Analysis of muscle responses elicited by transcranial stimulation of the cortico-spinal system in man.

Anodal stimulation of the central area of the scalp can produce visible twitches of various contralateral muscles. In a number of recent reports the technique of transcranial motor stimulation in man has been used to demonstrate abnormal evoked muscle responses in various diseases affecting descending motor pathways. The present paper deals with the systematic study of physiological influences and wave forms of these motor responses in normal subjects. It is demonstrated that they depend strongly on the stimulus strength, the placement of stimulating electrodes and the amount of background voluntary innervation. The presented normal values obtained under standardized conditions may serve as a basis for the decision on whether responses in an individual subject are abnormal or not.