The present study examined whether the attentional demand of rhythmic movements increased or decreased at a specific frequency by using the probe reaction time (RT) method. RTs of vocalization were measured on six subjects during bilateral reciprocal movements performed with the hands, feet, arms, or legs in synchrony with beats of a metronome from 80 to 200 per min. There was a frequency at which the lengthening of RT, compared to control condition, i.e., without movements, was minimum during

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... hythmic movements. This frequency was not different both among the body parts used and the subjects examined. It is assumed that there is an optimum frequency for rhythm-formation at which the attentional demand is minimum.