LOCALIZING THE SITE OF MAGNETIC BRAIN-STIMULATION BY FUNCTIONAL MRI

In order to locate the site of action of transcranial magnetic stimula tion (TMS) within the human motor cortices, we investigated how the op timal positions for evoking motor responses over the scalp corresponde d to the hand and leg primary-motor areas. TMS was delivered with a fi gure-8 shaped coil over each point of a grid system constructed on the skull surface, each separated by 1 cm, to find the optimal site for o btaining motor-evoked potentials (MEPs) in the contralateral first dor sal interosseous (FDI) and tibialis anterior (TA) muscles. Magnetic re sonance imaging scans of the brain were taken for each subject with ma rkers placed over these sites, the positions of which were projected o nto the cortical region just beneath. On the other hand, cortical area s where blood flow increased during finger tapping or leg movements we re identified on functional magnetic resonance images (fMRI), which sh ould include the hand and leg primary motor areas. The optimal locatio n for eliciting MEPs in FDI, regardless of their latency, lay just abo ve the bank of the precentral gyrus, which coincided with the activate d region during finger tapping in fMRI studies. The direction of induc ed cut-rent preferentially eliciting MEPs with the shortest latency in each subject was nearly perpendicular to the course of the precentral gyrus at this position. The optimal site for evoking motor responses in TA was also located just above the activated area during leg moveme nts identified within the anterior portion of the paracentral lobule. The results suggest that, for magnetic stimulation, activation occurs in the primary hand and leg motor area (Brodmann area 4), which is clo sest in distance to the optimal scalp position for evoking motor respo nses.