Nociceptive processing in the human brain of infrequent task-relevant and task-irrelevant noxious stimuli. A study with event-related potentials evoked by CO2 laser radiant heat stimuli.

Laser evoked potentials (LEPs) are nociceptive-related brain responses to activation of cutaneous nociceptors by laser radiant heat stimuli. We previously showed that LEP amplitude during the P2 period (approximately 400 ms) was increased by rare noxious stimuli, inside and outside the focus of spatial attention. It was postulated that this effect reflected a P3a response indexing an involuntary shift of attention. In the present study, LEPs were recorded in a three-stimulus oddball paradigm, commonly used to evoke P3a (or novelty-P3). CO(2) laser-induced noxious stimuli were delivered on one hand (80%, frequent). Two series of rare stronger-intensity deviant stimuli were randomly intermixed: target stimuli (10%) were delivered on the same hand while distractor stimuli (10%) were delivered on the other hand. Subjects were instructed to count targets. During an additional session, strong stimuli were delivered alone on one hand without instruction (100%, no-task stimuli). All stimulus types evoked a first positivity around 360 ms (P360). Targets and distractors elicited a late positive complex (LPC) around 465-500 ms. Topography of LPC to distractors was central and significantly more anterior than that of LPC to targets. Distractor LPC corresponds to P3a (or novelty-P3) indexing an involuntary orientation of attention toward an unexpected new/deviant event. It suggests that at least an early part of the LEP positivity (P360) is independent of P3-activities.