BILATERAL DISRUPTION OF CONDITIONED-RESPONSES AFTER UNILATERAL BLOCKADE OF CEREBELLAR OUTPUT IN THE DECEREBRATE FERRET

1. Lesions of the cerebellar cortex can abolish classically conditione d eyeblink responses, but some recovery with retraining has been obser ved. It has been suggested that the recovered responses are generated by the intact contralateral cerebellar hemisphere. In order to investi gate this suggestion, bilaterally acquired conditioned responses were studied after the unilateral blockade of cerebellar output. 2. Decereb rate ferrets were trained with ipsilateral electrical forelimb stimula tion (300 ms, 50 Hz, 1 mA) as the conditioned stimulus and bilaterally applied peri-orbital stimulation (40 ms, 50 Hz, 3 mA) as the uncondit ioned stimulus. The conditioned and unconditioned eyeblink responses w ere monitored by EMG recordings from the orbicularis oculi muscle. The output from one cerebellar hemisphere was blocked either by injecting small amounts of lignocaine (lidocaine; 0.5-1.0 mu l) into the brachi um conjunctivum, or by a restricted mechanical lesion of the brainstem rostral to the cerebellum. 3. As described by previous investigators, the unilateral blockade of cerebellar output abolished ipsilateral co nditioned responses. 4. More importantly, such blockade also abolished or strongly depressed contralateral conditioned responses. When mecha nical lesions of the brachium conjunctivum were made, contralateral re sponses, in contrast to ipsilateral responses, recovered within 1-2.5 h. 5. When the unconditioned stimulus was removed on one side, causing extinction of conditioned responses on this side, conditioned respons es were temporarily depressed on the trained side as well. 6. Unilater al interruption of cerebellar output had no clear effect on contralate ral unconditioned reflex responses. 7. The results demonstrate that on e cerebellar hemisphere in ferrets exerts a marked control of contrala teral conditioned eyeblink responses, probably via premotor neurones i nvolved specifically in conditioned, and not in unconditioned, respons es.