Abstract
Magnetic resonance imaging (MRI) provides a means to non-invasively investigate the neurological links with dyslexia, a learning disability that affects one’s ability to read. Most previous brain MRI studies of dyslexia and reading skill have used structural or diffusion imaging to reveal regional brain abnormalities. However, volumetric and diffusion MRI lack specificity in their interpretation at the microstructural level. Myelin is a critical neural component for brain function and plasticity, and as such, deficits in myelin may impact reading ability. MRI can estimate myelin using myelin water fraction (MWF) imaging, which is based on evaluation of the proportion of short T2 myelin-associated water from multi-exponential T2 relaxation analysis, but has not yet been applied to the study of reading or dyslexia. In this study, MWF MRI, intelligence, and reading assessments were acquired in 20 participants aged 10–18 years with a wide range of reading ability to investigate the relationship between reading ability and myelination. Group comparisons showed markedly lower MWF by 16–69% in poor readers relative to good readers in the left and right thalamus, as well as the left posterior limb of the internal capsule, left/right anterior limb of the internal capsule, left/right centrum semiovale, and splenium of the corpus callosum. MWF over the entire group also correlated positively with three different reading scores in the bilateral thalamus as well as white matter, including the splenium of the corpus callosum, left posterior limb of the internal capsule, left anterior limb of the internal capsule, and left centrum semiovale. MWF imaging from T2 relaxation suggests that myelination, particularly in the bilateral thalamus, splenium, and left hemisphere white matter, plays a role in reading abilities. Myelin water imaging thus provides a potentially valuable in vivo imaging tool for the study of dyslexia and its remediation.
Highlights
Dyslexia is a complex learning disability that affects one’s ability to read regardless of intelligence (Siegel, 1989a; Fletcher, 2009)
myelin water fraction (MWF) was significantly lower in the poor reader group relative to the good reader group in the left and right thalamus by 69% (p = 0.02) and 56% (p = 0.005), respectively, and in six white matter regions including left (−37%, p = 0.01) and right (−31%, p = 0.03) anterior limb of internal capsule, left (−22%, p = 0.02) and right (−16%, p = 0.03) centrum semiovale, left posterior limb of internal capsule (−21%, p = 0.02), and splenium of corpus callosum (−26%, p = 0.03; Figure 3)
Multicomponent T2 relaxation measurements have demonstrated that on average MWF is 20% lower in white matter and 46% lower in deep gray matter ROIs for children who are poor readers compared to good readers
Summary
Dyslexia is a complex learning disability that affects one’s ability to read regardless of intelligence (Siegel, 1989a; Fletcher, 2009). Since the earliest DTI papers demonstrating correlations of reading ability with DTI parameters (Klingberg et al, 2000; Nagy et al, 2004; Beaulieu et al, 2005; Deutsch et al, 2005; Niogi and McCandliss, 2006), a vast literature over the last 15 years has highlighted key white matter connections presumed to underlie the potentially inefficient communication or ‘‘disconnection’’ between brain regions in dyslexia (see Ben-Shachar et al, 2007; Vandermosten et al, 2012b; for comprehensive reviews and a meta-analysis of earlier DTI papers) These DTI reading findings are not limited to English with similar results across multiple languages (Qiu et al, 2008; Steinbrink et al, 2008; Thiebaut de Schotten et al, 2014; Zhang et al, 2014; Cui et al, 2016; Takeuchi et al, 2016; Zhao et al, 2016; de Moura et al, 2016; Vanderauwera et al, 2017; Su et al, 2018; Žaric et al, 2018; Moulton et al, 2019). While many of these studies reported DTI changes in the left temporoparietal white matter, other bilateral regions and the corpus callosum have shown associations with reading, implicating a broader network
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