Short trains of intra-epidermal electrical stimulation to elicit reliable behavioral and electrophysiological responses to the selective activation of nociceptors in humans

Currently, the study of nociception in humans relies mainly on thermal stimulation of heat-sensitive nociceptive afferents. To circumvent some limitations of thermal stimulation, it was proposed that intra-epidermal electrical stimulation (IES) could be used as an alternative method to activate nociceptors selectively. The selectivity of IES relies on the fact that it can generate a very focal electrical current and, thereby, activate nociceptive free nerve endings located in the epidermis without concomitantly activating non-nociceptive mechanoreceptors located more deeply in the dermis. However, an important limitation of IES is that it is selective for nociceptors only when very low current intensities are used. At these intensities, the stimulus generates a very weak percept, and the signal-to-noise ratio of the elicited evoked potentials (EPs) is very low. To circumvent this limitation, it was proposed that the strength of the nociceptive afferent volley could be increased through temporal summation, using short trains of repeated IES. Here, we characterized the intensity of perception and EPs elicited by trains of 2, 3 and 4 IES delivered using a 5-ms inter-stimulus interval. We found that both the intensity of perception and the magnitude of EPs significantly increased with the number of pulses. In contrast, the latency of the elicited EPs was not affected by the number of pulses, indicating that temporal summation did not affect the type of activated fibers and, therefore, that trains of IES can be used to increase the reliability of stimulus-evoked responses while still preserving its selectivity for nociceptors.