Coupled change in brain white matter integrity and fluid intelligence in later life

Research Question: Aging-related cognitive decline is a growing economic, medical, and social burden for countries with aging populations. Despite this increasing importance, a recent review concluded that little is known about the neurobiological substrates of cognitive decline. It is rare for studies to examine longitudinal trends in both cognitive ability and in neuroimaging measurements, and even rarer for them to include large, narrow-age samples.

Method: The Lothian Birth Cohort 1936 is a longitudinal study of ageing based in the UK. We used cognitive and neuroimaging measurements taken at mean ages 73 (n = 731) and 76 (n = 488) and a two-wave latent difference score model to investigate change in fluid intelligence, processing speed, and memory alongside change in white matter fractional anisotropy.

Results and conclusions: Two main results emerged: First, longitudinal decline in fluid intelligence and in white matter fractional anisotropy were significantly correlated, indicating that these two factors decline together. Second, higher fractional anisotropy at baseline predicted a shallower decline in processing speed across the next three years. There was no baseline-slope or slope-slope relation of fractional anisotropy to memory.

Discussion: These results, taken from a large, longitudinal sample, shed light on the neural basis of age-related cognitive decline, since they are among the first to investigate longitudinal associations between white matter and cognitive abilities. They have implications for hypotheses of 'brain reserve', and for theories that posit cortico-cortical 'disconnection' as a cause of cognitive ageing.