Neural basis of motor control and its cognitive implications.

It has recently been demonstrated that human subjects and nonhuman primates adapt their arm movements when subjected to complex patterns of disturbing forces. The presence of aftereffects following the removal of the disturbing forces indicates that adaptation takes place through the development of an internal model of the disturbing force. The experimental evidence described in this paper has identified some important properties of this internal model: (1) it is limited to a region surrounding that part of the space where the disturbances had been experienced; (2) there is an enhancement of the internal model that depends only on the passage of time; and (3) there is a process of consolidation of the internal model, which takes a minimum of four hours. Anatomically, the substrate of the internal model is distributed; the motor cortex, basal ganglia, and cerebellum are interconnected structures that are active to different degrees during the acquisition of motor skills. Recent investigation of the spinal cord has suggested the existence of modules that organize the motor output in a discrete set of synergies. The outputs of these modules combine by addition, and might thus form the building blocks for the internal models represented by supraspinal structures.