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 in cats during visually guided stepping on a horizontal circular ladder. Ninety per cent of both nuclear cells (53/59) and Purkinje cells (53/58) showed step-related rhythmic modulations of their discharge frequency (one or more 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 difference 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 relative to the step cycle. Nevertheless, for both Purkinje cells and nuclear cells population activity was significantly greater in swing than in stance; the difference was more marked for the nuclear population. 4. Some cells exhibited both step-related rhythmicity and visual responsiveness (28 of 67 tested, 42%), whilst others were rhythmically active during locomotion and increased their discharge rate ahead of saccadic eye movements (11 of 54 tested, 20%). The rhythmicity of cells that were visually responsive was typical of the rhythmicity seen in the whole locomotor-related population. The step-related rhythmicity of cells that also discharged in relation 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 acts as a powerful source of excitatory locomotor drive to motor cortex, and may thereby contribute to establishing the step-related rhythmicity of motor cortical (including pyramidal tract) neurones. More generally, the activity patterns of lateral cerebellar neurones provide for a role in the production of visually guided, co-ordinated eye and body movements.