Mu rhythm modulation during changes of visual percepts.

Cooperation between vision and somatomotor behavior, such as manual exploration of objects, suggests close functional coupling between the visual and sensorimotor systems. We observed this type of interaction in human volunteers during binocular rivalry while following the level of sensorimotor mu rhythm with a whole-scalp neuromagnetometer. The observers viewed a weak vertical grating in the lower visual field of one eye and a strong horizontal grating in the same spatial window of the other eye. When stationary, the weak grating was permanently invisible because of its low contrast and spatial frequency. A sudden brief drifting movement of the weak grating wiped out the dominant grating, and the weak grating became visible for less than the 3-s interval between the movements. The postcentral 8- to 15-Hz mu rhythm was found in six of nine observers, and its level increased transiently by 10-15%, starting about 450 ms after the beginning of the movement. The mu level was also enhanced by the actual disappearance of the stronger stimulus, when it occurred in random order with the rivalry stimuli. Identical visual motion, when not accompanied by a perceptual dominance change, produced only minor effects on the mu rhythm. Our results show that a change in visual percept, even with no real or imagined motor response, is associated with modified activity of the postcentral gyrus. This modification may reflect visuohaptic interactions and/or activity of the distributed cortical network implementing visually guided movements.