Central representations of human limb movement as revealed by studies of drawing and handwriting

In order to move, we must know 'where' to go and 'how' to get there. Since limb movement unfolds simultaneously in the space extrinsic to the subject ('where' to go) and in the intrinsic space of the joints and muscles ('how' to go), the brain must be able to represent the movement in both spaces and to map information from one space to the other. Here Francesco Lacquaniti reviews the experimental evidence in support of the hypothesis that these central representations consist of a limited, well-defined ensemble of movement properties, which are coded in specific coordinate systems. The discussion will be focused on drawing and handwriting movements, because much progress has recently been made in understanding them. However, most of the concepts apply to other arm movements as well.