Electrophysiological correlates of attentional orientation in humans to strong intensity deviant nociceptive stimuli, inside and outside the focus of spatial attention.

Laser evoked potentials (LEPs) are electrical brain responses to nociceptive heat stimuli. In a recent study [Legrain, V., Guérit, J.M., Bruyer, R. and Plaghki, L., Pain, 99 (2002) 21-39.], we found that amplitude at approximately 400 ms was increased by rare intensity deviant nociceptive stimuli (P400 effect). In that study, laser stimuli were randomly delivered on both hands, and subjects were focusing attention on one hand in order to detect rare stimuli. As the P400 effect was found for rare stimuli when spatial attention was directed both towards and away from the stimulated hand, it was postulated to represent a P3a component reflecting an involuntary orientation of attention to unexpected deviant stimuli. However LEPs to strong and weak intensity stimuli were averaged together and some effects could have been underestimated. So, we present a new interpretation of the P400 effect based on separate analyses of strong and weak intensity deviant stimuli. Indeed, the P400 effect was only observed for strong stimuli, and again on both attended and unattended hands. Thus, if the P400 effect reflects P3a, only strong deviant stimuli provided enough signals to induce attentional switching even when they were delivered outside the focus of spatial attention. It is suggested that attentional switching could have been triggered by neural systems having detected sharp increase of intensity. Weak deviant stimuli were not salient enough to induce attentional switching.