REVERSIBLE INACTIVATION OF MONKEY SUPERIOR COLLICULUS - I - CURVATUREOF SACCADIC TRAJECTORY

The neurons in the intermediate layers of the monkey superior collicul us (SC) that discharge before saccadic eye movements can be divided in to at least two types, burst and buildup neurons, and the differences in their characteristics are compatible with different functional cont ributions of the two cell types. It has been suggested that a spread o f activity across the population of the buildup neurons during saccade generation may contribute to the control of saccadic eye movements. T he influence of any such spread should be on both the horizontal and v ertical components of the saccade because the map of the movement fiel ds on the SC is a two-dimensional one; it should affect the trajectory of saccade. The present experiments used muscimol injections to inact ivate areas within the SC to determine the functional contribution of such a spread of activity on the trajectory of the saccades. The analy sis concentrated on saccades made to areas of the visual field that sh ould be affected primarily by alteration of buildup neuron activity. M uscimol injections produced saccades with altered trajectories; they b ecame consistently curved after the injection, and successive saccades to the same targets had similar curvatures. The curved saccades showe d changes in their direction and speed at the very beginning of the sa ccade, and for those saccades that reached the target, the direction o f the saccade was altered near the end to compensate for the initially incorrect direction. Postinjection saccades had lower peak speeds, lo nger durations, and longer latencies for initiation. The changes in sa ccadic trajectories resulting from muscimol injections, along with the previous observations on changes in speed of saccades with such injec tions, indicate that the SC is involved in influencing the eye positio n during the saccade as well as at the end of the saccade. The changes in trajectory when injections were made more rostral in the SC than t he most active burst neurons also are consistent with a contribution o f the buildup neurons to the control of the eye trajectory. The result s do not, however, support the hypothesis that the buildup neurons in the SC act as a spatial integrator.