Visuomotor deficits following ablation of monkey superior colliculus

Abstract

Whether ablation of the colliculus affected the generation of unrewarded, spontaneous saccades and to what extent any such effects might depend on a sensory loss was studied. Monkeys were trained on a stimulus-detection task that required response to a peripheral stimulus while fixation of a central target was continued. Unilateral surgical ablations produced a deficit in stimulus detection in the visual field contralateral to the ablation. This failure to respond to stimuli recovered during the 2nd postoperative week. Stimuli more eccentric than 20-30.degree. elicited the largest and most persistent deficit. There was no indication that the time course of recovery was modified by visual experience, since the amount of postoperative visual experience did not appear to speed recovery. A transient sensory loss evidently follows collicular ablation. Monkeys were tested for distractibility by presentation of a brief irrelevant peripheral stimulus while they were being rewarded for fixation of a central spot of light. After the lesion, monkeys made fewer saccades than before toward distracting stimuli appearing in the visual field contralateral to the ablation. A similar reduction in the frequency of saccades was observed during the stimulus-detection task. After the ablation, 2 monkeys that consistently made saccades toward stimuli in the field ipsilateral to the ablation showed a reduction in the number of saccades into the contralateral field. This decrease occurred even when the monkeys detected the stimuli. The pattern of spontaneous eye movements recorded while the monkey was looking around the experimental room was also modified even after recovery from the detection deficit. The greatest postoperative effect was that the monkeys tended to look toward the side ipsilateral to the ablation; the proportion of ipsilateral fixations and the proportion of time spent in ipsilateral eye positions increased. Only a slight change was found in the proportion of contralaterally directed and ipsilaterally directed saccades. Also, slight decreases in the frequency of eye movements in the light were evident. The visuomotor deficit that persists after recovery from the transient sensory loss can be characterized as a decrease in the frequency of saccades to visual targets contralateral to the lesion when the visually guided saccades are not required by the behavioral paradigm. The superior colliculus plays an important role in the selection of a visual target for a visually guided saccade. An effect of collicular lesions is a visual neglect that may be related to, but not necessarily limited to, the generation of saccadic eye movements.