Synchronisation of hand-foot coupled movements does not need interlimb feedback interactions

It has been prospected that synchrony of coupled limb oscillations may be controlled by mutual kinaesthetic feedback between the limbs [1, 5, 3]. Recent analysis of agonist-antagonist alternation in different mechanical contexts [2] led to hypothesise that oscillatory movements of each limb may be controlled by a neural mechanism that compares the intended position (encoded by a ‘central command’) with the actual position (encoded by the kinaesthetic afferences) and corrects for position errors. By this ‘private’ position controller, each limb may continuously adjust the movement to the central motor command, overcoming the mechanical contingencies. Through this mechanism, synchrony of coupled movements may be accomplished by independent linkage of each limb to a common clock signal, without need of an interlimb afferent feedback. To test this hypothesis, subjects were asked to rhythmically oscillate their hand (prone) and foot on the parasagittal plane, either alone or together (iso- and antidirectionally coupled), taking care of carefully synchronising the oscillation peaks to the metronome. Movement position error was expressed as the phase-lag between the metronome beat (intended oscillation peak) and the actual oscillation peak. Oscillations were performed at different frequencies (0.4-3.0Hz), in unloaded conditions and after inertial loading of either extremity. Wrist and ankle angular position and EMG from the respective flexors and extensors were recorded. With the limbs uncoupled and unloaded, the peak of both the hand and foot oscillations maintained a slight constant phase-delay with respect to the metronome over all frequencies (mean: hand 13.2°; foot -4.7°). This constancy was obtained by phase-advancing, at each frequency increment, the EMG activation with respect to the clock by the amount necessary to compensate for the simultaneous increase of the EMG-movement phase-lag, caused by limb mechanical impedance. If the EMG-movement phase-lag was further increased by loading the moving limb, the phase-advance of EMG activation, albeit increased, became insufficient for a complete compensation and the movement progressively phase-lagged the metronome. When limb oscillations were iso- or antidirectionally coupled, either in loaded or unloaded condition, all delays remained unchanged with respect to values measured when each limb was moved separately, showing that the synchrony control of either limb was uninfluenced by the simultaneous movement of the other limb. Thus, during both iso- and antidirectional hand-foot oscillations, each limb seems synchronised to a common clocking process [4] by a ‘private’ position control, with no need for a crossed feedback interaction between limbs. References 1. Baldissera, F, Cavallari, P, Marini, G, Tassone, G (1991). Differential control of in-phase and anti-phase coupling of rhythmic movements of ipsilateral hand and foot. Exp Brain Res, 83, 375-380. 2. Esposti, R, Cavallari, P, Baldissera, F (2005). Partition of voluntary command to antagonist muscles during cyclic flexion-extension of the hand. Exp Brain Res, 162, 436-448. 3. Ridderikhoff, A, Peper, CE, Beek, PJ (2005). Unravelling interlimb interactions underlying bimanual coordination. J Neurophysiol, 94, 3115-3125. 4. Turvey, MT, Schmidt, RC, Rosenblum, LD, Kugler, PN (1986). Fluctuations and phase symmetry in coordinated rhythmic movements. J Exp Psychol Hum Percept Perform, 12, 564–583. 5. Verschueren, SM, Swinnen, SP, Cordo, PJ, Dounskaia, NV (1999). Proprioceptive control of multijoint movement: bimanual circle drawing. Exp Brain Res, 127, 182-192.