Transcranial stimulation of the leg area of the motor cortex in humans

We used transcranial magnetic stimulation on nine normal volunteers to establish an effective way to stimulate the leg area of the motor cortex. Three types of coils: a large figure-eight coil, small figure-eight coil, and a round coil were used. Surface electromyographic activities were recorded from the left tibialis anterior muscle, and the latencies and amplitudes compared with those obtained by anodal electrical stimulation. The most stable responses were obtained when the large figure-eight coil was centered over the vertex and backward current was run through it or when the round coil was centered two to three centimeters anterior to the vertex with left-flowing current in it at the posterior widening. The latencies obtained under these stimulation conditions were the same as those obtained by electrical stimulation. We conclude that direct activation of the pyramidal cells occurs in the leg area of the motor cortex in all forms of magnetic and electrical stimulation.