No Evidence for Stronger Brain-Behavior Associations in the Bimanual Motor Network of Older Adults

Bimanual motor performance declines with age, is accompanied by cortical thinning and alterations in white matter microstructure within motor control networks. Aging is also associated with increased interindividual variability in behavioural and structural markers, and some studies report stronger brain-behaviour associations in older compared to younger adults. However, evidence for this remains mixed, and it is unclear whether young adults express similar relationships under comparable task demands. We tested whether bimanual performance is related differently to structural brain properties in younger and older adults. Twenty-three younger (22-27 years) and twenty-three older adults (65-70 years) performed a visually guided bimanual pinch-force task and underwent whole-brain structural magnetic resonance imaging (MRI). Fractional anisotropy (FA) and cortical thickness (CTh) were extracted from a pre-defined bilateral visuo-motor network and compared between groups. In regions showing significant age-related differences, we tested whether and how FA and CTh values correlates with bimanual performance within each age group. Older adults showed lower FA in the anterior portions of the bilateral superior longitudinal fasciculus (SLF) III and right inferior fronto-occipital fasciculus (IFOF), and lower CTh across most visuomotor regions, along with lower bimanual performance compared to young adults. Structure-function analyses in these areas revealed that bimanual performance correlated positively with FA in the anterior segment of the right SLF III in younger but not older adults. Furthermore, larger hemispheric asymmetries in cortical thickness between the dominant and non-dominant SMA correlated positively with larger intermanual performance differences between the dominant and non-dominant hand in younger but not older adults. These findings suggest that intrahemispheric white-matter integrity and interhemispheric cortical balance support efficient bimanual control in early adulthood, but they provide no evidence that structure-function relationships increase with age.