Interhemispheric communication consists of a complex balance of facilitation and inhibition that is modulated in a task-dependent manner. However, it remains unclear how individual differences in interhemispheric interactions relate to motor performance. To assess interhemispheric inhibition, we utilized the ipsilateral silent period technique (iSP; evoked by suprathreshold transcranial magnetic stimulation), which elicits inhibition of volitional motor activity. Participants performed three force production tasks: (1) unimanual (right hand) constant force, (2) bimanual constant force, (bimanual simultaneous) and (3) bimanual with right hand constant force and left hand sine wave tracking (bimanual independent). We found that individuals with greater IHI capacity demonstrated reduced mirror EMG activity in the left hand during unimanual right hand contraction. However, these same individuals demonstrated the poorest performance during the bimanual independent force production task. We suggest that a high capacity for IHI from one motor cortex to another can effectively prevent "motor overflow" during unimanual tasks, but it can also limit interhemispheric cooperation during independently controlled bimanual tasks.
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