Aim: 

Brief radiant laser pulses activate selectively A? and C skin nociceptors and elicit a number of transient brain responses (laser-evoked potentials, LEPs) in the electroencephalogram. It has been recently showed that LEPs do not reflect nociceptive-specific cortical processing, but the activity of a non nociceptive-specific network related to attentional reorientation towards salient somatosensory events (Legrain et al., 2009; Mouraux & Iannetti, 2009). Here we aim to dissect the specific effect of spatial expectancy on LEPs.

Methods: 

We recorded LEPs elicited by triplets of identical laser pulses (S1-S2-S3) in 10 healthy participants. Stimuli were delivered to the hand dorsum at a constant inter stimulus interval of 1 s. While S1 and S2 were always delivered to the same hand, S3 was delivered either on the same hand (condition 'same') or on the other hand (condition 'opposite'). Participants were either informed (condition 'expected') or not informed (condition 'unexpected') of the spatial location of S3.

Results: 

Consistent with previous results (Iannetti et al 2008), both the N2 and the P2 waves of S2-LEP were reduced as compared to S1-LEP. While the N2 of S3-LEP was unchanged compared to S2-LEP, the P2 of S3-LEP was significantly larger in the 'opposite' condition, both when the stimulus was 'expected' (+120.5%, p=0.0067) and 'unexpected' (+143.3%, p=0.0163).

Conclusion: 

Taking into account previous evidence ruling out neural refractoriness as an explanation for LEP habituation (Mouraux et al., 2004), the observed LEP dishabituation indicates that the magnitude of LEP reflects the increase in stimulus saliency determined by changes of the spatial location of the stimulus.