[Modulatory role of the cerebellar cortex during the classically conditioned nictitating membrane response in rabbit]

Classical conditioning of the rabbit nictitating membrane(NM) response was accomplished by standard delay procedure of paired tone (CS) and airpuff (US) stimuli. Upon reaching a criterion of 90% conditioned responses (CRs) in 3 consecutive blocks, an electrolytic lesion was made at the hemicerebellar VI cortical lobule on the training side. Results showed that the learned response of the dentate-interpositus (D-I) nuclei and the conditioned NM response were completely or almost completely eliminated without affecting the unconditioned response. However, cerebellar cortical lesion performed on animals being over 1 week overtrained produced no effect on CRs and the learned response of the D-I nuclei. Meanwhile, the spontaneous activity of the D-I nuclei during consolidation reduced in frequency as a result of the cortical lesion at the early stage of learning. Our findings indicate that the cerebellar cortex plays a modulatory role in the generation of the conditioned NM response and the development of the learned response of the D-I nuclei at an early stage of learning. With consolidation, the D-I nuclei become free of control from the cerebellar cortex and serve as the essential component of the memory trace for this model system.