Abstract

It has been recently demonstrated that unilateral electrolytic lesions of the dentate- interpositus nucleus completley prevent and abolish ipsilateral learning of the classically conditioned nictitating membrane response(NMR) without affecting the unconditioned response in the rabbit. But the lesion effects of the cerebellar cortex were not consistant. So the present experiment 1 was conducted to replicate the lesion effect of the dentate-interpositus nucleus in abolishing NMR conditioing with radio frequency lesion. Experiment 2 was performed to answer the question of whether the cerebellar cortical input to the nucleus is necessary for NMR conditioning. All animals were trained using standrad procedures for NMR conditioning. The conditioning involves a tone(1kHz, 85dB SPL, 350ms) as the conditioned stimulus with a coterminating periorbital electric shock(2mA, 5Oms) delivered at the right eye as the unconditioned stimulus. In experiment 1, 15 animals were trained one session a day until each of them reached the criteria of 8 consecutive CRs, after which one overtraining session was added. In experiment 2, 24 subjects were given the same training as in experiment 1. Following the overtraining, animals were lesioned on the right side. In experiment 1, RF lesions were made at or near the dentate-interpositus nucleus. In experiment 2, subjects were lesioned at simple lobule or other lobule by aspiration. After recovery, animals received 4 or 5 days of training on the right side to test for retention and recovery of conditioned responding. Then animals received an additional session in which training was switched to the left side. Results of experiment I showed that unilateral RF lesions of the dentate-interpositus nucleus abolished conditioning on the side of the lesion, leaving unconditioned responses to US intact. In experiment 2, unilateral lesions of the cerebellar cortex, simple lobule abolished conditioning on the side of the lesion but larger lesions of other cerebellar lobule did not impair NMR conditioning. These results indicate that critical neueral alteration during NMR conditioning occurs in the simple lobule rater than the dentate-interpositus nucleus.