Two phases of interhemispheric inhibition between motor related cortical areas and the primary motor cortex in human.

Interhemispheric inhibition (IHI) refers to the neurophysiological mechanism in which one hemisphere of the brain inhibits the opposite hemisphere. IHI can be studied by transcranial magnetic stimulation using a conditioning-test paradigm. We investigated IHI from 5 motor related cortical areas in the right hemisphere to the left primary motor cortex (M1). These areas are hand and face representations of M1, dorsal premotor cortex, somatosensory cortex, and dorsolateral prefrontal cortex. Test stimulus was delivered to the left M1 and conditioning stimulus (CS) was delivered to one of 5 motor related cortical areas in the right hemisphere. The time course of IHI, effects of different CS intensities and current directions on IHI were tested. Maximum IHI was found at interstimulus intervals of approximately 10 ms (short latency IHI, SIHI) and approximately 50 ms (long latency IHI, LIHI) for the motor related areas tested. LIHI could be elicited over a wide range of CS intensities, whereas SIHI required higher CS intensities. We conclude that there are 2 distinct phases of IHI from motor related cortical areas to the opposite M1 through the corpus callosum, and they are mediated by different neuronal populations.