The effect on corticospinal volleys of reversing the direction of current induced in the motor cortex by transcranial magnetic stimulation

Descending corticospinal volleys were recorded from a bipolar electrode ins erted into the cervical epidermal space of four conscious human subjects af ter monophasic transcranial magnetic stimulation over the motor cortex with a figure-of-eight coil. We examined the effect of reversing the direction of the induced current in the brain from the usual posterior-anterior (PA) direction to an anterior-posterior (AP) direction. The volleys were compare d with D waves evoked by anodal electrical stimulation (two subjects) or me dio-lateral magnetic stimulation (two subjects). As reported previously, PA stimulation preferentially recruited Il waves, with later I waves appearin g at higher stimulus intensities. AP stimulation tended to recruit later I waves (13 waves) in one of the subjects, but, in the other three, Il or D w aves were seen. Unexpectedly, the descending volleys evoked by AP stimulati on often had slightly different peak latencies and/or longer duration than those seen after PA stimulation. In addition the relationship between the s ize of the descending volleys and the subse quent EMG response was often di fferent for AP and PA stimulation. These findings suggest that AP stimulati on does not simply activate a subset of the sites activated by PA stimulati on. Some sites or neurones that are relatively inaccessible to PA stimulati on may be the low-threshold targets of AP stimulation.