Compensation-related utilisation of neural circuits hypothesis in cognitive control
Sharna Jamadar, Gary Egan
A decline in cognitive function is considered an inevitable consequence of the ageing process, even in healthy ageing adults. It is commonly believed that individuals can partly compensate for this decline by employing new strategies and/or additional brain regions to successfully perform cognitive tasks.
A. Switch vs. baseline activity averaged across group. Circle shows ROI. B. Older-young difference between groups (black). Positive values indicate older adults activated more than young adults, negative values indicate young adults activated more than older adults. Grey line shows example predicted curve from CRUNCH. Abbreviation: R DLPFC, right dorsolateral prefrontal cortex.
The compensation-related utilisation of neural circuits hypothesis (CRUNCH), proposes that during task performance, as task difficulty ('load') increases, more cortical regions will be activated. Older adults reach their load capacity sooner than younger adults, so at intermediate levels of task difficulty, they will recruit more neural resources than younger adults – the classic 'cognitive compensation' effect. At higher levels of load, the compensatory mechanism is no longer effective, leading to less activation in older versus younger adults.
In order to test CRUNCH, a design with at least four difficulty levels is required, which was modulated by increasing the number of tasks (and switches between them). Young (aged 23-26yrs) and older (aged 69-79yrs) adults completed an fMRI session and switched between 2, 3, 4 and 5 tasks in 4 separate functional and a structural MRI runs. To examine differences in activation between groups, an anatomical region-of-interest of the right dorsolateral pre-frontal cortex was created, a region of the brain involved in cognitive control in the task-switching paradigm. Older adults activated the dorsolateral pre-frontal cortex to a greater extent than younger adults. Older adults showed: (i) similar levels of activity as younger adults for 2 switch and 3 switch trials, (ii) greater activity than younger adults for 4 switch trials, and (iii) less activity than younger adults for 5 switch trials. This pattern is highly consistent with that predicted by CRUNCH. This is strong preliminary evidence that CRUNCH can account for cognitive compensation during cognitive control in older adults.
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