Are laser-evoked brain potentials modulated by attending to first or second pain?

By co-activating Aδ- and C-nociceptors, a brief and intense infrared laser stimulus may produce a double sensation referred to as 'first' and 'second' pain, evoke late laser-evoked brain potentials (LEPs) related to the processing of Aδ-fiber input, but fails to evoke any consistent activity related to C-fibers. Yet, ultra-late LEPs may be recorded if C-nociceptors are activated in isolation. Bromm and Treede (1985) reported that if a subject selectively focused his attention towards second pain, co-activation of Aδ- and C-nociceptors could elicit both late and ultra-late LEPs (Bromm B, Treede RD. Electroencephalogr Clin Neurophysiol 1985;61). They hypothesized that for C-fiber input to elicit consistent ultra-late LEPs, attention must be specifically focused towards that sensory channel. However, the significance of this study was limited by the fact it relied on results of a single subject and used latency-correction algorithms. In an attempt to replicate these findings, this study examined LEPs recorded while trained subjects focused their attention either towards first pain or towards second pain. Whether or not subjects attended second pain, laser stimuli co-activating Aδ- and C-nociceptors failed to elicit ultra-late LEPs, indicating that focusing attention towards C-fiber sensory input is not a sufficient condition for that input to evoke LEPs. However, selectively attending to first or second pain significantly modulated Aδ-fiber related late LEPs. When subjects attended first pain, a late parietal P3 component was recorded, possibly related to task closure. When subjects attended second pain, a prefrontal positive enhancement of the P2 component was observed. Whether it could reflect brain processes related to response-inhibition is discussed.