Stimulus methods for recording pain-related SEF

1. Introduction The cerebral representation of pain perception in humans is poorly understood compared with that of other sensations such as touch or vibration. This is mainly because of a lack of appropriate instrumentation for stimulation and recording [1]. To our knowledge, only a few systematic studies of somatosensory evoked magnetic field (SEF) following painful stimulation (pain-related SEF) have been conducted. Hari et al. [2] used painful electrical stimulation of tooth pulp. Huttunen et al. [3] and Hari et al. [4] used CO 2 gas stimulation of the nasal mucosa. Howland et al. [5] applied high-intensity painful electrical stimulation and reported that the equivalent current dipoles (ECD) were located at or near the second somatosensory cortex (SII) or the frontal operculum. We used the same method and found ECD in not only the SII but also the cingulate cortex [6-8]. A low power and long wavelength CO 2 laser beam induces sensations of pain or heat when applied to the skin. We have, therefore, recorded pain-related SEF following painful CO 2 laser stimulation to elucidate the mechanisms of pain processing in the human brain [9-12]. Bromm et al. [13] also reported SEF following painful CO 2 laser stimulation. We recently developed a new device which induced painful impact stimuli [14]. An airgun was placed outside a shielded room and small plastic bullets were fired at the arm and trunk at different intensities. The evoked pain-related SEF responses had a major component with characteristic polarity-reversal deflections, and the ECD were estimated in the SII of the bilateral hemispheres. In this paper, we describe two major methods for recording pain-related SEF, high-intensity painful electrical stimulation and painful CO 2 laser stimulation.