The spontaneous recruitment and suppression of degrees of freedom in rhythmic hand movements
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In most studies examining pattern switching in biological coordination, emphasis is placed on identifying the mechanisms underlying bifurcations in an already active set of components. Less well understood are the processes by which quiescent degrees of freedom (df) are recruited and active df suppressed. To examine such behavior, we studied four bimanual and two unimanual coordination patterns. Subjects produced the patterns in time with an auditory metronome whose frequency increased from 1.5 to 4.25 Hz in 0.25 Hz steps. Interlimb transitions from asymmetric to symmetric patterns in a motion plane occurred at critical cycling frequencies, f1. Spatial transitions, characterized by recruitment of y-(vertical) and suppression of x-(horizontal) motion, also occurred at critical cycling frequencies, f2. This recruitment-suppression process was either abrupt (2-3 cycles) or gradual (1 to 6 plateaus) where the finger-tips traversed an elliptical orbit in x, y space. Similar spatial transitions were observed in unimanual conditions. The results are discussed in reference to the problem of how task-specific coordination patterns are modified. The Hopf bifurcation is presented as a generic mechanism underlying the recruitment and suppression of df. Similarities between the four component bimanual pattern dynamics and the coordination dynamics of four limb patterns (e.g., in quadrupeds) are discussed.
