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 pe riod evoked by contralateral cortical stimulations. Ipsilateral TMS pr eceded the contralateral magnetic or electrical cortex stimulation by 0-50 ms. In all subjects, the duration of the silent period was decrea sed in interstimulus intervals of 20-30 ms when using magnetic ipsi- a nd contralateral stimuli. No change in the silent period was seen with ipsilateral magnetic and contralateral electrical stimulations. Decre ases of motor evoked potential amplitudes were an inconsistant phenome non. 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 e xcitation and inhibition may be mediated by different neuronal circuit s.