Aim: 

Nociception begins when epidermal free nerve endings of Ad small-myelinated and C unmyelinated peripheral nociceptors are activated. However, cortical processing of this nociceptive input is necessary for the emergence of a conscious painful experience. To characterize the cortical activity related to the emergence of that conscious experience we recorded event-related brain potentials (ERP) elicited by identical nociceptive afferent volleys that were either perceived or unperceived.

Methods: 

Identical infrared laser pulses, which selectively activate skin nociceptors, were delivered to the left hand as a pair of rapidly succeeding stimuli. After each trial, subjects reported whether they perceived one or two distinct pinprick sensations, associated with Ad nociceptor activation. The psychophysical feedback after each pair of stimuli was used to adjust the inter-stimulus interval (ISI) of the subsequent pair: when a single sensation was perceived, ISI was increased by 40 ms; when two distinct sensations were perceived, ISI was decreased by 40 ms. This adaptive algorithm ensured that the probability of perceiving the second stimulus of the pair tended towards p = 0.5. Single stimuli (1/3 of all trials) were randomly intermixed with pairs of stimuli.

Results: 

The magnitudes of the N2 and P2 waves elicited by S2 ('S2-LEP') were significantly reduced when S2 was not perceived. Single-trial analysis revealed that this reduction of magnitude in the 'S2-LEP/not perceived' condition was consistently observed across single trials. The magnitude reduction was also reflected in a change of distribution of scalp potentials. In contrast, the magnitude of the early-latency N1 wave elicited by S2 was similar regardless of whether or not S2 was perceived.

Conclusion: 

These findings suggest that the N1 wave closely reflects the ascending nociceptive input, while the N2 and P2 waves relate to brain processes involved in the emergence of a conscious painful experience.