Electrical-induced pain diminishes somatosensory evoked magnetic cortical fields

Objective: To investigate the effect of conditioning painful stimulation on the early somatosensory magnetic fields (SEF) of test stimulation, in order to clarify the location of the gating effect of pain on tactile response. Methods: We used a conditioning stimulus (CS) and test stimulus (TS) paradigm. The CS was applied at the left index finger followed by the TS at the left median nerve. The interstimulus interval between the CS and TS was varied from 100 to 1000 ms. There were two sessions corresponding to two intensities of the CS, painful CS (PCS) and non-painful CS (NPCS). Early components of SEF recorded 20 (IM) and 30 ms (2M) following the TS and the components obtained 20 (1m) and 30 ms (2m) following the CS were analyzed. Each value was compared between the two sessions. Results: PCS and NPCS attenuated the response of the 2M but not the 1M. The effect of PCS was significantly stronger and lasted longer than that of NPCS. The 1m and 2m components did not differ between PCS and NPCS in terms of amplitude and latency. Conclusions: Our data indicated that the early components of the median nerve SEF were affected by a preceding painful stimulation much more than a non-painful stimulus given on the median nerve, and that the sensory gating effect of a painful stimulation on tactile sensation lasted longer than that of a non-painful stimulation. Furthermore, our findings suggested the existence of a 'touch gate' (effect of pain on tactile sensation) at the level of the thalamus or primary somatosensory cortex (SI). Significance: The finding suggested that the touch gate might lie in the thalamus or SI. (C) 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.