Visual competition

Key Points Binocular rivalry occurs when one image is presented to a person's left eye and a different image to their right eye. The two images alternate in conscious perception. Rivalry is seen as a powerful tool with which to study visual perception and awareness, but there are many unanswered questions and controversies about its neural basis. The accumulating evidence suggests that multiple processes are involved in rivalry, and that these processes might be mediated by different neural substrates throughout the visual system. The successive periods of dominance of the two rivalling stimuli are unpredictable in duration, but the dynamics of the alternation can be altered by varying the relative strengths of the two stimuli. The stronger stimulus will then be suppressed for shorter periods. Conscious attention can prolong the periods of dominance of the attended stimulus, and stimulus events that capture involuntary attention can rescue a stimulus from suppression. Placing a stimulus in a congruent context can also lengthen the periods for which it is dominant. Local features that form a coherent global image can become entrained so that they become dominant or suppressed together, even if they are spread across the two eyes. The transitions that occur when one stimulus becomes suppressed and the other dominant tend to be gradual, rather than instantaneous — the newly dominant stimulus becomes visible at one point and then spreads, like a wave, across the visual field. There is considerable indirect, psychophysical evidence that relates to the possible mechanisms of rivalry. Visual sensitivity and oculomotor reflexes are reduced for stimuli that are presented during suppression. Some visual aftereffects, such as the tilt aftereffect, which is thought to arise from adaptation in orientation-specific neurons in area V1, are unaffected by suppression of the inducing stimulus, whereas others, such as some motion aftereffects that depend on global rather than local motion, are reduced by suppression. These findings support the idea that suppression during rivalry is a cortical phenomenon. More direct evidence comes from studies of human brain function using visual evoked responses, functional magnetic resonance imaging (fMRI) and magnetoencephalography, which have all shown that neural responses to visual stimuli are suppressed when awareness of those stimuli is suppressed. fMRI studies have found a reliable modulation of signal by suppression even in V1, al