Distribution of neural plasticity in cerebellum-dependent motor learning.

The cerebellum is essential for some forms of motor learning. Two examples that provide useful experimental models are modification of the vestibulo-ocular reflex and classical conditioning of the nictitating membrane response (NMR) in the rabbit. There has been considerable analysis of these behavioral models and of conditioning of the eyelid blink reflex, which is similar in several respects to NMR conditioning but with some key differences in its control circuitry. The evidence is consistent with the suggestion that storage of these motor memories is to be found within the cerebellum and its associated brainstem circuitry. The cerebellum presents many advantages as a model system to characterize the cellular and molecular mechanisms underpinning behavioral learning. And yet, localizing the essential synaptic changes has proven to be difficult. A major problem has been to establish to what extent these neural changes are distributed through the cerebellar cortex, cerebellar nuclei, and associated brainstem nuclei. Inspired by recent theoretical work, here we review evidence that the distribution of plasticity across cortical and cerebellar nuclear (or brainstem vestibular system) levels for different learning tasks may be different and distinct. Our primary focus is on classical conditioning of the NMR and eyelid blink, and we offer comparisons with mechanisms for modifications of the vestibulo-ocular reflex. We describe a view of cerebellar learning that satisfies theoretical and empirical analysis.