REVERSIBLE INACTIVATION OF MONKEY SUPERIOR COLLICULUS - II - MAPS OF SACCADIC DEFICITS

Neurons in the superior colliculus (SC) are organized as maps of visua l and motor space. The companion paper showed that muscimol injections into intermediate layers of the SC alter the trajectory of the moveme nt and confirmed previously reported effects on latency, amplitude, an d speed of saccades. In this paper we analyze the pattern of these def icits across the visual field by systematically comparing the magnitud e of each deficit throughout a grid of targets covering a large fracti on of the visual field. We also translate these deficits onto the SC m ap of the visual/movement fields to obtain a qualitative estimate of t he extent of the deficit in the SC. We found a consistent pattern of s ubstantially increased saccadic latency to targets in the contralatera l visual hemifield, accompanied by slight and inconsistent increases a nd decreases for saccades to the ipsilateral hemifield. The initial an d peak speed of saccades was reduced after the injection. The postinje ction amplitude of the saccades were either hypometric or normometric, but rarely hypermetric. Although errors in the initial direction of t he postinjection saccades were small, they consistently formed a simpl e pattern: an initial direction with minimal errors (a null direction) separating regions with clockwise and counterclockwise rotations of t he initial direction. However, the null direction did not go through t he center of the inactivated zone, as would be expected if the SC alon e were determining saccade direction, e.g., with a population code. On e hypothesis that can explain the misalignment of the null direction w ith the lesion site is that another system, acting in parallel with th e SC, contributes to the determination of saccadic trajectory.