O1 Comparison between intra-epidermal electrical and laser stimulations: An intracranial evoked potential study

Objectives Intra-epidermal electrical stimulation (IES) is considered to stimulate selectively and directly A δ fibres, by-passing the heat-mediated receptor process of laser stimulation. However, there has been only extra-cranial data to argue such fiber selectivity. Here, we recorded intracranial responses to IE and laser stimulations, to assess whether the two types of stimulation can demonstrate identical responses in precisely-localized brain regions. Methods We analysed intracranial evoked potentials to IE and laser (Nd:YAP) stimulations in 11 epileptic patients with stereotactically-implanted electrodes in the spinothalamic-receiving posterior insula (12 contacts) and affective-cognitive regions (amygdala, 8 contacts; anteromedial frontal region, 13 contacts). Results While both IE and laser stimuli were perceived as ‘pricking’ sensations, those elicited by IES were of lesser intensity and intrusiveness. In the posterior insula, IES-evoked and laser-evoked potentials were similar in waveform and amplitude. Peak-latencies were significantly shorter for IES than for laser, the mean latency difference being of 15.8 ms. In both the amygdala and anteromedial frontal regions, amplitudes were significantly attenuated for IES, as compared to laser. Discussion The latency difference in the posterior insula was concordant with the heat-transduction time of laser, suggesting the ability of IES to activate selectively the spinothalamic system. The depressed activations in the amygdala and anteromedial frontal regions, while preserved in the posterior sensory insula, were compatible with modest attentional-affective drive for IES. Conclusions IES can effectively activate the A δ -mediated sensory-encoding system (posterior insula) at non-nociceptive levels of stimulus intensity. Significance IES can minimize aversive reactions of patients while ensuring spinothalamic-specific sensory assessment.