The cerebellum is necessary for rabbit classical eyeblink conditioning with a non-somatosensory (photic) unconditioned stimulus

The present research investigated the acquisition of classically conditione d eyeblinks in rabbits using a light flash unconditioned stimulus (US), as well as the contribution of deep cerebellar nuclei to such an association. Two independent groups of animals experienced three phases of training: (1) pre-lesion delay conditioning using either a light- (Group 1) or an air pu ff-US (Group 2), (2) post-lesion testing of response performance, and (3) p ost-lesion acquisition to the opposite US. During the initial acquisition ( 720 trials), the groups did not differ with regard to their rate of learnin g or their overall level of responding. To assess the contribution of the c erebellum to the maintenance of responding, the interpositus nucleus was el ectrolytically lesioned and animals were given 8 days of additional trainin g. Both groups exhibited a profound reduction in conditioned responding (CR ) and showed no signs of recovery over the remainder of this phase (480 tri als). Animals were then shifted to the opposite US (same eye) and given 12 days of training to assess the effect of interpositus lesions on the acquis ition of CRs to a novel US. No learning was observed during this phase, reg ardless of whether animals experienced the light- or air puff-US. These res ults demonstrate: (1) the ability of a non-somatosensory stimulus to serve as a US during classical eyeblink conditioning; and (2) a common reliance o n deep cerebellar nuclei for both somatosensory- and non-somatosensory-base d reflexive motor learning. The findings are discussed in reference to the processing of conditioning stimuli within the brainstem-cerebellar circuitr y that underlies eyeblink conditioning. (C) 1999 Elsevier Science B.V. All rights reserved.