THE INFLUENCE OF FIXATIONAL EYE-MOVEMENTS ON THE RESPONSE OF NEURONS IN AREA MT OF THE MACAQUE

We analyzed the relationship between eye movements and neuronal respon ses recorded from area MT in alert monkeys trained to maintain visual fixation during the presentation of moving patterns. The monkeys made small saccades which moved the eyes with velocities that spanned the s ensitivity range of MT neurons. The saccades evoked changes in the neu ronal response that depended upon (1) the level of stimulus-evoked act ivity amidst which the saccade occurred and (2) the direction of the s accade relative to the preferred direction of the neuron. Most notably , saccades were able to suppress stimulus-evoked activity when they ca used retinal image flow that opposed the neuron's preference and were able to elicit a response or enhance weak activity when they caused fl ow in the neuron's preferred direction. On average, the disturbance la sted 40 ms beginning about 40 ms following saccade onset. Using these parameters, we simulated synthetic spike trains from an imaginary pair of similarly tuned neurons and determined that the interneuronal corr elation due to saccades should be negligible at all but the lowest ong oing firing rates. This conclusion was supported from our data by the observation that response variance for single MT spike trains was not measurably reduced during periods of stable gaze compared to periods w hen eye movement exceeded a stability criterion (0.1 deg during 0.5 s) . While the intrusions caused by saccades are too short-lived and infr equent to account for the variability of MT neuronal response (counter to the finding in V1 of Our et al., 1997), the clear directional sign al that they carry in area MT suggests that motion perception is not b locked during saccades by suppression at early stages in the visual pa thway.