1. Extracellular field potentials and single unit responses have been recorded from the inferior olive of the cat following stimulation of the surface of the contralateral paramedian lobule of the cerebellum, and of the ipsilateral cerebral cortex.2. Cerebellar stimulation results in antidromic invasion of inferior olivary neurones via the climbing fibres. These responses are followed by synaptic discharges which may be generated through climbing fibre recurrent collaterals.3. Precise histological controls have shown that these responses to stimulation of the paramedian lobule are located in the ventral lamella of the principal olive.4. Unifocal stimulation of the sensori-motor cortex with surface-anodal pulses evokes synaptically generated discharges of neurones in the ventral lamella, with a latency of 8-9 msec. The area of cortex yielding responses has been mapped at chosen stimulus intensities and the limitations of the maps have been discussed.5. It has been shown that the initial excitatory responses obtained from either cortex are followed by an inhibition which lasts about 100 msec, and gives way to a period of recovery or facilitation. This, in turn, is succeeded by a further period of inhibition. Possible neural substrates for these changes have been discussed.
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