Cerebral blood-flow changes induced by paired-pulse transcranial magnetic stimulation of the primary motor cortex

Positron emission tomography (PET) was used to assess changes in regional c erebral blood flow (CBF) induced by paired-pulse transcranial magnetic stim ulation (TMS) of primary motor cortex (M1). The study was performed in eigh t normal volunteers using two Magstim-200 stimulators linked with a Bistim module. A circular TMS coil was held in the scanner by a mechanical arm and located over the left M1. Surface electrodes were used to record motor evo ked potentials (MEPs) from the contralateral first dorsal interosseous musc le (FDI). Cortical excitability was evaluated in the relaxed FDI using a pa ired conditioning-test stimulus paradigm with two interstimulus intervals ( ISIs): 3 and 12 ms. The subjects were scanned three times during each of th e following four conditions: 1) baseline with no TMS (BASE); 2) single-puls e TMS (TMSsing); 3) 3-ms paired-pulse TMS (TMS3); and 4) 12-ms paired-pulse TMS (TMS12). CBF and peak-to-peak MEP amplitudes were measured over each 6 0-s scanning period. To assess TMS-induced changes in CBF, a t-statistic ma p was generated by first subtracting the single-pulse TMS condition from th e 3- and 12-ms paired-pulse TMS conditions and then correlating the CBF dif ferences, respectively, with the amount of suppression and facilitation of the EMG responses. A significant positive correlation was observed between the CBF difference (TMS3-TMSsing) and the amount of suppression of EMG resp onse, as well as between the CBF difference (TMS 12-TMSsing) and the amount of facilitation of EMG response. This positive correlation was observed in the left M1, left lateral premotor cortex, and right M1 in the case of 3-m s paired-pulse TMS, but only in the left M1 in the case of 12-ms paired-pul se TMS. The above pattern of CBF response to paired-pulse TMS supports the possibility that suppression and facilitation of the EMG response are media ted by different populations of cortical interneurons.