During binocular rivalry, physical stimulation is dissociated from conscious visual awareness. Human brain imaging reveals a tight linkage between the neural events in human primary visual cortex (V1) and the dynamics of perceptual waves during transitions in dominance during binocular rivalry. Here, we report results from experiments in which observers' attention was diverted from the rival stimuli, implying that: competition between two rival stimuli involves neural circuits in V1, and attention is crucial for the consequences of this neural competition to advance to higher visual areas and promote perceptual waves.

When dissimilar images are presented to the two eyes, they compete for perceptual dominance so that each image is visible in turn for a few seconds while the other is suppressed. Such binocular rivalry is associated with relative suppression of local, eye-based representations1, 2, 3, 4 that can also be modulated by high-level influences such as perceptual grouping3, 5, 6. However, it is currently unclear how early in visual processing the suppression of eye-based signals can occur. Here we use high-resolution functional magnetic resonance imaging (fMRI) in conjunction with a new binocular rivalry stimulus to show that signals recorded from the human lateral geniculate nucleus (LGN) exhibit eye-specific suppression during rivalry. Regions of the LGN that show strong eye-preference independently show strongly reduced activity during binocular rivalry when the stimulus presented in their preferred eye is perceptually suppressed. The human LGN is thus the earliest stage of visual processing that reflects eye-specific dominance and suppression.

When dissimilar images are presented to the two eyes, they compete for perceptual dominance so that only one image is visible at a time while the other one is suppressed. Neural correlates of such binocular rivalry have been found at multiple stages of visual processing, including striate and extrastriate visual cortex. However, little is known about the role of subcortical processing during binocular rivalry. Here we used fMRI to measure neural activity in the human LGN while subjects viewed contrast-modulated gratings presented dichoptically. Neural activity in the LGN correlated strongly with the subjects' reported percepts, such that activity increased when a high-contrast grating was perceived and decreased when a low-contrast grating was perceived. Our results provide evidence for a functional role of the LGN in binocular rivalry and suggest that the LGN, traditionally viewed as the gateway to the visual cortex, may be an early gatekeeper of visual awareness.

The primate visual system is believed to comprise two main pathways: a ventral pathway for conscious perception and a dorsal pathway that can process visual information and guide action without accompanying conscious knowledge. Evidence for this theory has come primarily from studies of neurological patients and animals. Using fMRI, we show here that even though observers are completely unaware of test object images owing to interocular suppression, their dorsal cortical areas demonstrate substantial activity for different types of visual objects, with stronger responses to images of tools than of human faces. This result also suggests that in binocular rivalry, substantial information in the suppressed eye can escape the interocular suppression and reach dorsal cortex.

Binocular rivalry is the alternating perception that occurs when incompatible stimuli are presented to the two eyes: one monocular stimulus dominates vision and then the other stimulus dominates, with a perceptual switch occurring every few seconds. There is a need for a binocular rivalry model that accounts for both well-established results on the timing of dominance intervals, and for more recent evidence on the distributed neural processing of rivalry. The model for binocular rivalry developed here consists of four parallel visual channels, two driven by the left eye, and two by the right. Each channel consists of several consecutive processing stages representing successively higher cortical levels, with mutual inhibition between the channels at each stage. All stages are architecturally identical. With n the number of stages, the model is implemented as 4n nonlinear differential equations using a total of eight parameters. Despite the simplicity of its architecture, the model accounts for a variety of experimental observations: 1. the increasing depth of rivalry at higher cortical areas, as shown in electrophysiological, imaging, and psychophysical experiments; 2. the unimodal probability density of dominance durations, where the mode is less than the mean; 3. the lack of correlation between successive dominance durations; 4. the effect of interocular stimulus differences on dominance duration; 5. eye suppression, as opposed to feature suppression. The model is potentially applicable to issues of visual processing more general than binocular rivalry.