INFLUENCE OF EYE POSITION ON ACTIVITY IN MONKEY SUPERIOR COLLICULUS

1. Most recording studies on the role of the monkey superior colliculu s (SC) in eye movement generation have so far indicated that the code of the recruited population of cells is a fixed vector command represe nting the desired saccadic eye displacement vector, irrespective of th e position of the eyes in the orbit. Experimental evidence from micros timulation, lesions, and neuroanatomy, however, suggests that the SC m ay have access to an eye position signal. 2. In this paper we have tes ted the hypothesis that SC activity is influenced by eye position, by recording from presaccadic burst neurons while monkeys made rapid eye movements in the light covering a large part of the oculomotor range. 3. In four alert rhesus monkeys, we obtained sufficient data from 57 S C single units. The activity of a substantial part of these cells (30/ 57) appeared to be significantly influenced by eye position. Although the tuning properties of these cells for saccade amplitude and directi on remained invariant for changes in eye position, the peak firing rat e of these units was systematically influenced by the position of the eyes in the head. 4. We have characterized this eye position dependenc e of a neuron's activity by a qualitative, model-independent, as well as by a quantitative model description (gain held), which takes into a ccount both the tuning properties of the cell for eye displacement vec tors and the dependence of eye position. 5. Although a majority of gai n fields had their eye position sensitivity vector roughly aligned wit h the optimal saccade vector direction (colinear gain field, 17/30), a substantial part of the gain fields had their eye position sensitivit y vectors in quite different directions, approximately homogeneously d istributed with respect to the cell's ON direction. 6. We conclude tha t the SC has access to a signal related to the position of the eyes in the orbit. Several hypotheses on the possible functional role of this signal, in relation to the neural code of the motor map, are discusse d.