Neural Coding of the Direction of Reaching and a Comparison with Saccadic Eye Movements

The neural control of movement has captured the imagination for a long time. For example, Descartes (1664) attached a special role to the pineal gland in eye-hand coordination (Fig. 1). It is only recently that knowledge has accumulated concerning the control of arm and eye movements by the central nervous system (CNS). These movements share common characteristics reflected in the neural mechanisms that control them. First, they are vectors whose direction and amplitude need to be specified, and, second, they are implemented by concomitant engagement of several muscles.The specification problem concerns the neural codes by which the direction and amplitude of the movement are generated in the brain. Recent studies have suggested that the specification of movement direction involves similar neural codes in both the arm and the eye movement systems: Information concerning the direction of the movement is distributed within a neuronal ensemble, and it is only at the level of the whole ensemble that the direction of movement is uniquely specified.