NEURONAL-ACTIVITY IN THE LATERAL CEREBELLUM OF THE CAT RELATED TO VISUAL-STIMULI AT REST, VISUALLY GUIDED STEP MODIFICATION, AND SACCADIC EYE-MOVEMENTS

1. The discharge patterns of 166 lateral cerebellar neurones were stud ied in cats at rest and during visually guided stepping on a horizonta l circular ladder. A hundred and twelve cells were tested against one or both of two visual stimuli: a brief full-field flash of light deliv ered during eating or rest, and a rung which moved up as the cat appro ached. Forty-five cells (40%) gave a short latency response to one or both of these stimuli. These visually responsive neurones were found i n hemispheral cortex (rather than paravermal) and the lateral cerebell ar nucleus (rather than nucleus interpositus). 2. Thirty-seven cells ( of 103 tested, 36%) responded to flash. The cortical visual response ( mean onset latency 38 ms) was usually an increase in Purkinje cell dis charge rate, of around 50 impulses s(-1) and representing 1 or 2 addit ional spikes per trial (1.6 on average). The nuclear response to flash (mean onset latency 27 ms) was usually an increased discharge rate wh ich was shorter lived and converted rapidly to a depression of dischar ge or return to control levels, so that there were on average only an additional 0.6 spikes per trial. A straightforward explanation of the difference between the cortical and nuclear response would be that the increased inhibitory Purkinje cell output cuts short the nuclear resp onse. 3. A higher proportion of cells responded to rung movement, sixt een of twenty-five tested (64%). Again most responded with increased d ischarge, which had longer latency than the flash response (first chan ge in dentate output ca 60 ms after start of movement) and longer dura tion. Peak frequency changes were twice the size of those in response to flash, at 100 impulses s(-1) on average and additional spices per t rial were correspondingly 3-4 times higher. Both cortical and nuclear responses were context dependent, being larger when the rung moved whe n the cat was closer than further away. 4. A quarter of cells (20 of 8 4 tested, 24%) modulated their activity in advance of saccades, increa sing their discharge rate. Four-fifths of these were non-reciprocally directionally selective. Saccade-related neurones were usually suscept ible to other influences, i.e. their activity was not wholly explicabl e in terms of saccade parameters. 5. Substantial numbers of visually r esponsive neurones also discharged in relation to stepping movements w hile other visually responsive neurones discharged in advance of sacca dic eye movements. And more than half the cells tested were active in relation both to eye movements and to stepping movements. These combin ations of properties qualify even individual cerebellar neurones to pa rticipate in the co-ordination of visually guided eye and limb movemen ts.