THE EFFECTS OF LATERAL GENICULATE-NUCLEUS, AREA V4, AND MIDDLE TEMPORAL (MT) LESIONS ON VISUALLY GUIDED EYE-MOVEMENTS

Visually guided saccadic eye movements to singly presented stationary targets form a bimodal distribution. After superior colliculus lesions , the so called ''express saccades'' that form the first mode of the d istribution are no longer obtained. The aim of this study was to deter mine what role several other neural systems play in the generation of express and regular saccades, with the latter being those that form th e second mode in the bimodal distribution. Lesions were made in the pa rvocellular and magnocellular portions of the lateral geniculate nucle us to disrupt either the midget system or the parasol system that orig inates in the retina and areas V4 and MT. The effects of the lesions w ere examined on the accuracy and latency of saccadic eye movements mad e to stationary and to moving visual targets. Following magnocellular and MT lesions deficits were observed in smooth pursuit and in the amp litude of saccades made to moving targets. However, none of the lesion s produced significant changes in the bimodal distribution of saccadic latencies to stationary targets. The results suggest that express sac cades and regular saccades are not selectively mediated by either the midget or the parasol systems or by areas V4 and MT. Neither are the f rontal eye fields involved as had previously been shown. We suggest th at the superior colliculus plays a central role in producing both expr ess and regular saccades by virtue of highly convergent input from num erous cortical structures.