Coil placement in magnetic brain stimulation related to skull and brain anatomy

The influence of coil position on the size of electromyographic responses evoked by transcranial magnetic brain stimulation was systematically evaluated. The position of the stimulation coil (11.6 cm outer diameter, Novametrix) was recorded by constructing individual grids on the skull surface using extracranial bony landmarks and was then related to underlying cerebral sulci by analysis of magnetic resonance images of the brain and skull. The largest responses in the right first dorsal interosseus (FDI) muscle, when using anticlockwise coil currents as viewed from above, were found, on average (5 subjects), to occur with the coil centred 2 cm behind and left of the vertex. For the right tibialis anterior (TA) muscle the largest responses were obtained, on average, with the coil centred around a point 4 cm anterior to and 4 cm left of the vertex. Between individuals, the optimal coil position for responses in a particular muscle varied up to 2 cm. This variability could be explained by the variability in the relationship between the central sulcus and the extracranial bony landmarks of each individual. Responses could be obtained whenever the windings of the coil passed over the appropriate region of the motor cortex, providing evidence that magnetically evoked responses do originate from activation of the precentral gyrus. The size of the cortically evoked responses for a given stimulus intensity depended on the direction of the coil currents passing over the motor area. For FDI muscles, the largest responses were obtained when the coil currents passed over the lateral part of the motor strip from forwards behind and transversely to the central sulcus.(ABSTRACT TRUNCATED AT 250 WORDS)