Abstract
Delays in sensorimotor loops have led to the proposal that reaching
movements are primarily under pre-programmed control and that sensory
feedback loops exert an influence only at the very end of a trajectory.
The present review challenges this view. Although behavioral data
suggest that a motor plan is assembled prior to the onset of movement,
more recent studies have indicated that this initial plan does not
unfold unaltered, but is updated continuously by internal feedback
loops. These loops rely on a forward model that integrates the sensory
inflow and motor outflow to evaluate the consequence of the motor
commands sent to a limb, such as the arm. In such a model, the probable
position and velocity of an effector can be estimated with negligible
delays and even predicted in advance, thus making feedback strategies
possible for fast reaching movements. The parietal lobe and cerebellum
appear to play a crucial role in this process. The ability of the
motor system to estimate the future state of the limb might be an
evolutionary substrate for mental operations that require an estimate
of sequelae in the immediate future.
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