Stability of bimanual finger tapping coordination is constrained by salient phases

In bimanual cyclical continuous movements, the relative timing of the most salient movement phase in each movement is a predominant constraint. This is the case for coordination when both movements have a single most salient phase (the relative-salience hypothesis). We tested whether the relative-salience hypothesis could explain results obtained for repetitive discrete movements, utilizing finger tapping. In experiment 1, participants performed unimanual alternate two-finger tapping with the metronome beat (i.e., one finger taps on the beat and the other finger taps off the beat). The stability of the tapping timing relative to the beat, which reflects the extent of salience, was higher in the index finger than the middle finger, and was lower in the ring finger than the middle finger. In experiment 2, participants performed four conditions of repetitive bimanual four-finger tapping (i.e., alternate two-finger tapping in each hand) without external pacing signals. Under all four conditions, a more stable pattern occurred when the timing of the more salient tapping in each hand was simultaneous rather than alternate, regardless of relative direction in the external space or movement coupling of the homologous fingers. The results indicated that bimanual four-finger tapping could be explained by the relative-salience hypothesis.