The importance of cerebellar cortex and facial nucleus in acquisition and retention of eyeblink/NM conditioning: evidence for critical unilateral regulation of the conditioned response.

Experiment 1 examined acquisition of the classically conditioned eyeblink response in rabbits (Oryctolagus cuniculus) during reversible cooling lesions of the cerebellar cortex (CX), facial nucleus (FN), or lateral parvocellular reticular formation (RF). Retention was then evaluated during periods of training during reversible cooling lesions and without cooling in rabbits that had acquired the conditioned response. We found that cooling the CX did not prevent acquisition, but did retard the acquisition rate. Cooling the FN during acquisition prevented the expression of the unconditioned and conditioned response, but did not prevent the acquisition when assessed during subsequent training without cooling. Cooling the RF had no effect on the acquisition or expression of the conditioned response. During subsequent retention testing, in well-trained animals, cooling the CX did not abolish the learned response. Cooling the FN abolished both the conditioned and the unconditioned response. The results from Experiment 1 indicate that the CX is more important for acquisition than retention of the conditioned response. The FN is not important for the acquisition of the conditioned response, but is essential for the expression of the conditioned and unconditioned response. Experiment 2 examined bilateral recordings from the cerebellum in well-trained rabbits, before and during interpositus or FN cooling. We found that cooling the interpositus abolished all learning related activity in the ipsilateral or contralateral cerebellum, but did not affect the stimulus evoked responses. Cooling the FN did not abolish stimulus evoked activity or learning related activity in the cerebellum. The results emphasize the critical importance of the ipsilateral cerebellum in classical eyeblink conditioning and suggest that the memory trace for this type of learning is mediated by unilateral circuitry.