Reversible lesions of the red nucleus during acquisition and retention of a classically conditioned behavior in rabbits.

Previous lesion, recording, and stimulation studies implicated the cerebellum and its associated brain-stem circuitry as essentially involved in classical conditioning of discrete, somatic muscle responses. This is a companion to our study of interpositus cooling, which showed that the formation of a memory was prevented. In the present study, we assess the red nucleus for its role in the plasticity associated with learning and memory by using local cooling as a reversible lesion technique. A cooling probe was implanted lateral to the red nucleus. Recording electrodes were implanted in the right red nucleus and the left interpositus nucleus. Animals were trained for 5 days with the cooling probe activated. No behavioral conditioned responses (CR) developed, and multiple unit recordings related to learning did not develop in the red nucleus. However, a learning related model did develop in the interpositus. After 5 days of training while cooling, animals were given 5 days of normal training (cooling probe inactive) to assess retention. Substantial savings were evident when normal training was given. CRs appeared quickly on the first day of normal training, and multiple unit models were present in both red nucleus and interpositus nucleus. These results support the idea that the red nucleus is a necessary efferent for the memory trace formed in the cerebellum.