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| Paper IPM / Cognitive / 7486 |
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Feed-forward motor plan is formed based on an internal representation of the external world. Formation of this plan requires implicit or explicit knowledge about the location of the target which is represented as allocentric or egocentric information (paillard 1987, 1991). The egocentric frame stems from the static and dynamical proprioceptive signals but the allocentric system derives from a memory-based internal structure, built from extracting the stable independent features. As it has been proposed, distance from an initial position of the limb to a visual target is specified by programming the timing and amplitude of force pulses. We investigated the role of time as an allocentric cue in forming a motor plan in a repetitive non-visual movement. Our experiment involved two phases: in the learning phase the subjects were asked to learn the distance between two targets by moving their hands between them while their hand movement was restricted to one dimension. In the test phase the end point target was removed and after a period of delay time (5s or 20s) the subjects had to reach the memorized target position. A tablet digitizer was used to extract the subjects? hand movement kinematics parameters. We found no correlation between time error rate and distance error rate(R Pearson: for 5 s delay 0.169 with P =0.148; for 20 s delay0.178 with P=0.133). In addition, Duration of the learning phase was equal to duration of the test phase (with the mean error of �47ms for 5 s delay trials and �53ms for 20 s delay trials) regardless of the observed distance error. This shows that the subjects may use the movement duration of learning phase as position independent information for reproducing the distance. We conclude that in the process of internalization of the external space, the nervous system tries to use an internal independent measure like time to code an external measure like distance. Considering time as an allocentric cue for motor planning, our results may imply that distance information could be coded into timing information in a repetitive motor task.
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