Background and Objective:
Previous research examining socioeconomic differences in brain white matter focused on fractional anisotrophy or mean diffusivity, which are open to several biological interpretations. Further, pathways linking socioeconomic conditions, white matter, and cognitive performance have rarely been investigated. We aimed to provide a more detailed neurobiological understanding of socioeconomic differences in brain anatomy and cognitive performance.

Methods:
We combined quantitative magnetic resonance imaging biomarkers indicative of tissue myelination and iron content with diffusion-weighted brain measures, cognitive performance (processing speed, cognitive flexibility, and fluid intelligence), and socioeconomic data (household income and occupational position) in a sample of community-dwelling individuals (n=751, aged 50 to 91 years). We adjusted the applied regression and structural equation models for the linear and non-linear effects of age, sex, education, cardiovascular risk factors, and depressive, anxiety, and substance use disorders.

Results:
Individuals from lower income households showed signs of advanced brain white matter ageing with greater mean diffusivity (MD), lower neurite density (intra-cellular volume fraction; ICVF), lower myelination (magnetization transfer saturation; MT), and lower iron content (transverse relaxation rate; R2*). The association between household income and MD was mediated by ICVF (B=0.084 (0.029, 0.141), p=0.003) and MT (B=0.019 (0.006, 0.034) p=0.009); MD partially mediated the association between household income and cognitive performance (B=0.017 (0.001, 0.033), p=0.040). Household income moderated the association between white matter microstructure and cognitive performance, such that greater MD, lower MT, or lower ICVF was only associated with poorer cognitive performance among individuals from lower income households. Individuals from higher income households showed preserved cognitive performance in the face of greater MD, lower MT, or lower ICVF.

Conclusion:
An individual’s household income relates to their white matter brain anatomy and associated cognitive performance in middle to older adulthood, and buffers the adverse effects of white matter decline.