To understand conscious vision, scientists must elucidate how the brain selects specific visual signals for awareness. When different monocular patterns are presented to the two eyes, they rival for conscious expression such that only one monocular image is perceived at a time while the other is temporarily suppressed from awareness. Despite extensive research, this phenomenon of binocular rivalry has remained highly controversial. Specifically, it is debated whether discrepant monocular patterns rival because of neural competition among pattern representations (Leopold & Logothetis, 1996) or monocular channels (Blake, 1989). Here we show that rivalry arises from interocular competition by monitoring fMRI activity in a monocular region of human primary visual cortex (V1) corresponding to the blind spot. This region receives input solely from the ipsilateral eye and not from the blind-spot eye. Subjects reported their dominant percept while viewing rivalrous orthogonal gratings in the visual location corresponding to the blind spot and its surround. As predicted by interocular rivalry, the monocular V1 blind-spot representation was strongly activated when the ipsilateral grating became perceptually dominant and sharply suppressed when the blind-spot grating became dominant. These awareness-related responses were as large as those observed during actual alternations between the ipsilateral and blind-spot grating, suggesting that rivalry is fully resolved in monocular visual cortex. Our findings provide the first physiological evidence that interocular competition mediates binocular rivalry. Furthermore, they suggest that V1 plays an important role in the selection and expression of conscious visual information.
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