Two right-handed and 2 left-handed participants drew circles in the horizontal plane with both hands simultaneously in either a symmetrical or an asymmetrical mode, at their preferred rate or as fast as possible. During symmetrical movements, the hands showed frequency and phase synchronization at both rates. During fast asymmetrical movements, the hands showed increased phase difference and phase variability, as well as transitions to symmetrical movements, and cases of frequency decoupling. Large distortions of the hand trajectory were also observed under fast asymmetrical movements. Trajectory distortions and movement direction reversals were confined to the nondominant hand. Under the assumption that circular trajectories are generated by properly timed orthogonal oscillations along the y-axis and the *-axis, these findings are accounted for by the characteristics of coupling between homologous functional oscillators of the 2 body sides.

Studies on drawing circles with both hands in the horizontal plane have shown that this task is easy to perform across a wide range of movement frequencies under the symmetrical mode of coordination, whereas under the asymmetrical mode (both limbs moving clockwise or counterclockwise) increases in movement frequency have a disruptive effect on trajectory control and hand coordination. To account for these interference effects, we propose a simplified computer model for bimanual circle drawing based on the assumptions that (1) circular trajectories are generated from two orthogonal oscillations coupled with a phase delay, (2) the trajectories are organized on two levels, "intention" and "motor execution", and (3) the motor systems controlling each hand are prone to neural cross-talk. The neural cross-talk consists in dispatching some fraction of any force command sent to one limb as a mirror image to the other limb. Assuming predominating coupling influences from the dominant to the nondominant limb, the simulations successfully reproduced the main characteristics of performance during asymmetrical bimanual circle drawing with increasing movement frequencies, including disruption of the circular form drawn with the nondominant hand, increasing dephasing of the hand movements, increasing variability of the phase difference, and occasional reversals of the movement direction in the nondominant limb. The implications of these results for current theories of bimanual coordination are discussed.

Spencer, Rebecca M. C., Richard B. Ivry, Daniel Cattaert, and Andras Semjen. Bimanual coordination during rhythmic movements in the absence of somatosensory feedback. J Neurophysiol 94: [2901][2902][2903][2904][2905][2906][2907][2908][2909][2910] 2005. First published July 13, 2005; doi:10.1152/jn.00363.2005. We investigated the role of somatosensory feedback during bimanual coordination by testing a bilaterally deafferented patient, a unilaterally deafferented patient, and three control participants on a repetitive bimanual circle-drawing task. Circles were drawn symmetrically or asymmetrically at varying speeds with full, partial, or no vision of the hands. Strong temporal coupling was observed between the hands at all movement rates during symmetrical drawing and at the comfortable movement rate during asymmetrical drawing in all participants. When making asymmetric movements at the comfortable and faster rates, the patients and controls exhibited similar evidence of pattern instability, including a reduction in temporal coupling and trajectory deformation. The patients differed from controls on measures of spatial coupling and variability. The amplitudes and shapes of the two circles were less similar across limbs for the patients than the controls and the circles produced by the patients tended to drift in extrinsic space across successive cycles. These results indicate that somatosensory feedback is not critical for achieving temporal coupling between the hands nor does it contribute significantly to the disruption of asymmetrical coordination at faster movement rates. However, spatial consistency and position, both within and between limbs, were disrupted in the absence of somatosensory feedback. I N T R O D U C T I O NStudies involving bimanual periodic movements have shown that two patterns of coordination, in-phase and antiphase, exhibit spontaneous stability. With respect to the sagittal plane of the body, in-phase movements are symmetric and typically involve the simultaneous activation of homologous muscles. Antiphase movements are asymmetric, with muscle activation patterns typically 180°out of phase. A fundamental observation in the motor control literature is that these two patterns are not equally stable. For in-phase movements, the variability of relative phase remains low and relatively constant across a large range of movement frequencies. In contrast, for antiphase movements, relative phase variability increases as frequency increases and, at a critical frequency, spontaneous transitions from anti-to in-phase movements are observed (reviewed in Schoener and Kelso 1988).Although the dynamics of hand coordination were originally developed for single-joint, oscillatory movements (Kelso 1984), many recent studies have used a two-dimensional bimanual circle-drawing task in which movements are made either symmetrically with one hand circling clockwise and the other, counterclockwise, or asymmetrically, with both hands circling clockwise or counterclockwise (Carson et al. 1997;Semjen et al. 1995)....

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