The role of inhibition in working memory performance associated with age

The current investigation addressed the view that inefficient inhibitory processes account for some of the age-related declines in performance on measures of working memory (WM). Two studies were designed that incorporated WM measures chosen from the neuroimaging literature. Study 1 examined age differences between young (18–29) a young-old (60–74) and an old-old group (75+) on versions of the n-back and item recognition tasks (Jonides et al. 1998; Smith & Jonides, 1997). The tasks included critical trials for which it was presumed that inhibitory processes were particularly important for successful performance. According to the inhibitory deficit view, older participants should show particular difficulty on these trials. Results generally supported this hypothesis. Not only did the older adults have more difficulty than the young adults on these tasks overall, they also showed relatively more difficulty on those trials that were theorized to rely more on the integrity of inhibitory processes for successful performance. No differences were found between the two older groups. Study 1 also addressed the suggestion of some researchers that inhibition is partly subserved by the prefrontal cortex (PFC). In particular, performance on the WCST, often described as a putative measure of frontal lobe functioning was correlated with performance in the 2-back version of the n-back task, lending some support for the relationship between the PFC and inhibitory mechanisms. Study 2 investigated performance on the item recognition task in relation to age-related information processing speed changes as assessed by two perceptual speed measures (Salthouse, 1996). Additionally, in an attempt to vary the amount of interference across trials, Study 2 also included a comparison between categorized and unrelated word lists. Because of prior strong associative bonds between category members, it was expected that the categorized list condition would particularly impair performance in the older adults due to inefficient inhibitory mechanisms. It was also hypothesized that although age-related changes in processing speed would account for some of the differences in this working memory task, (a) it would not account for all of age-related variance, and (b) it would account for less of the variance in the categorized list condition than the unrelated condition, presumably because inhibition was also important to performance, particularly in the categorized condition. The category manipulation did not achieve the desired results; contrary to expectations, performance was better in the categorized list condition compared to the unrelated condition. Hierarchical regression analysis indicated that perceptual speed accounted for much of the age-related variance in the item recognition task. Nonetheless, the interpretation of such findings as necessarily supporting a processing speed explanation of age-related changes in cognition is called into question by recent findings indicating that age differences on perceptual speed measures are partly a reflection of differences in susceptibility to interference effects from visual distraction. This outcome suggests that the fundamental factor in regression analysis may be inhibitory efficiency rather than processing speed. Further studies are needed to evaluate the relative role of processing speed and inhibition to age changes in performance on WM tasks.