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Abstract:
When we make brief reaching arm movements or saccadic eye
movements, trajectories tend to be straight and smooth with
symmetrical bell-shaped velocity profiles that become
progressively asymmetrical with increased duration. A similar
phenomenon occurs in tongue movement during articulation, and
this suggests that there may be some underlying principle to
movement control. We propose motor commands are perturbed by
signal dependent noise (SDN). With SDN larger control signals
increase noise which accumulates during a movement leading to
more inaccuracy. Since faster movements require large control
signals, SDN imposes a speed accuracy trade-off. We show that,
for a given movement duration, there is a unique optimal
trajectory that minimises inaccuracy, or equivalently minimises
duration for a given level of endpoint error. This trajectory is
very similar to empirically observed arm movements and saccades,
being smooth and symmetrical for brief durations and asymmetrical
for long durations. We further propose that when there is a fixed
speed-accuracy trade-off, there is a unique duration for a given
amplitude of movement, which is in good agreement with the
stereotyped duration-amplitude relationship ('main sequence')
empirically observed in saccades. The similarity of tongue and
lip movements to arm and eye movements leads us to suggest that
the speed and accuracy of speech articulation may also be
constrained by the same kind of motor noise.
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