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 st imulus feature, small fixational eye movements persist. We have recorded th e impulse activity of single neurons in primary visual cortex (V1) of macaq ue monkeys while their fixational eye movements moved the receptive-field a ctivating region (AR) over and around a stationary stimulus. Three types of eye movement activation were found. (1) Saccade cells discharged when a Fi xation:ll saccade moved the AR onto the stimulus, off the stimulus, or acro ss the stimulus. (2) Position/drift calls discharged during the intersaccad ic (drift) intervals and were not activated by saccades that swept the AR a cross the stimulus without remaining on it. To activate these neurons, it w as 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 othe r cell types. (3) Mixed cells fired bursts of activity immediately followin g saccades and continued to fire at a lower rate during intersaccadic inter vals. The tendency of each neuron to fire transient bursts or sustained tra ins of impulses following saccades was strongly correlated with the transie ncy of its response to stationary flashed stimuli. For one monkey, an extra retinal influence accompanying fixational saccades was identified. During n atural 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 si ze and their selectivity for sign of contrast and retinal position. However , saccade neurons transmit information that is ambiguous with respect to th e spatial details of the retinal image because they are activated whether t he 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 construct ing a stable world in spite of incessant retinal image motion, as well as i n suppressing potentially confusing input associated with saccades.