The cerebellum: Comparative and animal studies

The cerebellum has a uniform cellular structure and microcircuitry, but the size of its subdivisions varies greatly among vertebrates. This variability is a challenge to anatomists to attempt to relate size differences to differences in characteristic behaviour. Here we review the early work of Lodewijk Bolk on the mammalian cerebellum and relate his observations to unfolded maps of the rodent cerebella. We further take insights from the comparative anatomy of the bird cerebella and find that cerebellar enlargement in large brains is not a passive consequence of overall brain enlargement, but is related to specific behaviour. We speculate that for some rodents (e.g., squirrels), primates and some large-brained birds (crows, parrots and woodpeckers), specifically enlarged cerebella are associated with either the elaboration of forelimb control (squirrels and primates) or in the case of the birds with beak control. The elaboration of such motor behaviour combined with increased visual control could have helped to furnish manipulative skills in these animals. Finally, we review the connections of the mammalian cerebellum and show that several pieces of experimental evidence point to an important function of the cerebellum in sensory control of movement reflex adjustment, and motor learning.