Abstract
The neural correlates of developmental dyslexia have been investigated intensively over the last two decades and reliable evidence for a dysfunction of left-hemispheric reading systems in dyslexic readers has been found in functional neuroimaging studies. In addition, structural imaging studies using voxel-based morphometry (VBM) demonstrated grey matter reductions in dyslexics in several brain regions. To objectively assess the consistency of these findings, we performed activation likelihood estimation (ALE) meta-analysis on nine published VBM studies reporting 62 foci of grey matter reduction in dyslexic readers. We found six significant clusters of convergence in bilateral temporo-parietal and left occipito-temporal cortical regions and in the cerebellum bilaterally. To identify possible overlaps between structural and functional deviations in dyslexic readers, we conducted additional ALE meta-analyses of imaging studies reporting functional underactivations (125 foci from 24 studies) or overactivations (95 foci from 11 studies ) in dyslexics. Subsequent conjunction analyses revealed overlaps between the results of the VBM meta-analysis and the meta-analysis of functional underactivations in the fusiform and supramarginal gyri of the left hemisphere. An overlap between VBM results and the meta-analysis of functional overactivations was found in the left cerebellum. The results of our study provide evidence for consistent grey matter variations bilaterally in the dyslexic brain and substantial overlap of these structural variations with functional abnormalities in left hemispheric regions.
Highlights
Developmental dyslexia is a severe difficulty in learning to read accurately and fluently that affects 5–17% of all children and often persists into adulthood (e.g., [1])
The present study aims at closing this gap by quantitatively meta-analyzing the results of published voxel-based morphometry (VBM) studies comparing dyslexic and control participants
activation likelihood estimation (ALE) Meta-Analysis of VBM Studies In the meta-analysis of VBM studies reporting grey matter reductions in dyslexic readers, we found six clusters of significant convergence between the studies
Summary
Developmental dyslexia is a severe difficulty in learning to read accurately and fluently that affects 5–17% of all children and often persists into adulthood (e.g., [1]). The most widely accepted explanation for the origin of this disorder is an underlying deficit in the representation and processing of speech sounds [2,3,4]. This phonological deficit is associated with deficient graphemephoneme decoding skills that are crucial in the beginning stages of reading development. Phonological deficits can be found in the majority of persons with reading difficulties, dyslexia is a heterogeneous condition [5,6]. Several other sensorimotor deficits have been associated with the disorder resulting in the emergence of alternative theories describing phonological deficits as secondary consequences of basal auditory [7], visual [8], attentional [9] and/or motor problems [10]
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