Balancing load, memory, and movement: Effects of cognitive reserve on dual-task performance in older adults

Cognitive reserve (CR) is thought to mitigate age-related cognitive decline by supporting more efficient or flexible allocation of neural resources during cognitively demanding situations, yet its neural and behavioral signatures during challenging dual-task contexts remain unclear. This study examined how CR moderates prefrontal cortical activation and motor performance during cognitive-motor dual tasking in healthy older adults using working-memory tasks of differing cognitive load. Forty-two participants completed single- and dual-task conditions pairing finger tapping with either a double number sequence (DNS) or an n-back (NBK) task. Functional near-infrared spectroscopy captured changes in prefrontal oxygenated and deoxygenated hemoglobin (ΔHbO2, ΔHbR), while behavioral outcomes included tapping speed, accuracy, variability, and cognitive task accuracy. A multidimensional composite CR score was calculated using education, occupation, bilingualism, and physical activity and participants were categorized into higher and lower CR groups via median split. Dual tasking during higher cognitive load (NBK) elicited greater prefrontal activation compared to single-tasking and lower load (DNS). Significant three-way interactions revealed that individuals with lower CR showed disproportionately elevated ΔHbO₂ (p = .009) and ΔHbR (p = .011) during NBK dual-tasking, suggesting reduced neural efficiency. A parallel interaction in motor performance indicated that only the lower CR group exhibited marked slowing of tapping speed during NBK dual-tasking (p = .016), mirroring the neural findings, while accuracy declined with increasing cognitive load across groups. These results demonstrate that CR moderates neural and behavioral responses under high cognitive-motor demands and suggest that tasks requiring continuous working-memory updating may be particularly sensitive for detecting subtle reserve-related differences in healthy aging.