Exogenous and endogenous components of ultralate (C-fibre) evoked potentials following CO 2 laser stimuli to tiny skin surface areas in healthy subjects

Abstract

CO 2 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/mm 2 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).