Bilateral control of the orbicularis oculi muscle by one cerebellar hemisphere in the ferret.

Reports that lesions of the anterior interpositus nucleus in the cerebellum or of the cerebellar cortex abolish classically conditioned eyeblink responses were originally taken to indicate that the cerebellum is the locus of learning. This interpretation has recently been questioned by reports that conditioned responses may recover after ipsilateral cerebellar lesions. It cannot be excluded, however, that the recovered responses were produced by the intact contralateral hemisphere. In order to determine if cerebellar outflow to the orbicularis oculi muscle is bilateral, we stimulated both the brachium conjunctivum and a cortical area in the c3 zone of the cerebellar cortex which controls eyeblink. Both kinds of stimulation elicited EMG activity in both the ipsi- and the contralateral eyelids. The results thus show that there is a bilateral control of eyeblink from each cerebellar hemisphere and they raise the possibility that recovery of conditioning after ipsilateral cerebellar lesions is due to the intact hemisphere. Reports of such recovery after unilateral lesions are therefore inconclusive.