In each tapping task, motor performance was indexed by tapping rate, tapping variability, and the completion ratio. The mean tapping rate was calculated in each tapping task as the number of all index finger extensions crossing the lower light beam per second. The tapping variability was calculated as the standard deviation of the normalized movement intervals across a trial. The movement intervals were obtained as the time intervals between adjacent movement onsets. Concerning the variation of movement rates between subjects, time intervals were normalized to the maximum interval in a trial. We then computed the mean tapping variability across trials, per task, for each subject. The completion ratio was computed as the ratio of index finger extensions reaching the level of the upper photoelectric sensor divided by the total number of index finger extensions.
