Changes of inhibitory interneurons during transcallosal stimulations.

The present study was performed in order to determine the influence of ipsilateral transcranial magnetic stimulations (TMS) on the silent period evoked by contralateral cortical stimulations. Ipsilateral TMS preceded the contralateral magnetic or electrical cortex stimulation by 0-50 ms. In all subjects, the duration of the silent period was decreased in interstimulus intervals of 20-30 ms when using magnetic ipsi- and contralateral stimuli. No change in the silent period was seen with ipsilateral magnetic and contralateral electrical stimulations. Decreases of motor evoked potential amplitudes were an inconsistent phenomenon. The results indicate that ipsilateral TMS in activate inhibitory cortical interneurons, probably via transcallosal pathways. Different time courses and different degrees of inhibition indicate that motor excitation and inhibition may be mediated by different neuronal circuits.