• Valéry Legrain, Raymond Bruyer, Jean-Michel Guérit, and Léon Plaghki.
• Unité de Neurosciences Cognitives (NESC), Faculté de Psychologie et des Sciences de l'Education, Université catholique de Louvain, Louvain-la-Neuve, Belgium. Valery.Legrain@psp.ucl.ac.be
• Clin Neurophysiol. 2005 Sep 1; 116 (9): 2165-74.

ObjectiveRecent laser evoked potential (LEP) studies showed that unattended rare intensity-deviant nociceptive stimuli enhance the LEP vertex positivity P2 ('P400 effect'). It was hypothesized to reflect an involuntary switch of attention to nociceptive events. If true the P400 effect (1) should be produced when attention is focused on a task in another sensory modality (primary task), and (2) should be modulated by the primary task difficulty.MethodsSubjects had to count the number of visual symbols presented on a screen. In a difficult condition, symbols were digits 1-4 (interference between amount and meaning). In an easy condition, symbols were letters X (no interference). Nociceptive CO2 laser stimuli were simultaneously delivered on the left hand. Occasional stronger deviant stimuli (16%) were presented at random. In additional sessions, the strong stimuli were presented alone in homogenous series (100%).ResultsLEP amplitude at about 400 ms was larger for rare deviant than for homogenous stimuli. Visual task difficulty decreased LEP amplitude at this latency. Deviant stimuli seemed also to interfere with performance in the visual task.ConclusionsThe results give evidence for considering the P400 effect as reflecting an involuntary attentional shift to nociceptive events.SignificanceThe study provides electrophysiological evidences for an intrusive capacity of pain to attract attention and to decrease behavioural performance in concurrent processes. In turn, such an attentional shift is tampered if attention is very engaged in a concomitant task.

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