Abstract

This study examined effects of lesions of the cerebellar cortical lobule VI (HVI) on the rabbit's conditioned nictitating membrane responses and the neural activity of the cerebellar interpositus nucleus (INT). Rabbits implanted with a recording electrode in INT were trained with a tone conditioned stimulus(CS) and an air puff unconditioned stimulus(US) for 4 days. They showed robust behavioral conditioned responses(CR) and time-amplitude neural model of learned responses in INT. Unilateral electrolytic lesions of lobule HVI were then made through 4 electrodes implanted before the training. The complete lesions of lobule HVI abolished not only the behavoiral CR but also the conditioned increase in neural activity of INT in the CS period that is considered as an essential neuronal plasiticity for the CR. Unconditioned responses(UR) and neural activities in the US period were not affected by the lesions. The results suggest that the input from HVI to INT plays an important role in the retention of the CR and the conditioned-related neural activity in INT. The INT thus seems not to be the only site that represents the learned response but to consist an element of the neural circuit that forms the memory trace for generating the CR. As the animals with lesions of HVI further retrained, they reacquisited the CR, though the CR amplitude and the learning rate were profoundly reduced. The neural modeling in INT was also slightly recovered. It indicates that HVI is ordinarily involved in the retention of the CR and the neural modeling in INT and that the classical conditioning is possible through the neural circuit even not including HVI.