Recovery of saccadic dysmetria following localized lesions in monkey superior colliculus

Damage to the monkey superior colliculus (SC) produces deficits in the generation of saccadic eye movements. Recovery of the accuracy of saccades is rapid, but saccadic latency and peak velocity recover slowly or not at all. In the present experiments we revisited the issue of recovery of function following localized lesions of the SC using three methodological advances: implantation of wire recording electrodes into the SC for the duration of the experiment to ensure that we were recording from the same site on the SC map on successive days; quantification of changes in saccadic accuracy, latency, and velocity using a standard grid of target points in the visual field contralateral to the SC lesion; measurement of movement field size to quantitatively determine any changes following the lesion. We confirmed a decrease in saccadic accuracy following electrolytic lesions of the SC, and we found that this dysmetria recovered within about 4 days. Saccadic latency increased for saccades to the lesion area and this deficit persisted. Peak saccadic velocity decreased immediately after the lesion and decreased further during the 10 days to 2 weeks of the experiment. We found no indication of an expansion of the movement fields of neurons adjacent to the lesion area. This lack of reorganization suggests that movement field changes within the SC cannot mediate the recovery in accuracy of the saccade. The persistence of the latency and velocity deficits despite the recovery of amplitude deficits indicates that saccadic latency and peak velocity are dependent upon the SC whereas saccadic amplitude is not.