Inhibitory decline with age: The influence of failed strategy.

Don't think of a white bear. Doesn't work so well, does it? Yet under some circumstances, people appear to be able to do precisely this: as described last week, young adults are thought (by some) to actually suppress the neural activity related to to-be-ignored stimuli, and even delay the peak of this neural activity, relative to a situation in which stimuli are to be just passively-viewed. In a follow-up paper at Nature Neuroscience, Gazzaley et al report that cognitively-intact older adults (60-77 years of age) show an impairment in this ability, without concomitant impairments in the enhancement effects normally observed to to-be-attended stimuli.

Gazzaley et al report that normal aging is not only accompanied by impairments in working memory for the to-be-attended stimuli, but also correlated with it, perhaps suggesting a critical role for these suppression effects in good performance, and in age-related decline in performance. In addition, those older adults showing large behavioral impairment, but not those who were better matched to the younger subjects, rated the to-be-ignored stimuli as more familiar than the younger adults, indicating that perhaps they failed to inhibit processing of those to-be-ignored stimuli.

On the other hand, some aspects of the design seem to indicate that young adults use an oculomotor strategy to accomplish suppression. That is, they might have been looking at uninformative parts of the stimuli or closing their eyes altogether, which would be expected to yield a large number of saccade/blink related artifacts in the EEG, or the presence of high-amplitude "alpha" waves, respectively. Both phenomena were observed in previous work, with 7 out of 18 young adults being excluded for these reasons. In the current study with older adults, EEG was not used - so we cannot know whether similar numbers of older adults would have been excluded for these regions, or whether they might not have used this strategy.

Nonetheless there are reasons to believe that poor strategy use could be more central to age-related decline than inhibitory deficits - a number of previous studies indicate that older adults are impaired at cognitively monitoring their own performance and at identifying and selecting optimal strategies. For example, this study by Brigham & Pressley showed that 60-88 year-old adults were less able to detect the advantage conferred by an explicitly taught strategy for studying word lists, and were less likely to apply it when learning subsequent lists, than younger adults (24-39 years old). Even if older adults actually discovered the advantageous oculomotor strategy for the Gazzaley paradigm, they may have failed to use it. Alternatively (or additionally) older adults may have been more likely to forget the instructions on any particular trial sequence, and to default to a strategy of attempting to remember everything, leading to greater familiarity of to-be-ignored items.

Nonetheless, the study remains a powerful demonstration of the age-related decline that can be observed on tasks with high working memory demands, and is strongly suggestive of a age-related deficit that is specific to trials with inhibitory demands. The open question is whether these deficits reflect a core underlying "inhibitory" mechanism, or whether they rely on impaired "metamemory," impaired strategy discovery, or impaired strategy use among older adults.