Laser evoked potential amplitude and laser-pain rating reduction during high-frequency non-noxious somatosensory stimulation

Objective: To investigate the mechanism subtending the analgesic effect of high frequency non-painful somatosensory stimulation. Methods: Laser evoked potentials (LEPs) and laser-pain rating were obtained from healthy subjects to stimulation of different parts of the body. LEPs were recorded at baseline and during non-painful electrical stimulation of the superficial branch of the right radial nerve (RRES). Results: RRES reduced N2/P2 LEP amplitude to right radial (F_(8,10) = 82.4, p < 0.001), left radial (F_(8,10) = 22.2, p < 0.001), and right ulnar (F_(8,10) = 7.2, p = 0.008) stimulation, while the N2/P2 amplitude to left ulnar territory stimulation remained unchanged (F_(8,10) = 3.6, p = 0.07). The laser-pain rating was reduced by RRES to bilateral radial territory stimulation (p < 0.05). In a control experiment, laser-pain rating and LEPs to left foot stimulation were not modified by RRES (p > 0.05). Conclusions: Our study confirms that the non-nociceptive afferents dampen the nociceptive input. The spatial pattern of this interaction suggests that, when conditioning higher frequency non-painful stimulation is used, the inhibition takes place at the spinal cord. Significance: Our experimental design reproduces what happens when non-painful somatosensory stimuli are used to reduce pain, such as rubbing a wound or during transcutaneous electrical nerve stimulation. Therefore, in these situations the analgesia is likely to occur at the spinal cord level.