Cerebral midline structures in bimanual coordination

In six healthy right-handed volunteers, we compared the cerebral activation pattern related to unimanual right- and left-hand movements and to bimanua l in-phase and anti-phase movements using functional magnetic resonance ima ging (fMRI). Internally paced unimanual finger-to-thumb opposition movement s led to a strong contralateral activation of primary sensorimotor areas in all six subjects. Midline activity was lateralized to the left side during right-hand movements, but to both sides during left-hand movements. Activi ty patterns of bimanual in-phase movements resembled the combined activity patterns of the two unimanual conditions: right and left hemispheric activa tions of the primary sensorimotor cortices and predominantly left-sided med ial frontal activity. In contrast, during anti-phase movements, we observed a clear increase in activity, in both right and left frontal midline areas and in right hemispheric, mainly dorsolateral premotor areas compared to i n-phase movements. These results indicate that frontal midline activity is not specific for bimanual movements per se. It can already be involved duri ng simple unimanual movements but becomes progressively more involved durin g more complex aspects of movement control.