Neurobiology of dyslexia
Introduction
Developmental dyslexia, an unexplained difficulty in word reading accuracy and/or fluency, affects 5–12% of children [1, 2]. Dyslexia is associated with many undesirable outcomes, including reduced educational attainment and academic self-esteem [3]. Furthermore, children with dyslexia tend to read far less outside of school than their peers [4], resulting in a widening gap in reading skills. Over the past 15 years, neuroimaging has made visible and quantifiable the brain differences that are associated with dyslexia; here, we review progress in the past few years in understanding the biological basis of dyslexia at a neural systems level.
Reading is a complex and slowly learned skill requiring the integration of multiple visual, linguistic, cognitive, and attentional processes. Neuroimaging methods including functional magnetic resonance imaging (fMRI), electroencephalography (EEG, and event-related potentials or ERPs), and magnetoencephalography (MEG), have revealed the brain regions most consistently involved in single word reading. In typically reading adults, these regions are lateralized to the language-dominant left hemisphere, and include inferior frontal, superior and middle temporal, and temporo-parietal regions [5]. In addition, experienced readers recruit an area of the left fusiform gyrus, termed the visual word form area (VWFA), which becomes preferentially engaged for orthographic (print) processing with reading experience [6, 7, 8•]. This reading network (Figure 1) develops over years as children gain both specific reading skills and other abilities relevant to reading (e.g., [9]). White-matter pathways that connect the components of the reading network can be quantified in size and strength by diffusion tensor imaging (DTI). Major tracts involved in reading include the left arcuate/superior longitudinal fasciculus, which connects frontal and temporal language regions, the inferior longitudinal fasciculus, which connects occipital and temporal lobes, and the corona radiata, which connects cortex to subcortical structures [10].
Section snippets
Psychological bases of dyslexia
Because reading involves multiple linguistic, visual, and attentional processes, it is probable that variable patterns of weakness may contribute to reading difficulty across children. Although it is unlikely that there is a single causal mechanism of dyslexia, some frequent likely causes have been identified (Table 1). The best understood cause for dyslexia is a weakness in phonological awareness (PA) for spoken (auditory) language that predicts and accompanies dyslexia [11]. Whereas learning
Functional and structural brain differences in dyslexia
Meta-analyses of primary research findings have identified broad patterns of functional and structural differences between typical and dyslexic readers. The most common functional brain differences, in children and adults, are reduced activations (hypoactivations) in left temporal, parietal, and fusiform (VWFA) regions [19, 20, 21, 22]. In most cases, these hypoactivations arise from comparisons between two tasks or conditions, and thus reflect a lack of differential sensitivity to reading
Brain basis of phonological awareness (PA) deficits
Impaired PA in dyslexia could reflect either a deficit in representing phonetic sounds and/or a deficit in access to and manipulation of those sounds (e.g., for mapping phonemes to print). Previously, a review of behavioral studies of dyslexia concluded that phonetic representations are intact, but access to those representations may be impaired [41]. Recently, a neuroimaging study with adults found that phonetic representations, as measured by multivoxel pattern analysis of activations in
Conclusion
Progress in understanding the cognitive neuroscience of dyslexia may be approaching translation from basic research to intervention for children who will struggle to read. Remediation is known to be most effective in beginning readers, so early and accurate identification may promote effective intervention for children before they experience prolonged reading failure. Neuroimaging has identified biomarkers that enhance or outperform current behavioral measures in predicting long-term reading
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgments
This work was supported by a grant from the National Institutes of Health/National Institute of Child Health and Human Development (Grant #R01 HD067312). We thank Rachel Romeo for assistance with manuscript preparation.
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