Visual stimulus change and the orienting reaction: event-related potential evidence for a two-stage process.

In a previous study it was found that infrequent deviant visual stimuli, in a series of standards, elicited event-related potentials (ERPs) with enhanced P2-N2s and P3 amplitudes, suggesting that these parameters reflect processes related to the orienting reaction (OR). In the present study a similar oddball series was presented against the background of a second class of stimuli. With respect to the latter stimuli, subjects had to perform either a very involved (hard) or an easy task. EEG was recorded to oddball (probe) stimuli from Oz, Pz, Cz, and Fz. Analysis of average ERPs revealed that, in the easy condition, deviant probes elicited both enhanced P2-N2s and enhanced P3s, relative to the standards. In contrast, in the hard condition P2-N2, but not P3, was enhanced by stimulus change. In addition, overall P3 amplitude to probes was smaller in the hard condition (sequence-independent load effect). Analysis of single-trial ERPs (SERPs) with orthogonal polynomial trend analysis largely replicated these effects. In addition, SERP analysis also revealed a sequence-independent load effect on P2, as well as a decreasing P3 to deviant stimuli in the Easy condition, which was observed at Cz and Fz, but not at Pz or Oz. The results are interpreted as suggesting that P2-N2 and P3 reflect different stages of the OR, one of automatic and one of capacity-limited processing.