Understanding how age-related changes in prefrontal function affects episodic retrieval and strategic organization

The goal of the current study was to determine whether age-related decline in temporal order memory performance is due to functional changes in right prefrontal regions related to episodic retrieval processes, or, to functional changes in right prefrontal regions related to post-retrieval strategic organization processes. To achieve this goal, an event-related fMRI study was conducted in which 8 young and 9 elderly subjects were scanned while performing verbal recognition memory (R), temporal order memory (T) and reverse alphabetizing (A) tasks. A difficulty manipulation was also incorporated into the experiment to control for age-related differences in task effort. The behavioral results from this study indicated that older subjects displayed lower accuracy performance during T and difficult versions of the A tasks, compared to young subjects. Within and between group activation analyses were conducted on the fMRI data obtained from both groups. The fMRI results for young subjects indicated that right superior frontal activity was related to episodic retrieval processes since this region was only active during the performances of R and T tasks. Right middle frontal activity was related to strategic organization since it was commonly active during both A and T tasks in young subjects. The within group analysis of the older subjects indicated that the contributions of these right prefrontal regions differed with age. Older subjects engaged right middle and inferior frontal regions exclusively during episodic memory tasks. Right superior frontal (BA I0) activity was not observed in the within group results of older subjects. Older subjects also did not engage any additional right prefrontal regions during the performance of temporal order memory tasks. Instead, they engaged left middle and inferior frontal regions to a greater extent than young subjects across all tasks. Left middle frontal activity was interpreted as reflecting semantic retrieval and/or linguistic manipulation. Left inferior frontal activity was interpreted as reflecting maintenance of verbal information in WM across all tasks. Therefore, the current results suggest that older subjects had a primary deficit in right superior frontal function during EM tasks and consequently compensated by recruiting bilateral middle and inferior frontal regions to perform EM tasks and strategic organization tasks.