Spontaneous bimanual independence during parallel tapping and sawing

The performance of complex polyrhythms – rhythms where the left and right hand move at different rates – is usually the province of highly trained individuals. However, studies in which hand movement was guided haptically show that even novices can perform such tasks with no or only brief training. In this study, we investigated whether novices are able to tap by matching different rates of a metronome while operating a saw, based on the assumption that saw movement could be controlled without paying primary attention to it. Results showed that saw movement was executed at spontaneous and consistent cycle duration of 0.44 [0.20] s to 0.51 [0.19] s across single- and bimanual conditions, with no significant effect of the condition on the cycle duration (p = 0.315). Free tapping was executed at a matching cycle duration of 0.48 [0.22] s. In the bimanual conditions, we found that for a ratio of 4:3 (4 taps against 3 sawing cycles per measure), the observed and predicted ratio of 0.75 was not significantly different (p = 0.369), supporting our hypothesis. However, for a ratio of 3:4, observed and predicted ratio differed (p = 0.016). Our findings show that bimanual independence when performing complex polyrhythms can in principle be achieved if the movement of one hand can be performed without paying much – if any – attention to it, possibly driven by an intrinsic timing which leads to arm movements converging spontaneously on a cycle duration of around 0.5 s when not under conscious control.