Pain-related magnetic fields evoked by intra-epidermal electrical stimulation in humans

Objectives: We recently developed a new method for the preferential stimulation of Aδ fibers in humans. The aim of the present study was to examine whether this method can serve as an appropriate stimulus in a magnetoencephalographic study. Methods: We recorded somatosensory-evoked magnetic fields (SEFs) following intra-epidermal electrical stimulation applied to the hand and elbow. Superficial parts of the skin were electrically stimulated through a needle electrode whose tip was inserted in the epidermis. Results: In all 13 subjects, the equivalent current dipole was estimated in the secondary somatosensory cortices (SII). In 5 out of 13 subjects, simultaneous activation of the primary somatosensory cortex (SI) in the hemisphere contralateral to the stimulation was identified. The mean peak latencies of magnetic fields corresponding to contralateral SI, SII and ipsilateral SII activation following hand stimulation were 162, 158 and 171 ms, respectively. The respective latency following elbow stimulation was 137, 139 and 157 ms, respectively. Estimated peripheral conduction velocity was 15.6 m/s. Conclusions: All the results were consistent with previous findings in pain SEF studies. We concluded that our novel intra-epidermal electrical stimulation is useful for pain SEF studies since it does not need special equipment and is easy to control.