Modulation of pursuit eye movements by stimulation of cortical areas MT and MST

1. Many cells in the superior temporal sulcus (STS) of the monkey that represent the foveal region of the visual field discharge during pursuit eye movements. Damage to these areas produces a deficit in the maintenance of pursuit eye movements when the target towards the side of the brain with the lesion. In the present experiments, we electrically stimulated these areas to better localize and understand the mechanisms underlying this directional pursuit deficit. 2. Monkeys were trained to pursue a moving target using a step-ramp task in which the target first stepped to an eccentric position and then moved smoothly across the screen. Trains of stimulation were applied after the monkey had begun to pursue the target to study stimulation effects of maintenance of pursuit. 3. Stimulation during pursuit frequently produced eye acceleration toward the side of the brain stimulated. Eye speed increased during pursuit toward the side stimulated and decreased during pursuit away from the side stimulated. This increase in velocity toward the side of the brain where stimulation presumably activated cells is consistent with the decrease in pursuit velocity toward the side of the brain after cells were removed by chemical lesions. 4. The increase or decrease in pursuit speed following stimulation produced a slip of the target on the retina. The pursuit system seemed to be insensitive to this slip during the period of stimulation, however, since the effect of stimulation during pursuit of a stabilized image (open-loop condition) was similar to that resulting from stimulation under normal pursuit conditions (closed-loop). This insensitivity to visual motion during stimulation suggests that the stimulation substitutes for that visual input. 5. The separation of eye and target position that resulted from stimulation did produce catch-up saccades. This provides added evidence that alteration of middle temporal area (MT) and medial superior temporal area (MST) modifies visual-motion but not visual-position information. 6. Stimulation that produced eye acceleration during pursuit produced only a slight effect during fixation of a stationary target. The effectiveness of the stimulation also increased as the speed of the pursuit increased between 5 and 25 degrees/s. These observations, which show that pursuit velocity altered the effect of stimulation, suggest that the stimulation acted on visual motion processing before information about the pursuit movement itself is incorporated. Since this stimulation produces directional pursuit effects, we hypothesize that the directional bias for pursuit originates in the visual signal conveyed to the pursuit system.(ABSTRACT TRUNCATED AT 400 WORDS)