Abstract
Age-related differences in white matter (WM) microstructure have been linked to lower performance in tasks of processing speed in healthy older individuals. However, only few studies have examined this link in a longitudinal setting. These investigations have been limited to the correlation of simultaneous changes in WM microstructure and processing speed. Still little is known about the nature of age-related changes in WM microstructure, i.e., regionally distinct vs. global changes. In the present study, we addressed these open questions by exploring whether previous changes in WM microstructure were related to subsequent changes in processing speed: (a) 1 year later; or (b) 2 years later. Furthermore, we investigated whether age-related changes in WM microstructure were regionally specific or global. We used data from four occasions (covering 4 years) of the Longitudinal Healthy Aging Brain (LHAB) database project (N = 232; age range at baseline = 64–86). As a measure of WM microstructure, we used mean fractional anisotropy (FA) in 10 major WM tracts averaged across hemispheres. Processing speed was measured with four cognitive tasks. Statistical analyses were conducted with bivariate latent change score (LCS) models. We found, for the first time, evidence for lagged couplings between preceding changes in FA and subsequent changes in processing speed 2 years, but not 1 year later in some of the WM tracts (anterior thalamic radiation, superior longitudinal fasciculus). Our results supported the notion that FA changes were different between regional WM tracts rather than globally shared, with some tracts showing mean declines in FA, and others remaining relatively stable across 4 years.
Highlights
It is widely recognized that fluid cognitive abilities decline during the course of aging (Park et al, 2002; Deary et al, 2009; Salthouse, 2010), with substantial variability observed between individuals (e.g., Wilson et al, 2002)
Age-related cortical disconnection caused by the degradation of the microstructure of myelinated axonal fiber bundles that make up the white matter (WM) of the brain has been identified as one potential neural mechanism for age-related deficits in processing speed (Madden et al, 2009, 2012; Bennett and Madden, 2014)
We evaluated whether a global fractional anisotropy (FA) factor capturing the shared variance across the WM tracts of interest was tenable
Summary
It is widely recognized that fluid cognitive abilities decline during the course of aging (Park et al, 2002; Deary et al, 2009; Salthouse, 2010), with substantial variability observed between individuals (e.g., Wilson et al, 2002). The investigation of the neural mechanisms underlying changes in processing speed is highly relevant in order to advance our understanding of age-related decline in fluid cognitive abilities in general. The microstructural properties of these WM fiber pathways can be estimated in vivo with diffusion-weighted magnetic resonance imaging (DW-MRI), a neuroimaging method that allows to measure the rate and directionality of water diffusion in brain tissues (Jones et al, 2013). LCS models estimate LCS between subsequent measurement scores that represent true change separated from measurement error (McArdle, 2009). These models allow the estimation of dynamic within-person associations between two change processes
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