SUMMARY1. We have examined the role of the anterior interpositus nucleus (AIP) of the cerebellum in Pavlovian conditioning of the nictitating membrane response (NMR) of the rabbit with the use of reversible brain lesions produced by the local anaesthetic lidocaine. Previous experiments have demonstrated that destructive lesions of the AIP prevent the performance of conditioned NMRs (CRs). Microinjections of lidocaine into the AIP were used in the present experiment to determine whether the deficit in the performance of CRs resulted from a deficit in learning or memory.2. A 3-phase procedure was employed to determine whether associative learning required the function of the AIP. In phase 1, rabbits were trained to make CRs to a flashing-light conditioned stimulus (CS) that was paired with an air-puff unconditioned stimulus (UCS) directed at the cornea. In phase 2, the AIP was anaesthetized during a session of conditioning in which a tone CS was paired with the UCS. Presentations of the light CS were interpolated throughout the tone conditioning in order to monitor the degree to which CRs were impaired by lidocaine. Phase 3 occurred after the effects of the lidocaine had dissipated and consisted of a test of retention to determine whether learning occurred during phase 2 but could not be expressed because of a performance deficit resulting from the inactivation of the AIP.3. Infusion of lidocaine into the AIP abolished CRs to the light CS and prevented the performance of CRs to the tone CS in phase 2. The effect of the infusion was specifically due to a conduction block of neurons and/or fibres in the lateral aspect of the AIP. The infusion of lidocaine into regions surrounding the AIP did not affect CRs elicited by the light CS or prevent acquisition of CRs to the tone. Infusions of saline directly into the AIP did not impair the performance of CRs to either the tone or light CS. Quantitative analysis of diffusion revealed that the abolition of CRs was accompanied by anaesthetization of the AIP.4. The retention test in phase 3 indicated that learning occurred normally during phase 2 when the AIP was inactivated and performance was abolished. When the function of the AIP was restored and performance had recovered, the subjects demonstrated a frequency of CRs to the tone CS that was indistinguishable from control subjects whose performance had never been impaired. MS 8897 J. P. WELSH AND J. A. HARVEY 5. The CRs observed during the retention test provided an unequivocal measure of associative learning. Control subjects that received explicitly unpaired presentations of the tone and air puff during the infusion of lidocaine responded at baseline levels to the tone when the function of the AIP was restored.6. We conclude that the AIP is not essential for Pavlovian conditioning of the NMR because learning occurs normally when the AIP is inactivated. Therefore, the AIP can neither be a site where memories for Pavlovian conditioning are stored nor a site that is essential for sensory processing which leads to associat...

This study reexamined the effects of unilateral damage to cerebellar hemispheral lobule VI on the rabbit's conditioned nictitating membrane (NM) response. Extensive unilateral removal of hemispheral lobule VI in 11 rabbits impaired ipsilateral conditioned responses as reflected by reductions of 52% in mean frequency and 53% in mean amplitude during test trials on the first postoperative session. The decreases in the amplitude and frequency of conditioned responses were highly correlated (r = 0.82). The frequency of conditioned responses recovered to control levels but their amplitudes remained reduced such that the correlation between these two measures of responding was no longer significant by the 12th postoperative conditioning session. The decrease in the amplitude of conditioned responses was not accompanied by changes in onset latency or rise time. There was no significant impairment of conditioned responses in surgical controls and animals with only partial damage to hemispheral lobule VI. It was concluded that hemispheral lobule VI plays an important role in the regulation of motor centers in the brainstem so as to facilitate the initiation and optimum execution of the conditioned NM reflex. This cortical regulation of the conditioned NM response may contain learned elements; however, these cannot be resolved with lesion methods, nor has their existence been proven in this or other lesion studies. Nevertheless, the results of this study do demonstrate that the cerebellar cortex cannot be considered as the single locus necessary for NM conditioning.

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