Comparison of descending volleys evoked by monophasic and biphasic magnetic stimulation of the motor cortex in conscious humans

The descending spinal volleys evoked by monophasic and biphasic magnetic st imulation of the motor cortex were recorded from a bipolar electrode insert ed into the cervical epidural space of four conscious human subjects. The r esults suggest that both phases of the biphasic pulse are capable of activa ting descending motor output. The pattern of recruitment of descending acti vity depends on the intensity of the stimulus and the relative threshold of each volley to each direction of current flow.