Models of the cerebellum and motor learning
This article reviews models of the cerebellum and motor learning starting with the landmark papers by Marr and Albus and going through present times. The unique architecture of the cerebellar cortex is ideally suited for pattern recognition, but how is pattern recognition incorporated into motor control and learning systems? The present analysis begins with a discussion of what the cerebellar cortex needs to regulate through its anatomically defined projections to premotor networks. Next we examine various models of how the microcircuitry in the cerebellar cortex may be used to achieve its regulatory functions. Having thus defined what it is that Purkinje cells in the cerebellar cortex must learn, we then evaluate theories of motor learning. We examine current models of synaptic plasticity, credit assignment and the generation of training information, indicating how they could function cooperatively to guide the processes of motor learning. Lesion studies carried out in the 19th century demonstrated that the cerebellum is important for coordinating movements (Florens, 1824). However, mechanistic models of the cerebellum awaited an analysis of the histology of the cerebellum (Braitenberg & Atwood, 1958), and combined analyses of histology and electrophysiology (Marr, 1969; Albus, 1971). The clear orthogonal relationships between parallel and climbing fibers and the dendritic trees of Purkinje cells convinced Braitenberg that the cerebellum functions as a timing organ. He viewed the parallel fibers as delay lines and climbing fibers as clock read-out mechanisms, with Purkinje cells firing only when there was a coincidence of a parallel fiber volley and a climbing fiber activation. In one of his examples, a Purkinje cell innervating an antagonist muscle fired at an appropriate time delay to terminate a movement at its intended target. Even though parallel fibers have small diameters and low conduction velocities, the time delays only amount to a few milliseconds, rather short for most problems in motor control. Another limitation of this theory is its requirement for foci of synchronized activation in the granular layer. For these and other reasons, Braitenberg’ s timing theory has not been as vigorously pursued as the learned pattern recognition theories developed subsequently by Marr (1969) and Albus (1971).