Interhemispheric facilitation of the hand motor area in humans

1. We investigated interhemispheric interactions between the human hand mot or areas using transcranial cortical magnetic and electrical stimulation. 2. A magnetic test stimulus was applied over the motor cortex contralateral to the recorded muscle (test motor cortex), and an electrical or magnetic conditioning stimulus was applied over the ipsilateral hemisphere (conditio ning motor cortex). We investigated the effects of the conditioning stimulu s on responses to the test stimulus. 3. Two effects were elicited at different interstimulus intervals (ISIs): e arly facilitation (ISI = 4-5 ms) and late inhibition (ISI greater than or e qual to 11 ms). 4. The early facilitation was evoked by a magnetic or anodal electrical con ditioning stimulus over the motor point in the conditioning hemisphere, whi ch suggests that the conditioning stimulus for early facilitation directly activates corticospinal neurones. 5. The ISIs for early facilitation taken together with the time required fo r activation of corticospinal neurones by I3-waves in the test hemisphere a re compatible with the interhemispheric conduction time through the corpus callosum. Early facilitation was observed in responses to I3-waves, but not in responses to D-waves nor to I1-waves. Based on these results, we conclu de that early facilitation is mediated through the corpus callosum. 6. If the magnetic conditioning stimulus induced posteriorly directed curre nts, or if an anodal electrical conditioning stimulus was applied over a po int 2 cm anterior to the motor point, then we observed late inhibition with no early facilitation. 7. Late inhibition was evoked in responses to both I1- and I3-waves, but wa s not evoked in responses to D-waves. The stronger the conditioning stimulu s was, the greater wits the amount of inhibition. These results are compati ble with surround inhibition at the motor cortex.