Laser-induced somatosensory evoked potentials: evidence of photosensitivity in peripheral nerves.

Irradiation of the skin overlying the median nerve at the wrist in humans with a low power (1 mW; 632.5 nm) helium-neon laser produced a somatosensory evoked potential obtained at Erb's point. This evoked potential had a latency equal to that observed after electrical stimulation of the same nerve. Prolonged exposure to laser (20 min, 3.1 Hz) resulted in a large (10-90%) decrease in the amplitude of the electrical evoked potential. Since this laser produces no detectable thermal change, the results imply that photochemical reactions alter neuronal activity.