UC Riverside

The number of individuals experiencing age-related cognitive decline will increase as the population of older adults continues to rise, with the fastest growing segment being oldest-old adults aged 85+ years. These cognitive deficits can be at least partly attributed to age-related declines in the underlying neural substrates (e.g., white matter microstructure), which can be measured in living individuals using magnetic resonance imaging (MRI). However, MRI studies examining the association between cognitive and brain aging across the older adult lifespan rarely use samples that extend into advanced age. To address this limitation, this dissertation studied neurocognitive aging within oldest-old adults using diffusion MRI and integrative review methodologies. Chapter 1 used traditional single tensor diffusion imaging to examine the linearity of age-related declines in white matter microstructure across 108 adults ages 65-98 years. Results indicated accelerated brain-wide white matter microstructure declines into advanced age, with declines in medial temporal white matter mediating the negative effect of age on episodic memory performance. Chapter 2 used more advanced multicompartment diffusion imaging to assess relations between white matter microstructure and associative learning performance within a subset of 22 oldest-old adults from Chapter 1. Results indicated preserved associative learning abilities into the 10th decade of life that were supported by better microstructure of white matter connections between the prefrontal cortex and dorsal striatum. Findings from Chapters 1 and 2 were independent of diagnoses of cognitive impairment no dementia in oldest-old adults, suggesting that they were not driven by advanced age-related cognitive dysfunction. Finally, Chapter 3 examined whether these findings align with those from other MRI studies of oldest-old adults as well as predictions of select neurocognitive aging theories. Despite there being some continuity across the older adult lifespan, results indicated that older adults also have unique cognitive and neural profiles during the eighth through tenth decades of life. Together, this collection of findings (1) supports the notion that advanced age affects white matter microstructure and in turn cognitive ability and (2) highlights the importance of considering oldest-old adults in modern neurocognitive aging research.