LASER-EVOKED POTENTIALS - EXOGENOUS AND ENDOGENOUS COMPONENTS

The aim of this study was to distinguish the exogenous component (rela ted to the physical properties of the stimulus) and the endogenous com ponent (reflecting event-related cognitive processing) of the laser-ev oked potential (LEP. Short painful radiant heat pulses generated by a CO2-laser were applied to the dorsum of the right and left foot. LEPs were recorded with 5 scalp electrodes in the midline versus linked ear lobes in 26 healthy subjects. In order to identify the exogenous compo nent, the LEP was recorded during a standardised distraction task (rea ding a short story). To identify the endogenous component P3 for the L EP, a 2-stimulus oddball paradigm was used (20% probability of targets ). When the task of the oddball paradigm consisted of pressing a butto n, a movement-related long-latency negativity (N1200) was recorded in frontal leads that was absent in a counting task, The LEP of targets, frequent non-targets and during distraction was dominated by a single large positivity. The amplitude of this positivity was task-dependent and increased the more attention the subject payed to the laser stimul i (distraction < neutral < non-target < target). The laser-evoked posi tivity during distraction had a peak latency of about 400 msec (P400) and a maximum amplitude at the vertex, which was independent of inter- stimulus interval. The P3 following laser stimulation had a significan tly later peak at about 570 msec (P570) and a different scalp topograp hy with a parietal maximum. Its amplitude decreased when the interstim ulus interval was reduced from 10 to 6 sec. Under neutral instructions , the LEP positivity consisted of a superposition of both the exogenou s P400 and the endogenous P570.