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Abstract
The principles underlying the production of goal directed movements are largely unknown, as evidenced by the lack of artificial systems matching the real-world performance of biological organisms. The ability to transform in real time a vaguely defined goal (skiing downhill) into a detailed motor act (synchronized movements at multiple joints) appropriate under a wide range of previously unencountered circumstances (details of the terrain, snow conditions, etc.) is something we take for granted (after a period of painful learning). This is reminiscent of the subjective ease with which we perceive visually cluttered environments, or follow a conversation in a noisy room.
This thesis argues that the classic view of motor control as a unidirectional process, starting with a planning stage (a learned mapping from goals into actions or a straightforward interpolation connecting a number of intermediate postures into a detailed trajectory) followed by execution of the motor plan, provides an oversimplified and inadequate account of the versatility of everyday human performance. We propose that this standard model of motor control as well as the standard bottom-up models of perception, are inadequate because both perception and motor control are actually “inverse” problems, and are better treated as such.
After some theoretical considerations of goal directed multijoint movements, we develop a computational model of sensory-motor processing in intermediate point tasks (including reaching) based on stochastic optimal control theory. In this model planning end execution are not separate stages, but are both integrated into a tight sensory-motor loop which constantly adjusts the ongoing movement to better achieve the specified goal. The model provides a natural account for a number of experimental findings reported here (as well as previously observed phenomena); taken together these experimental results strongly indicate that the motor system updates online its internal estimates of both the environment and the moving limb, and is always ready to modify its “plan” in favor of a different movement that better achieves the goal under the new circumstances. The observations include eye-hand synchronization patterns, corrections for undetected saccade-triggered visual perturbations and hand inertia anisotropies, effects of desired accuracy on hand kinematics, movement segmentation, speed-curvature relationships. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)
