Preceding stimulus sequence effects on the oddball-P300 in young and healthy older adults.

The present study examined the mechanisms underlying stimulus sequence effects on the parietal P300 and frontal P3a elicited by infrequent targets in an oddball paradigm and compared these effects between young and older adults. Fifty young and 50 healthy older adults completed a simple visual oddball task including an infrequent (20%) target stimulus. A temporo-spatial principal component (PCA) analysis was used to disentangle the parietal P300 and frontal P3a from overlapping ERP components. The typical age-related anterior-shift of the P300 was, according to the PCA, due to a disproportionately large contribution of a frontal P3a in older adults. Up to about five successive standards preceding a target, the results replicated the typical increase in parietal P300 amplitude to targets with increases in preceding standards. A novel finding was that with further increases in preceding standards, P300 amplitude to targets gradually decreased. This pattern is consistent with a combination of relatively automatic, temporary expectations and higher level, conscious expectations driving P300 sequential effects, with different dynamics over the course of successively presented standards. However, an expectancy-based account of these sequential effects was challenged by an analogous analysis of RTs. Sequential effects on the frontal P3a differed from the parietal P300: The P3a was largest when a target followed another target and continually decreased with the number of intervening standards. Notably, sequential effects were strikingly similar for young and older adults, and implications for the functional significance of morphological changes in P300 due to aging are discussed.