Ultra-Late Evoked Potentials Following Stimulation of Tiny Skin Surface Areas in Humans

C-fiber input to the central nervous system is considered to represent an important antecedent for the experience of secondary pain. Clinically more important, this input is suggested to play a major role in the chronification of pain. However, there are only limited possibilities to investigate this input selectively, i.e., without simultaneous activation of C-fibers. One method that allows the selective activation of cutaneous A- and C-fiber afferents in humans is the stimulation of tiny skin surface areas (about 0.23mm²). We used this method for stimulation of the dorsum of the left hand and the left foot using a thulium-YAG laser stimulator. 12 volunteers were stimulated while EEG were recorded simultaneously. Additionally, reaction times (RTs), ratings of the perceived intensity of the stimulus, and descriptions of the perceptions (e.g., warm, burning) were registered for each single stimulus. Laser-evoked brain potentials (LEPs) were calculated for shorter and longer RTs on the basis of individual distributions of RTs and interpreted as belonging to A- and C-fibers, respectively. LEPs calculated on the basis of the stimulation of A of shorter RTs showing a waveform typical for late LEPs, i.e.,, a negative peak around 220 ms and a positive peak around 320, ms both with a maximum in Cz. As expected, latencies were longer for foot as compared to hand stimulation. LEPs calculated on the basis of longer RTs show a waveform typical for ultra-late LEPs with two components irrespective from the place of stimulation. A negative component peaked around 920 ms whereas a positive component peaked around 1,120 ms. Both components had their maximum at electrode Cz. As expected, latencies were again longer for foot as compared to hand stimulation. However, a different A- and C-fiber stimulation was found for the hand vs. foot distribution of A stimulation. C-fiber responses were significantly more often observed to stimulation to the hand as compared to stimulation to the foot. In conclusion, Tm-YAG laser stimulation of tiny skin surface areas offers an opportunity to investigate central nervous responses to selective stimulation of A- and C-fibers in humans at different sites of the body. Supported by IZKF Jena.