Transcranial electric stimulation of motor pathways: a theoretical analysis.

The response to transcranial electrical stimulation of the brain is an important means of assessing motor pathways in the anesthetized patient. The purposes of this study were to elucidate the pattern of axonal excitation produced by transcranial stimulation and to demonstrate how this pattern is affected by changes in the conductivity or geometry of the skull-CSF-brain complex. To this end, analytic solutions to the problem of electrodes placed on a three shell spherical model were obtained under constant current conditions. The potentials, currents and fields generated were computed and the "activating function" was computed for an idealized set of radially organized axons in order to estimate the degree of membrane depolarization produced by stimulation. The degree to which electromagnetic/radiation effects change these solutions was also estimated. The pattern of stimulation was only slightly dependent on the conductivity and the thickness of the CSF layer. Axons very close to the anode were stimulated with lowest threshold at the brain surface. Axons further away were stimulated with higher thresholds and the point of maximum stimulation moved nearer the center of the sphere. Near the cathode, stimulation was maximal about 5-7 degrees away from the edge of the electrode but the peak magnitude of the activating function was generally 20 times lower than over the anode.