Comparison of descending volleys evoked by transcranial magnetic and electric stimulation in conscious humans

Objectives: The present experiments were designed to compare the understanding of the transcranial electric and magnetic stimulation of the human motorcortex.Methods: The spinal volleys evoked by single transcranial magnetic or electric stimulation over the cerebral motor cortex were recorded from a bipolar electrode inserted into the cervical epidural space of two conscious human subjects. These volleys were termed D- and I waves, according to their latency. Magnetic stimulation was performed with a figure-of-eight coil held over the right motor cortex at the optimum scalp position, in order to elicit motor responses in the contralateral FDI using two different orientations over the motor strip. The induced current flowed either in a postero-anterior or in a latero-medial direction.Results: At active motor threshold intensity, the electric anodal stimulation evoked pure D activity. At this intensity, magnetic stimulation with the induced current flowing in a posterior-anterior direction evoked pure I₁ activity. When a latero-medial induced current was used, magnetic stimulation evoked both D and I₁ activity. Using electric anodal stimulation, at a stimulus intensity of 9% of the stimulator output above the active motor threshold (corresponding approximately to 1.5 active motor threshold), a small I₁ wave appeared only in subject 1. Using magnetic stimulation with a posterior-anterior induced current, at a stimulus intensity of 21% of maximum stimulator output above the active motor threshold (corresponding approximately to 1.8 times threshold in subject 1 and to two times threshold in subject 2), a small D wave appeared in subject 1 but not in subject 2.Conclusions: Present results demonstrate that, in conscious humans at threshold intensities, electric stimulation evokes D waves and magnetic stimulation (with a posterior-anterior induced current) evokes I waves, while magnetic stimulation (with a latero-medial induced current) evokes both activities.