Roles of Cerebellar Cortex and Nuclei in Motor Learning: Contradictions or Clues?

population of granule cells, whose activity is also influThe cerebellum, with its relatively simple and regular enced by the inhibitory Golgi cells. One numeric fact synaptic organization, has yielded much about its contriillustrates these dramatic differences in computing bution to brain function and its internal information propower; each nucleus cell receives around 10 mossy cessing. A central theme that has emerged is the cerefiber synapses (mf→nuc) and is influenced by 10 granbellum’s role in the adaptation or learning of movements. ule cell synapses onto Purkinje cells (gr→Pkj). Ideas about cerebellar-mediated motor learning began Early ideas about cerebellar-mediated motor learning in the 1960s, most notably with the seminal theory prowere based on thesenotable anatomicalcharacteristics. posed by Marr (1969). The basic tenets of this theory In 1969, Marr proposed a theory suggesting how plasticare supported by numerous studies. In particular, analyity in the cerebellar cortex at the gr→Pkj synapses could sis of two forms of motor learning, adaptation of the mediate motor learning. Three basic components of this vestibulo-ocular reflex (VOR) and Pavlovian eyelid contheory were: (i) the mossy fiber/granule cell system enditioning (EC), has revealed much about cerebellar concodes the contexts in which movements occur, with the tributions to motor learning and the cerebellar informaabundance of granule cells providing a rich representation processing involved. tion; (ii) climbing fibers signal that the movement conThe cerebellum is comprised of two anatomical comtrolled by its Purkinje cell should change, and (iii) these ponents, the cerebellar cortex and nuclei. Despite much climbing fiber inputs induce plasticity at coactive gr→Pkj progress, their relative contributions to motor learning synapses, improving subsequent movement perforremain a fundamentally important issue that is largely mance in the context encoded by that pattern of granule