Physiology of clinical dysfunction of the cerebellum.

Experimental and theoretical research into cerebellar function has begun to converge toward understanding the cerebellum as a "controller" in the engineering sense. The purpose of a controller is to convert high-level intent commands and information describing the current state of a system into low-level control signals suitable for maintaining or changing system behavior. The cerebellar subsystem appears to play this role for parts of the body and other parts of the brain. As with engineering controllers, fundamental functions include stabilization at a fixed posture or state, adjustment of movement or transition amplitude, facilitation of movement/transition speed and crispness of launch and braking, improvement of resistance to disturbances, coordination of control across multiple degrees of freedom, and assistance with estimation and/or prediction of current and future system states. As with adaptive engineering controllers, the cerebellar subsystem also readily tunes itself over time. At a more detailed level, many of the specific actions of cerebellar circuits can be understood in terms of proportional (P), integrator-like (I), and differentiator-like (D) signal processing which are fundamental components of many engineering control systems. This chapter presents an integrated, mechanistic view of ataxia, tremor, and several cerebellar oculomotor signs in terms of PID control and the neural centers that appear to subserve these functions. It also suggests the manner in which impairments in motor learning, perception, and cognition that are associated with cerebellar dysfunction may be viewed from a similar perspective.