A comparison of the effects of microstimulation in area MT on saccades and smooth pursuit eye movements
Purpose. Tracking a moving target that appears in the periphery involves two kinds of eye movements, saccades and smooth pursuit. Both kinds of movements require a signal of target velocity. The saccadic system must use a target velocity signal to compensate for the motion of the target, and the pursuit system must use such a signal to match eye velocity to target velocity. We conducted a microstimulation experiment in two rhesus monkeys to compare the contribution of motion signals in MT to saccades and pursuit. Methods. We stimulated extrafoveal MT while monkeys performed a tracking task in which a target appeared in the multiunit receptive field of the site and moved either in the preferred direction of the site, the null direction, or not at all (saccade only trials). We stimulated (20-80 μamps, 200 Hz) from target onset until the monkey made a saccade to the target. We examined the effects of microstimulation on the endpoint of the saccade and on the average eye velocity over the first 60 ms of pursuit after the saccade. Results. Stimulation altered the velocity of pursuit in 71 out of 123 experiments, and altered the endpoints of saccades in 78 experiments. Both types of effects occurred in 50 experiments. The effects on saccades were quantified in terms of the velocity compensation, or the target velocity for which the saccade would be appropriate. For both pursuit and saccades, the effects of microstimulation were suggestive of vector averaging of the visual target and an electrically induced velocity signal. The nature of the effects on saccades and pursuit within a given experiment could be quite dissimilar, but across the whole population of experiments, the two types of effects were weakly correlated. Conclusions. The weak correlation between effects on pursuit and saccades suggests that the pathways from MT to the pursuit and saccade systems overlap to some extent, but the fact that the effects can occur independently and can be dissimilar to each other suggests a surprising degree of segregation of these pathways.
Groh, JM; Born, RT; Newsome, WT
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