Significance of shape and size of the stimulating coil in magnetic stimulation of the human motor cortex.

Three different stimulating coil designs were evaluated for magnetic motor cortex stimulation by comparing threshold stimulus intensities at different sites over the scalp for exciting upper and lower limb muscles. Little difference was found between stimulation characteristics of two circular coils of different size, the smaller coil delivering slightly more focal stimuli. A twin coil composed of two single circular coils in series arranged side-by-side, produced significantly more powerful and more focal stimuli. It proved to be superior for exciting the leg muscles, in that less output energy was needed. For all coils, the orientation of the inducing current over the presumed motor area was the most critical stimulation parameter, and a sagittal current in anterior direction or coronal towards the stimulated hemisphere was optimal for the upper limb or lower limb muscles, respectively.