The hypothesis that plasticity essential for motor learning is resident in the cerebellum is supported by studies showing that the integrity of parts of the olivo-cerebellar system (in particular, anterior interpositus nucleus, AIP) is critical for the acquisition and retention of conditioned nictitating membrane responses (CRs) - a simple form of motor learning. Some have argued that such lesions prevent performance, rather than learning. While permanent lesions cannot differentiate between these functions, reversible inactivations of AIP with muscimol (GABA-A agonist), reveal that AIP is essential for both. Experiments here evaluate the role of the AIP in extinction, the effects of AIP inactivation first on acquisition and then on extinction, and in post-conditioning consolidation processes. Experiment 1: Rabbits that had previously acquired, were first given four sessions of extinction training during inactivation, and then a further four sessions of extinction training without inactivation. They showed no CRs during inactivation, and CRs were at unextinguished levels after inactivation. Responses then extinguished normally. Extinction was therefore prevented. Experiment 2: Rabbits were given acquisition training first with inactivation of the AIP, and then without. They showed no CRs under muscimol, but acquired CRs gradually after inactivation. Muscimol therefore prevented acquisition. Subsequently, inactivations of the same site, AIP, in these animals also prevented extinction of CRs. Experiment 3: Pharmacological interventions after conditioning often disrupt acquisition. If the AIP is associated with learning-related plasticity, will AIP inactivations prevent post-conditioning consolidation processes associated with such plasticity. AIP was inactivated immediately after conditioning and learning was unaffected. When muscimol was infused subsequently during training it effectively blocked performance of CRs. So the blockades were effective, but they did not prevent muscimol-dependent consolidation processes in AIP. Inactivations of AIP with muscimol prevent extinction and acquisition, but not post-training consolidation processes. These findings are discussed in the context of the cerebellar learning hypothesis, and the anatomy and physiology of the cerebellum
