Rhythmic neuronal activity in the lateral cerebellum of the cat during visually guided stepping

1. The discharge patterns of 117 lateral cerebellar neurones were studied i n cats during visually guided stepping on a horizontal circular ladder. Nin ety per cent of both nuclear cells (53/59) and Purkinje cells (53/58) showe d step-related rhythmic modulations of their discharge frequency (one or mo re periods of 'raised activity' per step cycle of the ipsilateral forelimb) . 2. For 31% of nuclear cells (18/59) and 34% of Purkinje cells (20/58) the d ifference between the highest and lowest discharge rates in different parts of the step cycle was >50 impulses s(-1). 3. Individual neurones differed widely in the phasing of their discharges r elative to the step cycle. Nevertheless, for both Purkinje cells and nuclea r cells population activity was significantly greater in swing than in stan ce; the difference was more marked for the nuclear population. 4. Some cells exhibited both step-related rhythmicity and visual responsive ness (28 of 67 tested, 42%), whilst others were rhythmically active during locomotion and increased their discharge rate ahead of saccadic eye movemen ts (11 of 54 tested, 20%). The rhythmicity of cells that were visually resp onsive was typical of the: rhythmicity seen in the whole locomotor-related population. The step-related rhythmicity of cells that also discharged in r elation to saccades was generally below average strength compared with the cortical and nuclear populations as a whole. 5. The possibility is discussed that the rhythmicity of dentate neurones ac ts as a powerful source of excitatory locomotor drive to motor cortex, and may thereby contribute to establishing the step-related rhythmicity of moto r cortical (including pyramidal tract) neurones. More generally, the activi ty patterns of lateral cerebellar neurones provide for a role in the produc tion of visually guided, co-ordinated eye and body movements.