Kinesthetic memory in distance reproduction task: importance of initial hand position information

The role of the initial hand position in planning and implementation of a goal-directed movement is a matter of debate. We designed a non-visually guided repetitive reaching movement task to investigate the role of proprioceptive information of the initial or end hand position in implementation of kinesthetic memory. The experimental design consisted four experiments. Each experiment consisted two phases: the training phase and the test phase. In the training phase the subjects were asked to learn and memorize the distance between start and end points by moving their hands ten times between them. In the test phase, the end point was removed, and starting from the same initial position, the subjects were asked to reproduce the memorized distance. In all experiments, varied conditions called "mask phase" preceded the test phase: a 5-s delay in experiment 1, shifting both initial and end points while keeping the distance constant in experiment 2, shifting only the end point, while keeping the initial point constant in experiment 3 and shifting only the initial point and keeping the end point constant in experiment 4. In comparison to experiment 1 (which was considered as the control group), no significant error in distance reproduction occurred in experiment 3, while experiments 2 and 4 showed significant error. These findings mean that although the distance information was available during the mask phase of experiment 2, subjects could not efficiently use this information to reproduce the distance. The other two experiments suggest that this error was caused by changing the initial hand position and not the end point. We conclude that in a kinesthetic-guided distance reproduction task, the initial hand position is more important for implementation of distance memory rather than the end point.