved.