Cortical representation of different motor rhythms during bimanual movements.

The cortical control of bimanual and unimanual movements involves complex facilitatory and inhibitory interhemispheric interactions. We analysed the part of the cortical network directly related to the motor output by corticomuscular (64 channel EEG-EMG) and cortico-cortical (EEG-EEG) coherence and delays at the frequency of a voluntarily maintained unimanual and bimanual rhythm and in the 15-30-Hz band during isometric contractions. Voluntary rhythms of each hand showed coherence with lateral cortical areas in both hemispheres and occasionally in the frontal midline region (60-80 % of the recordings and 10-30 %, respectively). They were always coherent between both hands, and this coherence was positively correlated with the interhemispheric coherence (p < 0.01). Unilateral movements were represented mainly in the contralateral cortex (60-80 vs. 10-30 % ipsilateral, p < 0.01). Ipsilateral coherence was more common in left-hand movements, paralleled by more left-right muscle coherence. Partial corticomuscular coherence most often disappeared (p < 0.05) when the contralateral cortex was the predictor, indicating a mainly indirect connection of ipsilateral/frontomesial representations with the muscle via contralateral cortex. Interhemispheric delays had a bimodal distribution (1-10 and 15-30 ms) indicating direct and subcortical routes. Corticomuscular delays (mainly 12-25 ms) indicated fast corticospinal projections and musculocortical feedback. The 15-30-Hz corticomuscular coherence during isometric contractions (60-70 % of recordings) was strictly contralaterally represented without any peripheral left-right coherence. Thus, bilateral cortical areas generate voluntary unimanual and bimanual rhythmic movements. Interhemispheric interactions as detected by EEG-EEG coherence contribute to bimanual synchronization. This is distinct from the unilateral cortical representation of the 15-30-Hz motor rhythm during isometric movements.