Evidence for frontally mediated controlled processing differences in older adults.

Advanced aging is associated with slower and less flexible performance on demanding cognitive tasks. Here we used rapid event-related functional magnetic resonance imaging to explore differences between young (n = 65) and older adults (n = 75) during memory retrieval. Methods were optimized to afford exploration of both amplitude and timing differences in neural activity. Although many correlates of retrieval were similar between the groups, including medial and lateral parietal responses to successful recognition, older adults showed increased recruitment of frontal regions relative to young adults when retrieval demanded heavy use of control processes. This effect was not significant during less effortful retrieval. Moreover, the timing of increased recruitment in older adults occurred at relatively late stages of the retrieval event, suggesting a strategy shift. One possibility is that older adults fail to engage appropriate top-down attentional sets at early stages of the retrieval event; as a consequence, frontally mediated processing is extended at late stages to compensate. This strategy shift, which we conceptualize in a framework called the "load-shift" model, may underlie the often observed retention of high-level cognitive function during advanced aging but at the cost of less flexible and slower performance on demanding cognitive tasks.