Effects of novelty on event-related potentials: aging and stimulus replacement

Irrelevant unexpected stimuli elicit various types of inhibitory processes modulated by ongoing activity. An index of the inhibitory processes, namely the N2b and P3a event-related potentials (ERP), shows marked age-related differences.

In this experiment, we attempted to separate the effects of inhibitory and working memory processes in older and younger participants on cognitive processes, and we investigated the effect of irrelevant events at different stages of task-related cognitive processes.

In a successive letter-matching task, either the first or the second member of letter pairs were infrequently replaced by a novel stimulus in 2 age groups while ERPs were recorded.

Novel stimuli had different effects on ERP in the 2 age groups. In the group of young participants, novel stimuli replacing the first member of the letter pair elicited a long-lasting anterior negative ERP component. In the older group, such stimuli elicited the N2b-P3a complex. In the case of novelties replacing the second member of the pair, the N2b-P3a complex emerged in both groups. Task-relevant letters in both groups elicited the P3b (late positive component). P3b had more a diffuse scalp distribution in the older group.

In the different stages of the letter-matching task, ERPs elicited by novel stimuli differed only qualitatively in the older group, whereas in the younger group we obtained differences in the characteristics of the ERPs. The long-lasting anterior negativity in the younger group is considered a correlate of inhibitory activity towards the subsequent events initiated by the central executive system. This process is compromised in the older age group.