Abstract
Objective: The present study investigates associations between brain white matter tract integrity and cognitive abilities in community-dwelling older people (N = 655). We explored two potential confounds of white matter tract−cognition associations in later life: (a) whether the associations between tracts and specific cognitive abilities are accounted for by general cognitive ability (g); and (b) how the presence of atrophy and white matter lesions affect these associations. Method: Tract integrity was determined using quantitative diffusion magnetic resonance imaging tractography (tract-averaged fractional anisotropy [FA]). Using confirmatory factor analysis, we compared first-order and bifactor models to investigate whether specific tract-ability associations were accounted for by g. Results: Significant associations were found between g and FA in bilateral anterior thalamic radiations (r range: .16−.18, p < .01), uncinate (r range: .19−.26, p < .001), arcuate fasciculi (r range: .11−.12, p < .05), and the splenium of corpus callosum (r = .14, p < .01). After controlling for g within the bifactor model, some significant specific cognitive domain associations remained. Results also suggest that the primary effects of controlling for whole brain integrity were on g associations, not specific abilities. Conclusion: Results suggest that g accounts for most of, but not all, the tract−cognition associations in the current data. When controlling for age-related overall brain structural changes, only minor attenuations of the tract−cognition associations were found, and these were primarily with g. In totality, the results highlight the importance of controlling for g when investigating associations between specific cognitive abilities and neuropsychology variables.
Highlights
Cognitive ability is associated with many important life outcomes
In the bi-factor model (Figure 4), in which g is controlled for, most of the significant associations with specific cognitive ability factors are markedly attenuated, and become nonsignificant (Table 2). These results suggest that, in the current battery of tests, the associations between specific factors of cognitive ability and white matter tract integrity are largely driven by g, and not by separable specific cognitive ability variance. (Insert Table 2 about here) As can be seen in Table 2, the strongest associations with g are found for bilateral uncinate fasciculi (Left = 0.19; Right = 0.26; p
We provide further evidence that failure to control for g when investigating the associations between specific cognitive abilities and neuroimaging biomarkers could result in misleading, spurious or inflated associations with the specific cognitive factors
Summary
Recent research suggests communicative white matter pathways are an important aspect of its neurostructural foundation (Deary, Penke and Johnson, 2010). The retention of cognitive functioning is of particular importance to successful ageing (Deary et al, 2009); with a growing body of research exploring the associations between cognitive ability and measures of white matter integrity in the brain (see Madden et al 2012 for a recent review). Diffusion tensor MRI (DTI) is a widely used technique for studying brain connectivity (Sullivan and Pfefferbaum, 2006), providing biomarkers of white matter integrity, in particular fractional anisotropy (FA) which measures the directional coherence of water molecule diffusion. FA shows a gradual mean decrease, indicative of decreasing white matter tract integrity (Wozniak and Lim, 2006). We focus on studies which have utilised DTI tractography as this method is applied in the current study (see Methods)
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