Intermediate cerebellum and conditioned eyeblinks. Parallel involvement in eyeblinks and tonic eyelid closure.

The intermediate cerebellum (the intermediate cerebellar cortex and interposed nuclei) and associated brainstem circuits are essential for the acquisition and expression of classically conditioned eyeblinks in the rabbit. The purpose of the present experiment was to determine whether these circuits are also involved in adaptive eyelid closure learned in an instrumental paradigm. For that purpose, rabbits with unrestrained eyelids were trained in two tasks: (1) classical conditioning of the eyeblink; and (2) a new instrumental task in which they avoided delivery of an aversive stimulus by maintaining tonic eyelid closure. To examine the involvement of the intermediate cerebellum in these two types of learned behavior, the cerebellar interposed nuclei were injected with the GABAA agonist muscimol and with the GABAA antagonist picrotoxin. Inactivating the interposed nuclei with muscimol abolished classically conditioned eyeblinks and severely affected the rabbit's capacity to maintain tonic eyelid closure. On the other hand, reducing inhibition with picrotoxin failed to interrupt the learned responses and increased the amplitude of eyelid closure. These data indicate that the cerebellar interposed nuclei control both phasic classically conditioned eyeblinks and tonic instrumental eyelid closure. To account for this new finding, a "hybrid" hypothesis combining the cerebellar learning hypothesis and the performance hypothesis is proposed.