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

Abstract

Elderly adults may master challenging cognitive demands by additionally recruiting the cross-hemispheric counterparts of otherwise unilaterally engaged brain regions, a strategy that seems to be at odds with the notion of lateralized functions in cerebral cortex. We wondered whether bilateral activation might be a general coping strategy that is independent of age, task content and brain region. While using functional magnetic resonance imaging (fMRI), we pushed young and old subjects to their working memory (WM) capacity limits in verbal, spatial, and object domains. Then, we compared the fMRI signal reflecting WM maintenance between hemispheric counterparts of various task-relevant cerebral regions that are known to exhibit lateralization. Whereas language-related areas kept their lateralized activation pattern independent of age in difficult tasks, we observed bilaterality in dorsolateral and anterior prefrontal cortex across WM domains and age groups. In summary, the additional recruitment of cross-hemispheric counterparts seems to be an age-independent domain-general strategy to master cognitive challenges. This phenomenon is largely confined to prefrontal cortex, which is arguably less specialized and more flexible than other parts of the brain. working memory | subjective task difficulty | lateralization | prefrontal cortex | cognitive aging

Neural Correlates of Working Memory Training: Evidence for Plasticity in Older Adults

“…Overall, these results are consistent with CRUNCH and replicate previous findings (Berlingeri et al, 2013;Cappell et al, 2010;Heinzel et al, 2014;Holler-Wallscheid et al, 2017;Kennedy et al, 2015;Schneider-Garces et al, 2010). According to CRUNCH, the extent of compensationrelated activity varies with both the level of task demand and the resources available to meet that demand.…”

“…Also in line with CRUNCH, the voxel-wise ANOVA at Time1 (see Fig. S1) showed bilateral modulation of brain activity by load in both groups, suggesting that contralateral recruitment is not unique to older adults, but an age-independent compensation mechanism engaged for difficult tasks (see also Holler-Wallscheid et al, 2017).…”

“…1a). Older adults reach this apparent resource ceiling at lower loads than younger adults (Cappell et al, 2010), although younger adults can also show compensatory neural activation when task demands are sufficiently high (Holler-Wallscheid, Thier, Pomper, & Lindner, 2017).…”

“…While accumulating evidence clearly supports the idea of load-dependent over-activation in older adults, particularly when able to reach performance levels comparable with younger adults (e.g., Berlingeri, Danelli, Bottini, Sberna, & Paulesu, 2013;Cappell et al, 2010;Heinzel et al, 2014;Holler-Wallscheid et al, 2017;Kennedy et al, 2015;Schneider-Garces et al, 2010) (for a recent meta-analysis see Li et al, 2015), the hypothesis that over-activation signals compensation has sometimes been challenged. It could be argued that the correlational nature of the available evidence makes it impossible to ascertain whether increased brain activity is used in the service of improved performance (Schneider-Garces et al, 2010).…”

Bidirectional Frontoparietal Oscillatory Systems Support Working Memory

“…Stimuli presented to the right visual hemifield are initially encoded by the left hemisphere (and vice versa) and transferred via the splenium within 15 msec to engage both hemispheres, permitting both unilateral and rapid bilateral processing of visual stimuli. Recent functional magnetic resonance imaging (MRI) data indicate that bilateral PFC is recruited for cognitively challenging tasks across the lifespan [19], suggesting that working memory recruits bilateral PFC when both hemispheres are available. Collecting EEG data in a lateralized visual hemifield design is a method to examine whether the unilaterally lesioned brain quickly compensates across hemispheres [15, 16].…”