The role of the superior colliculus in saccade initiation: a study of express saccades and the gap effect

Neural mechanisms underlying the initiation of saccadic eye movements were studied by recording the activity of neurons in the superior colliculus of rhesus monkeys that had extensive experience on the gap task using targets restricted to one visual field. The superposition of visual activation upon the increased excitability occurring on gap trials facilitates the occurre nce of a motor burst with extremely short latency; the motor burst is tight ly coupled to saccade onset for the full range of saccadic reactions times, both regular and express. We found no evidence that express saccades are a special class of saccades triggered directly by visual responses. The low frequency activity, necessary for the occurrence of express saccades, neith er initiates express saccades nor serves as an accurate predictor of the di rection or latency of saccades. Based upon these findings, the hypothesis t hat the motor burst of collicular neurons serves as a signal for triggering saccade onset can now be extended to express saccades. (C) 2000 Elsevier S cience Ltd. All rights reserved.