Intermediate representations in the formation of arm trajectories.

Psychophysical evidence shows that the planning of an arm trajectory is specified by the central nervous system in extrinsic coordinates. The complex issue of translating the planning of arm movements into muscle forces is discussed in relation to the recent discovery of structures in the brainstem and in the spinal cord. These structures represent discrete maps of motor behavior. Remarkably, the force outputs, produced by activating different zones of the map, sum vectorially. This vectorial combination of motor outputs is a mechanism for producing a vast repertoire of motor behaviors in a simple fashion.