Task-dependent intermanual coupling of 8-Hz discontinuities during slow finger movements.

During slow finger movements, small discontinuities are visible at approximately 8-10 Hz. We have recorded from eight normal subjects whilst they performed index finger flexion-extension movements with the left and right hand. Movements were either performed in-phase (both fingers flexing or extending together), or anti-phase (flexion on one side coinciding with extension on the other). Coherence calculated between left and right finger velocity was significantly above zero at approximately 8 Hz for the in-phase condition, but was significantly smaller for anti-phase movements (mean coherence across all subjects at 8.7 Hz was 0.031 for in-phase, 0.010 for anti-phase). We also calculated a 'phase coherence' measure which, unlike conventional coherence analysis, was sensitive only to phase synchronization and not to amplitude co-variations. For the in-phase task, phase coherence values were smaller than coherence, but still significantly different from zero around 8 Hz; for the anti-phase task, phase coherence was not significant in this band. Measures calculated from EMG recordings yielded similar conclusions to those using finger velocity, indicating that the results were not simply due to mechanical cross-talk. Neural oscillators generating approximately 8-Hz movement discontinuities on each side of the body are therefore selectively coupled during the in-phase task. A wavelet-based analysis further suggested that intermanual coupling modulated during in-phase task performance; coupling was maximal at the start and the end of a movement. We conclude that the systems producing approximately 8-Hz movement discontinuities and those responsible for intermanual coupling are likely to share common neural elements.