The laser-evoked P600 may reflect 'Context Closure' rather than 'Context Updating'

Mouraux, A.1, Guérit, J. M.2, and Plaghki, L.2
1Laboratoire de neurophysiologie (NEFY), Université catholique de Louvain, Belgium; 2Unité de réadaptation (READ), Université catholique de Louvain, Belgium

Laser-evoked potentials (LEPs) are brain responses related to the activation of cutaneous A∂ nociceptors. Occurring between 120-450 ms for hand stimulation, LEPs mainly consist of a negative-positive complex (N2-P2) maximal at CZ. Peaking around 600 ms, an additional parietal positivity is often described. As its amplitude is enhanced when evoked by rare target stimuli presented in an oddball paradigm, investigators proposed that this P600 is equivalent to the P3B evoked by other modalities. The P3B is hypothesized to reflect either updating of working memory following arrival of new information (‘Context Updating’ model) or closure of information processing occurring when expectations are terminated (‘Context Closure’ model). To test these hypotheses, eleven subjects participated in two experiments. Laser stimuli were applied to the hand dorsum. In the first experiment, trials consisted of single stimuli. After each trial, subjects were asked to rate intensity of perception. In the second experiment, subjects were told that each trial would consist of two consecutive stimuli occurring with varying stimulus onset asynchrony (SOA). Instructions were to rate separately intensity of both perceptions. In four trial categories, two consecutive stimuli occurred with four different SOAs (range 280-2100 ms). In a fifth category, one single stimulus occurred. Results showed that stimuli evoked clear P600 responses only when ending the sensory detection task (i.e. single stimuli when a single stimulus was expected, second of two consecutive stimuli when two were expected). These observations give clear support to the ‘Context Closure’ model for the P600 component.