Reaching: Coding in motor cortex

The Handbook of Brain Theory and Neural Networks - 1995-01-01Georgopoulos AP
A common and behaviorally meaningful movement is reaching to targets in space. Reaching involves well-coordinated motion about the shoulder and elbow joints for transporting the hand in space and bringing it to a desired location. A reaching movement can be regarded as a vector, from its origin to its target, with direction and amplitude. The results of several studies support the view that these two parameters reflect separate processing constraints (Georgopoulos, 1991). First, accuracy of pointing is much better for direction than for amplitude; second, when subjects are forced to make a motor response at a time shorter than the usual reaction time, the direction and amplitude of the motor trajectory are affected differently; and third, peripheral sensory neuropathy affects deferentially the direction and amplitude of the movement (Ghez et al., 1990). Moreover, the generation of arm movements is not a stereotypic process but seems to involve processing that is subject to interference by distracting sensory and cognitive loads (Frens and Erkelens, 1991); this is in contrast to visually evoked saccades which are unaffected under such conditions. This susceptibility of the arm movement-generating process to distracting (or competing) processes underscores the complexity of the central nervous processes that are involved in this function.