Size and distribution of movement fields in the monkey superior colliculus.

A gradient of response magnitude was observed across the movement fields (the range of eye movements which alter the discharge frequency of a nueron) of neurons in the intermediate and deeper layers of the superior colliculus. A vigorous discharge preceded movements with a particular direction and amplitude but reduced responses preceded movements which deviated from this direction and/or amplitude. Movement field size is a function of the amplitude of the optimal movement. Neurons discharging prior to small saccades have small and sharply tuned fields. Neurons discharging prior to large saccades have large movement fields and tuning is relatively coarse. Movement fields are topographically organized within the superior colliculus. Neurons discharging prior to small saccades are located anteriorly; neurons firing before large saccades are found caudally. Neurons near the midline discharge prior to up movements and neurons located laterally fire before downward movements. Movement fields of superior colliculus neurons are also characterized by a temporal gradient. The interval between spike discharge and the onset of a saccade is greater for movements near the center of the movement field than for movements to the periphery of the field. Results are interpreted as supporting the foveation hypothesis of superior colliculus function. It is suggested that precise saccadic movements are not produced by the discharge of a small population of finely tuned neurons but result from the weighted sum of the simultaneous movement tendencies produced by the activity of a large population of less finely tuned neurons.