DISTINGUISHING BETWEEN THE EFFECTS OF FREQUENCY AND AMPLITUDE ON INTERLIMB COUPLING IN TAPPING A 2 3 POLYRHYTHM/

Rhythmic interlimb coordination is characterized by attraction to stab le phase and frequency relations. Sudden, unintended transitions betwe en such coordination patterns have been observed in iso-and multifrequ ency tasks when movement frequency was gradually increased. These tran sitions have been accounted for by modeling the two limbs as nonlinear ly coupled oscillators. The prevailing form of the coupling function i s based on time derivatives, but an alternative formulation can be der ived by incorporating time delays. These time delays may be related to the neurophysiological delays associated with the use of kinesthetic afferences. The two ways of deriving coupling functions for interlimb coordination allow for different predictions with respect to the effec ts of movement frequency and amplitude on the strength of interaction between the limbs. In the current experiment, the effects of amplitude and frequency were dissociated experimentally, so as to arrive at an empirically motivated choice between the two ways of formalizing inter limb coupling. Subjects tapped the polyrhythm 2:3 at five different fr equencies under three amplitude conditions. Whereas no effects of ampl itude were observed, the strength of interaction between the hands dec reased with increasing movement frequency. These results support the t ime-delay version of the model, in which differential (loss of) stabil ity of coordination modes results from differential dependence on move ment amplitude, but overall coupling strength is related reciprocally to movement frequency squared. This version of the model was related t entatively to three proposed aspects of interlimb coordination: (1) ne urophysiological delays associated with the use of kinesthetic afferen ces; (2) rate-dependent decrease pattern stability; and (3) differenti al entrainment influences of kinesthetic signals.