N1 and P2 of frequent and rare event-related potentials show effects and after-effects of the attended target in the oddball-paradigm.

The effects of serial order of the stimuli on event-related potentials (ERPs) in an auditory oddball paradigm with 700 ms ISI were studied in 19 normals, recording from Fz, Cz, Pz and combined ears. N1 and P2 to the last preceding frequent stimulus, the rare (attended target) stimulus, and the following two frequent stimuli were evaluated using 6 reference-independent measures: latency (time of maximal potential range between any two locations), amplitude of maximal potential range, global field power, vertex (Cz) current source density, location of extreme potential, and location of potential centroid. Exploratory statistics were used to determine differences of interest. Eighteen of the 36 comparisons for N1, and 4 of the 36 comparisons for P2 showed double-ended P-values of less than 5%, 6 times the overall incidence expected by chance. For the ERP evoked 1400-ms post-target, global field power and location of N1, and latency and location of P2 still differed from pre-target ERP values. This suggests a new and temporarily persisting change of brain state following the target stimulus. The first 3 measures showed 'undershoot' below pre-target levels, contradicting a simple 're-habituation' model. All 6 measures of N1, as well as latency and location of P2 increased or anteriorized for the attended target; the location changes indicate that processing the attended target activates additional neural processes, and does not only increase the activation of the same neural processes which operate on frequent stimuli.