Age-specific activation patterns and inter-subject similarity during verbal working-memory maintenance and Cognitive Reserve

Cognitive Reserve, according to a recent consensus definition of the
NIH-funded Reserve and Resilience collaboratory
(https://reserveandresilience.com/), is constituted by any mechanism
contributing to cognitive performance beyond, or interacting with, brain
structure in the widest sense. To identity multivariate activation
patterns fulfilling this postulate, we investigated a verbal Sternberg
fMRI task and imaged 181 people with age coverage in the ranges 20-30 (44
participants) and 55-70 (137 participants). Beyond task performance,
participants were characterized in terms of demographics, and
neuropsychological assessments of vocabulary, episodic memory, perceptual
speed, and abstract fluid reasoning. Participants studied an array of
either 1, 3, or 6 upper-case letters for 3 seconds (=encoding phase), then
a blank fixation screen was presented for 7 seconds (=maintenance phase),
to be probed with a lower-case letter to which they responded with a
differential button press whether the letter was part of the studied array
or not (=retrieval phase). We focused on identifying maintenance-related
activation patterns showing memory-load increases in pattern score on an
individual-participant level for both age groups. We found such a pattern
that increased with memory load for all but one person in the young
participants (p<0.001), and such a pattern for all participants in
the older group (p<0.001). Both patterns showed broad topographic
similarities; however, relationships to task performance and
neuropsychological characteristics were markedly different and point to
individual differences in Cognitive Reserve. Beyond the derivation of
group-level activation patterns, we also investigated the inter-subject
spatial similarity of individual working-memory rehearsal patterns in the
older participants’ group as a function of neuropsychological and task
performance, education and mean cortical thickness. Higher task accuracy
and neuropsychological function was reliably associated with higher
inter-subject similarity of individual-level activation patterns in older
participants.