VISUAL RESPONSES WITH AND WITHOUT FIXATION - NEURONS IN PREMOTOR CORTEX ENCODE SPATIAL LOCATIONS INDEPENDENTLY OF EYE POSITION

The ventral premotor cortex (PMv) of the macaque monkey contains neuro ns that respond both to visual and to tactile stimuli. For almost all of these ''bimodal'' cells, the visual receptive field is anchored to the tactile receptive field on the head or the arms, and remains stati onary when the eyes fixate different locations. This study compared th e responses of bimodal PMv neurons to a visual stimulus when the monke y was required to fixate a spot of light and when no fixation was requ ired. Even when the monkey was not fixating and the eyes were moving, the visual receptive fields remained in the same location, near the as sociated tactile receptive field. For many of the neurons, the respons e to the visual stimulus was significantly larger when the monkey was not performing the fixation task. In control tests, the presence or ab sence of the fixation spot itself had little or no effect on the respo nse to the visual stimulus. These results show that even when the monk ey's eye position is continuously changing, the neurons in PMv have vi sual receptive fields that are stable and fixed to the relevant body p art. The reduction in response during fixation may reflect a shift of attention from the visual stimulus to the demands of the fixation task .