The Sustaining Brain: The impact of engagement in occupational and recreational activities on white matter integrity and cognitive processing speed in older adults

This thesis investigates cognitive ageing through the lens of processing speed, white matter integrity, and cognitive reserve, addressing the global challenge of sustaining cognitive vitality in ageing populations. As life expectancy rises, understanding the mechanisms underpinning cognitive decline and resilience becomes critical. Three research questions guide the inquiry: how to conceptualize brain health using longitudinal data, the risks of age-related cognitive decline, and the influence of occupational and recreational experiences on cognitive performance. Employing a systematic review of 15 global studies (N = 9,785, ages 9–80+) and longitudinal analyses of the Irish Longitudinal Study on Ageing (TILDA; N= 8,504, 50+), the thesis integrates neurocognitive theory with quantitative social science methods. Study 1 establishes processing speed as a robust proxy for white matter integrity, with age-varying associations suggesting compensatory mechanisms. Study 2’s latent change score modelling confirms processing speed’s predictive power for dementia risk, validating its role as a foundational cognitive marker. Study 3’s structural equation modelling reveals that occupational complexity enhances processing speed but does not moderate white matter effects, challenging simplistic cognitive reserve models. Study 4’s best subset regression identifies engagement activities (e.g., reading, lectures) as predictors of sustained processing speed and executive function, highlighting environmental modifiability. These findings affirm the processing speed theory’s relevance while revealing cognitive reserve’s complex, multifaceted role. Together these studies contribute to a lifespan perspective on brain health, empirical support for processing speed’s clinical utility, and actionable insights for public health interventions. This thesis bridges neural and environmental perspectives, and lays a foundation to promote cognitive resilience, envisioning a future where ageing populations thrive with independence and clarity.