Sensorimotor transformations underlying the organization of arm movements in three-dimensional space.

Coordinated movements in three-dimensional space involve sensorimotor transformations between extrinsic and intrinsic coordinates. It is hypothesized that a key aspect underlying the organization of such movements is the need to simplify these transformations by means of suitable approximations and the imposition of constraints. Motor tasks involving the drawing of circles and ellipses in different planes were analyzed from this perspective, and some rules are presented whereby the plane of motion and the slant of an ellipse can be specified in a simple way in terms of intrinsic parameters. It is shown that these rules can be generalized to hold for more complicated wrist motions if one assumes that they consist of segments of elliptical arcs.