Essential neuronal pathways for reflex and conditioned response initiationin an intracerebellar stimulation paradigm and the impact of unconditionedstimulus preexposure on learning rate

It has been demonstrated previously that pairing of tone CS and intracerebe llar stimulation of lobule HVI white matter as the US produces conditioning that is robust and in many ways similar to that obtained with an airpuff U S. The first study in this report addressed the effect of interpositus lesi ons on conditioned performance in rabbits trained with white matter stimula tion as the US. It was found that interpositus lesions effectively eliminat ed the CR irrespective of the behavioral response measured. In addition, it was shown that the interpositus lesions also abolished the UR, providing s trong evidence that the effects of the electrical stimulation were confined to the cerebellum and did not require the activation of brainstem structur es. The second experiment examined performance on US-alone trials of varyin g durations. Response initiation within 100 ms of the US onset, regardless of US duration, indicated that reflex generation could not be due to reboun d excitation of the interpositus following termination of Purkinje cell inh ibition of that structure but instead likely reflects orthodromic activatio n of interpositus neurons via climbing fiber and/or mossy fiber collaterals . The impact of US preexposure on associative conditioning in this paradigm was also determined. Animals which received only 108 US-alone trials were massively impaired during subsequent training compared to rabbits that rece ived fewer than 12 US-alone trials. (C) 1999 Academic Press.