Span, CRUNCH, and Beyond: Working Memory Capacity and the Aging Brain

Abstract

Neuroimaging data emphasize that older adults often show greater extent of brain activation than younger adults for similar objective levels of difficulty. A possible interpretation of this finding is that older adults need to recruit neuronal resources at lower loads than younger adults, leaving no resources for higher loads, and thus leading to performance decrements [Compensation-Related Utilization of Neural Circuits Hypothesis; e.g., Reuter-Lorenz, P. A., & Cappell, K. A. Neurocognitive aging and the compensation hypothesis. Current Directions in Psychological Science, 17, 177–182, 2008]. The Compensation-Related Utilization of Neural Circuits Hypothesis leads to the prediction that activation differences between younger and older adults should disappear when task difficulty is made subjectively comparable. In a Sternberg memory search task, this can be achieved by assessing brain activity as a function of load relative to the individual’s memory span, which declines with age. Specifically, we hypothesized a nonlinear relationship between load and both performance and brain activity and predicted that asymptotes in the brain activation function should correlate with performance asymptotes (corresponding to working memory span). The results suggest that age differences in brain activation can be largely attributed to individual variations in working memory span. Interestingly, the brain activation data show a sigmoid relationship with load. Results are discussed in terms of Cowan’s [Cowan, N. The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24, 87–114, 2001] model of working memory and theories of impaired inhibitory processes in aging.

The cognitive neuroscience of ageing

“…At higher levels of load, this compensatory mechanism is no longer effective, leading to equivalent or less activation in older adults relative to young. Data consistent with this idea have been reported in PFC 65 , and in both PFC and parietal cortex 66,67 during working memory tasks that varied in the number of items that had to be kept in mind. In these studies, older adults had more activation at low levels of working memory load, where performance was equivalent to that of younger adults, but less activity and lower accuracy at higher loads.…”

Bilateral recruitment of prefrontal cortex in working memory is associated with task demand but not with age

“…Here, we also describe the results for these additional task-related areas, because they have been consistently reported to be involved in WM processes in young subjects (5,16,19,20). However, we had no prior hypotheses with respect to their pattern of lateralization.…”

“…WM spans were assessed for each WM domain by means of a measurement termed "throughput." This term, introduced by Schneider-Garces et al (16), captures how many of the presented WM items a person is able to successfully keep in memory. The following Eq.…”

“…The following Eq. 1 [adapted from SchneiderGarces et al (16)] defines the throughput in our experiment as a function of the objective memory load, the subjective performance (hits) and chance level: throughput = ðhits-chance levelÞ ð1-chance levelÞ × memory load.…”

“…Actually, in the studies that have shown the HAROLD phenomenon (2)(3)(4)(5)11), all subjects worked on the same memory tasks despite the fact that cognitive performance of older adults deteriorates, and WM is one of the most affected cognitive functions (12)(13)(14). This cognitive deterioration might make a task subjectively more demanding for older subjects, so that they need the recruitment of contralateral counterparts in prefrontal cortex to perform as well as the young subjects (14)(15)(16). However, the presence of bilateral activation patterns in the elderly does not necessarily mean that this mechanism is age-specific.…”