"Initial-" and "change-orienting reactions": an analysis based on visual single-trial event-related potentials.

The present study was concerned with brain potentials elicited by, respectively, the first of a series of stimuli ("initial-orienting reaction", I-OR), and infrequent deviants ("change-orienting reaction", C-OR). Single-trial event-related potentials (ERPs) to visual stimuli were estimated from recordings at Oz, Pz, Cz and Fz. The design included both a habituation series as well as a series of occasional deviant trials against a background of standards. This was done with both task-relevant and neutral stimuli, and in two interstimulus interval (ISI) conditions: 2.45 s and 8.45 s. In the latter ISI condition, skin conductance responses (SCRs) were recorded as well. Decrease (Habituation) in the habituation series was found for a non-specific N1, a posteriorly distributed P3, and the SCR, but not for P2-N2. Deviant stimuli produced an enhancement of the central P2-N2, the P3, the N1 (on the first few deviant trials only, in both ISI conditions), and the SCR (with task-relevant stimuli only). Elongation of ISI delayed both short-term and long-term decrease of P3, but had no effect on enhancement of P2-N2 due to stimulus deviance. It was concluded that, with respect to ERP parameters, the I-OR is marked by the N1, whereas the C-OR coincides with the P2-N2.