Selective activation of visual cortex neurons by fixational eye movements: implications for neural coding.
During normal vision, when subjects attempt to fix their gaze on a small stimulus feature, small fixational eye movements persist. We have recorded the impulse activity of single neurons in primary visual cortex (V1) of macaque monkeys while their fixational eye movements moved the receptive-field activating region (AR) over and around a stationary stimulus. Three types of eye movement activation were found. (1) Saccade cells discharged when a fixational saccade moved the AR onto the stimulus, off the stimulus, or across the stimulus. (2) Position/drift cells discharged during the intersaccadic (drift) intervals and were not activated by saccades that swept the AR across the stimulus without remaining on it. To activate these neurons, it was essential that the AR be placed on the stimulus and many of these cells were selective for the sign of contrast. They had smaller ARs than the other cell types. (3) Mixed cells fired bursts of activity immediately following saccades and continued to fire at a lower rate during intersaccadic intervals. The tendency of each neuron to fire transient bursts or sustained trains of impulses following saccades was strongly correlated with the transiency of its response to stationary flashed stimuli. For one monkey, an extraretinal influence accompanying fixational saccades was identified. During natural viewing, the different eye movement classes probably make different contributions to visual processing. Position/drift neurons are well suited for coding spatial details of the visual scene because of their small AR size and their selectivity for sign of contrast and retinal position. However, saccade neurons transmit information that is ambiguous with respect to the spatial details of the retinal image because they are activated whether the AR lands on a stimulus contour, or the AR leaves or crosses the contour and lands in another location. Saccade neurons may be involved in constructing a stable world in spite of incessant retinal image motion, as well as in suppressing potentially confusing input associated with saccades.