Comparison between intra-epidermal electrical and laser stimulation using direct intracranial recording of evoked potentials

Abstract

It has long been argued that the intra-epidermal electrical stimulation (IES) can stimulate A-delta fibres. The IES directly stimulates free nerve endings, therefore bypassing the temperature conduction delay of around 20 ms needed for laser-induced activation. However, there is no direct evidence to prove its selectivity to A-delta; this hampers its clinical application in comparison to the laser stimulation, which is currently the gold standard in this field. Here, we performed direct intracranial recordings in response to both IES (10 pulses at 49 Hz; 0.2–0.3 mA) and laser stimulation (Nd:YAP; 80–100 mJ/mm 2 ). Seven (7) recording contacts were located in the posterior insula, 4 in the pericentral insula (posterior short gyrus 2, anterior long gyrus 1, central sulcus 1), 5 in the orbitofrontal cortex, 4 in the amygdala, 3 in the prefrontal cortex, and 2 in the perigenual anterior cingulate. Mean difference in peak latencies was 15.3 ms, consistent with peripheral heat transduction time. The amplitude of local insular responses was not significantly different between laser and intra-epidermal stimulation (97.7 ± 45.0 and 77.6 ± 33.0 μV, respectively; P = 0.24). However, when looking outside the insula, the magnitude of responses was significantly smaller for the IES-evoked potentials in the amygdala, perigenual cingulate and prefrontal/orbitofrontal cortices. We conclude that IES can effectively activate the A-delta system at non-nociceptive levels, and induce sizeable sensory insular responses; conversely, IES barely activates limbic, cingulate and prefrontal structures contributing to the cognitive and arousal value of the stimulus.