Neuronal activity in the lateral cerebellum of trained monkeys, related to visual stimuli or to eye movements.

1. The responses of neurones in the lateral cerebellar cortex to visual stimuli and to eye movements were recorded in rhesus monkeys trained to perform visually guided arm and eye movements in a tracking task. 2. Twenty-two of 134 units recorded (16%) modulated their discharge in response to a bright Xenon flash. They were mainly located in the dorsal paraflocculus. Among those identified as Purkinje cells both simple spike and climbing fibre responses to the flash were seen. (72% of the units were related to arm movements; these were centred in the paramedian lobule, and have been described fully in Marple-Horvat & Stein (1987).) 3. The visual responsiveness of one of the units varied according to the phase of the monkey's task. Around the time that the target stepped, which was the monkey's cue to move, its sensitivity to other stimuli disappeared. 4. Only two neurones responded to the movements of the tracking target. These responses were conditional upon the monkey using visual signals to guide his movements; they did not respond to the target step if he moved before the target did. 5. Fourteen units (10%) located in crus I and II and lobulus simplex correlated strongly with the velocity of horizontal eye movements. Only one of these also responded to visual stimuli. 6. Thus most neurones were found to carry only visual, or eye movement, or limb movement information rather than combinations of these signals; they were located in different but overlapping regions of lateral cerebellar cortex. Visually responsive neurones are probably involved in planning the visual goal of movements, while eye and arm movement neurones probably help to create co-ordinative structures for executing voluntary eye and arm movements.