Cerebellum Predicts the Future Motor State

Abstract

Feed forward control and estimates of the future state of the motor system are critical for fast and coordinated movements. One framework for generating these predictive signals is based on the central nervous system implementing internal models. Internal models provide for representations of the input-output properties of the motor apparatus or their inverses. It has been widely hypothesized that the cerebellum acquires and stores internal models of the motor apparatus. The results of psychophysical, functional imaging and transcranial magnetic stimulation studies in normal subjects support this hypothesis. Also, the deficits in patients with cerebellar dysfunction can be attributed to a failure of predictive feed forward control and/or to accurately estimate the consequences of motor commands. Furthermore, the computation performed by the cerebellar-like electrosensory lobes in several groups of fishes is to predict the sensory consequences of motor commands. However, only a few electrophysiological investigations have directly tested whether neurons in the cerebellar cortex have the requisite signals compatible with either an inverse or forward internal model. Our studies in the monkey performing manual pursuit tracking demonstrate that the simple spike discharge of Purkinje cells does not have the dynamics-related signals required to be the output of an inverse dynamics model. However, Purkinje cell firing has several of the characteristics of a forward internal model of the arm. A synthesis of the evidence suggests that the cerebellum is involved in integrating the current state of the motor system with internally generated motor commands to predict the future state. KeywordsPrimate; Cerebellum; Purkinje cell; Simple spike; Internal models ReviewInternal models are widely discussed as a mechanism by which the central nervous system (CNS) generates feed forward commands and/or makes predictions about the upcoming states. Forward internal models predict the state of the system, either the motor variables or the sensory output, as a consequence of the current state of the system and the motor commands. Inverse dynamics models, in response to the desired trajectory, generate the signals required to produce the joint torques/forces used to control a movement. Numerous psychophysical findings support the hypothesis that the CNS uses internal models. Prominent examples include the anticipatory changes in grip forces during predictable manipulations of object load [11,17] and our ability to adapt and generalize reaching movements in novel dynamic environments [38,42]. Furthermore, sensory feedback loops have delays that are too long and gains too low to control fast and coordinated movements.Correspondence to: Timothy J. Ebner. Many investigators postulate that the cerebellum acquires and stores internal models of the motor system [16,18,26,37,44]. Support for these hypotheses is largely based on deficits in patients with cerebellar dysfunction or on functional imaging studies. Consistent with the inverse d...

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