White matter microstructure and cognitive function

Abstract

In recent years, diffusion-weighted magnetic resonance imaging (DW-MRI) has been increasingly used to explore the relationship between white matter structure and cognitive function. This technique uses the passive diffusion of water molecules to infer properties of the surrounding tissue. DW-MRI has been extensively employed to investigate how individual differences in behavior are related to variability in white matter microstructure on a range of different cognitive tasks and also to examine the effect experiential learning might have on brain structural connectivity. Using diffusion tensor tractography, large white matter pathways have been traced in vivo and used to explore patterns of white matter projections between different brain regions. Recent findings suggest that diffusion-weighted imaging might even be used to measure functional differences in water diffusion during task performance. This review describes some research highlights in diffusion-weighted imaging and how this technique can be employed to further our understanding of cognitive function.

Figure 1.

Principles of diffusion. (A) The diffusion tensor ellipsoid represents the probability that water molecules within a voxel will diffuse in a given direction. (B) Fractional anisotropy (FA) is calculated from the diffusion tensor. Areas of high anisotropy have a more elongated probability distribution, reflecting the higher likelihood of diffusion in one direction. (C) Areas of lower anisotropy have a more spherical distribution. (D) This FA image has been colored to illustrate the principal directions of diffusion within different white matter pathways. (E) Here, the principal direction of diffusion within each voxel (red line) is shown overlaid on an FA map (lighter gray indicates higher anisotropy). Within white matter, the principal direction of diffusion is often aligned parallel to the length of the tract.

Figure 2.

Exploring individual differences using diffusion-weighted imaging. (A) Fractional anisotropy (FA) in white matter of a region within the genu of the anterior corpus callosum showed a correlation between FA and metacognitive ability that was statistically significant. Correlations are shown overlaid on the average FA image of all participants (adapted from Fleming and others 2010). (B) On a mean FA map (gray scale), the locations of differences in radial diffusivity between Royal Air Force pilots and controls are shown in red and circled in white. In the parietal region of interest (ROI), higher values of radial diffusivity were found in the control group, whereas in the dorsomedial frontal ROI, higher values of radial diffusivity occurred in the pilot group. All participants were assessed using the Eriksen flanker task. In the parietal cluster, mean radial diffusivity was negatively correlated with accuracy, incongruence cost, and pure cost across the group. In the medial frontal cluster, mean radial diffusivity was positively correlated with incongruence cost and benefit and negatively with postconflict adaptation change in pure cost. (C) After a six-week training period on a visuomotor task (juggling), a significant increase in FA was found in the white matter adjacent to the posterior parietal cortex (adapted from Scholz and others 2009).

Figure 3.

Applications of diffusion tensor tractography. (A) Analysis of pathways connecting Broca area (B), Wernicke area (W), and Geschwind area (G) revealed a significant left hemisphere lateralization in the majority of participants. Those individuals with Broca-Wernicke pathways in the right hemisphere were superior at recalling words based on semantic associations (adapted from Catani and others 2007). (B) Group-averaged tractography results, normalized and co-registered in MNI space, superimposed on coronal, axial, and sagittal slices of the SPM T1 template image, together with the group functional magnetic resonance imaging results, illustrating the locations of frontal cortical eye fields: supplementary eye field (SEF) and frontal eye field (FEF). Tract connecting contralateral SEF shown in red and ipsilateral SEF to FEF connections shown in green (adapted from Anderson and others 2011). (C) Probabilistic tractography was used to parcellate the posterior parietal cortex by identifying brain areas with similar cortical projections. Colors correspond to specific cortical projection sites (adapted from Neubert and others 2010).