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| Paper IPM / Cognitive / 7485 |
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Reaching to a target point needs representation of target information. The nature of this information is a matter of debate. Some theories suggest that distance from initial position is coded as timing of muscle forces however other ones prefer to consider the muscle tension-length relationship as the source of target location information. We investigated the interference of distance information and movement initial and end-point position in kinesthetic memory. We designed a repetitive non visual task in which in the learning phase subjects learned the distance between two targets (defined by two sticks) by moving their index fingers between the sticks 10-12 times back and forth, and in the test phase starting from the initial target they had to reproduce the distance following a delay period of 6-8 sec. Movement information was digitized by a tablet digitizer. To investigate the role of initial position in distance reproduction, the learning phase was followed by a Position Mask Phase in which the subjects had to perform a repetitive task for 4 trials with the same distance but a varying initial position (the temporal duration of the mask phase was 6-8 sec). After ward, in the test phase the subjects fingers were brought back to the initial position of the learning phase and they were asked to reproduce the learned distance. Comparing the result of these two situations showed that masking the initial position while the distance is constant leads to significant error in reproducing the distance initiating from primary initial point ( Mixed 2*2 ANOVA F(1,274)=15.408 P<0.001 ). This means that although the distance information is available during changing the position, subjects could not use this information accurately for reproducing the distance. To rule out the probable role of varied end point in generation of this error, we designed a condition in which the learning phase was followed by a Distance Mask Phase in which the initial position remained constant but the distance was either shorter or longer than the learned distance. Again a significant error was seen in reproduction of the distance (Mixed 2*2 ANOVA F (1,275) =8.42 P=0.005), but the error did not obey the expected kind of end point based errors (overshooting in longer masks and undershooting in shorter masks). The subjects overshot the target point regardless of the mask being shorter or longer and the reproduction error was not different in the long and short masks situations (Independent sample T-test df=138 P=0.67). Hence we suggest that this error has been raised from distraction of distance information rather than interference of distracting end point information. We conclude that the distance information and the initial position information are essential in an accurate distance reproduction task.
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