Exogenous and endogenous components of ultralate (C-fibre) evoked potentials following CO2 laser stimuli to tiny skin surface areas in healthy subjects

CO2 laser stimulation of tiny skin surface areas on the hand dorsum generate ultralate laser evoked potentials (LEPs) with a major positivity maximal at the vertex at a latency of about 1 s. These potentials follow selective and direct activation of C-fibres in the superficial layers of the skin. To identify the endogenous P3 component in ultralate LEPs, we used a 2-stimulus oddball paradigm (20% probability of targets). Ultralate LEPs were recorded in eight healthy subjects with 19 channels EEG and EOG. Laser stimuli (n = 200/subject, < 0.5 mm diameter, 5 ms duration, 9.4 mJ/mm2 energy density) were applied in a pseudo-random order to the proximal phalanx of either middle finger (target) or index (non-target) of the left hand. Vigilance was maintained during recording sessions. Subject's task was to press a button to any kind of sensation perceived at the target zone. We observed a negative-positive (N965-P1139) complex maximal at Cz for all stimulus conditions. This complex was of significantly larger amplitude for rare events. For the target responses, a second distinct positive peak was observed with a mean latency of 1343 +/- 103.5 ms with maximum amplitude at Pz. This latest peak may represent an endogenous P3-like component as it is linked to the subject's detection of the target stimulus (rare events).