How the brains of young and old human adults cope with increased working memory demands

Abstract

Working memory tasks are generally more difficult for older adults due to decreasing working memory capacity that is evoked by changes in the ageing brain. To successfully cope with this increased challenge in cognitive demand, elderly adults additionally activate the contralateral counterparts of specific prefrontal brain regions, whereas young adults do engage them unilaterally. At first glance, this strategy seems at odds with the idea of lateralized cognitive functions in cerebral cortex. In this dissertation, I investigated whether bilateral recruitment is a general strategy of the human brain to respond to increased working memory demands that is independent of age, task content and cerebral region. To answer these questions, we conducted our main experiment in which a group of young and a group of old adult participants worked on verbal, spatial, and object-based working memory tasks that had been individually tailored to push each subject to her or his capacity limit in each working memory domain. Simultaneously, we used functional magnetic resonance imaging to measure brain activity associated with working memory maintenance and to compare this activity between cross-hemispheric counterparts of the respective brain regions. Our results clearly indicate that language-related regions such as Broca’s area in the left ventrolateral prefrontal cortex, the left supplementary motor area, right lobule VI and crus1 of lobule VII of the cerebellum, and the left ventral premotor cortex maintained their lateralized activation patterns across the two age groups despite our highly challenging working memory tasks. In contrast, dorsolateral prefrontal cortex and anterior prefrontal cortex showed bilateral activation in difficult conditions across all working memory domains and this was true for both age groups. To further confirm that also young adults shift from a unilateral to a bilateral recruitment of these prefrontal brain regions in easy vs. highly demanding working memory tasks, respectively, we conducted an additional control experiment that engaged both the verbal and the spatial working memory domain. The results of this control experiment demonstrated that dorsolateral prefrontal cortex and anterior prefrontal cortex are unilaterally recruited during the easy task variants and – together with the results of the main experiment – they showed that this unilaterality transforms into bilaterality in difficult tasks. The additional activation of contralateral cerebral counterparts seems to be a strategy of the brain to cope with increased cognitive challenges independent of age and working memory task content. This phenomenon mainly emerges in prefrontal cortex – a brain structure that is less specialized and more flexible than other parts of the brain.