Expert musicians show experience-dependent reduced asymmetry in the structure of motor-related brain areas and in the maximum tapping frequency between the hands. Therefore we hypothesized that a reduced handskill asymmetry is strongly related to rapid and rhythmical bimanual coordination and developed a dynamical model including a symmetry-breaking parameter Δω, for human bimanual coordination. We conducted unimanual and bimanual drumming experiments to test the following model predictions. 1) The asymmetry in the maximum tapping frequency is more pronounced in nondrummers than that in drummers. If so, 2) a larger number of phase wanderings (i.e., succession of taps by the same hand), 3) larger SD of the relative phase between the hands (SD φ), and 4) larger deviation of mean relative phase (mean φ) from 180° would be observed in nondrummers than that in professional drummers during antiphase bimanual drumming at the maximum speed. In a unimanual tapping task, the asymmetry in maximum tapping frequency of nondrummers was more pronounced than that of professional drummers. In a bimanual coordination task, phase wanderings were observed only in nondrummers and SD φ of the nondrummers is significantly larger than that of professional drummers. On the other hand, there was no significant difference between the mean φ of the two groups. All these observations were successfully reproduced by changing Δω, which corresponded to the asymmetry in the maximum tapping frequency. These results support the hypothesis indicating that the prominent bimanual coordination pattern emerges spontaneously after a nonspecific change in Δω or symmetry restoration of the nonlinear dynamical systems.
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