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Abstract:
Transcranial magnetic stimulation (TMS) of the brain with a figure-of-eight coil causes a focal stimulation of the cortical surface. However, the exact stimulation site with respect to the coil is not yet clear. In an isolated peripheral nerve preparation not the electric field maximum in the center of the coil causes the excitation but rather the negative spatial derivative some millimeters besides the maximum (Maccabee 93). In the cortex, on the other hand, it has been argued that the field maximum itself causes the depolarization due to bend in the axons (Amassian 92). The visual and motor cortex in three subjects was mapped with monophasic pulses and two current directions. Coil positions, i.e. midpoint at the junction of the two coil windings relative to the head were monitored online and registered with a custom-made stereotactic measuring device with a precision of ± 1 mm. Subjects reported the site and size of phosphenes in the visual field that were evoked by stimulating the occipital lobes (horizonal current directions). A phosphene map from different stimulation sites was measured with a highly reproducible vertical region above the inion where subjects reported phosphenes in the left and right visual field. Reversing the current of the coil shifted the zone of bilateral phosphenes horizontally by about 10 mm. In the motor cortex a similar shift was found in a center-of-gravity map determined for a small hand muscle (current directions perpendicular to the central sulcus). The data show that the focus of depolarization in the cortex, like that in the peripheral nerve, is not under the midpoint of the coil but shifted towards the negative spatial derivative of the induced electric field.
