Anodal and cathodal stimulation of the upper-limb area of the human motor cortex

In 18 neurologically normal subjects the corticofugal volleys evoked by anodal and cathodal electrical stimulation of the motor cortex were recorded using epidural electrodes at the high-thoracic and low-thoracic regions of the spinal cord during surgery for scoliosis. At and just above threshold, anodal and cathodal stimulation of the upper-limb area and motor cortex produced a D wave that propagated to the low-thoracic region. The stimulus intensity required to produce D waves was significantly lower with anodal stimulation. I waves were recorded at higher stimulus intensities than the D wave but not more readily with cathodal stimulation. There was no significant difference in the extent to which stimulus intensity had to be increased above D-wave threshold to produce I waves with the two stimulus polarities, and the number of I waves was the same when the stimulus was increased by the same amount above D-wave threshold. After withdrawal of isoflurane, I waves could not be recorded when the stimulus intensity was below D-wave threshold with either stimulus polarity. Anodal stimulation over the upper-limb area remained more effective than cathodal stimulation in producing both D and I waves. These results indicate that, at threshold, regardless of anaesthesia, anodal and cathodal stimuli access upper-limb corticospinal neurons directly at a similar site, the anodal stimuli being more effective. In addition, the results suggest that some corticospinal neurons in the upper-limb area of motor cortex have projections to lumbar segments.