Abstract
Developmental dyslexia can be distinguished as different cognitive subtypes with and without phonological deficits. However, despite some general agreement on the neurobiological basis of dyslexia, the neurofunctional mechanisms underlying these cognitive subtypes remain to be identified. The present BOLD fMRI study thus aimed at investigating by which distinct and/or shared neural activation patterns dyslexia subtypes are characterized. German dyslexic fourth graders with and without deficits in phonological awareness and age-matched normal readers performed a phonological decision task: does the auditory word contain the phoneme/a/? Both dyslexic subtypes showed increased activation in the right cerebellum (Lobule IV) compared to controls. Subtype-specific increased activation was systematically found for the phonological dyslexics as compared to those without this deficit and controls in the left inferior frontal gyrus (area 44: phonological segmentation), the left SMA (area 6), the left precentral gyrus (area 6) and the right insula. Non-phonological dyslexics revealed subtype-specific increased activation in the left supramarginal gyrus (area PFcm; phonological storage) and angular gyrus (area PGp). The study thus provides the first direct evidence for the neurobiological grounding of dyslexia subtypes. Moreover, the data contribute to a better understanding of the frequently encountered heterogeneous neuroimaging results in the field of dyslexia.
Highlights
The present paper investigates the neurobiological underpinnings of reading problems in developmental dyslexia
The fMRI study reported here sought to elucidate the neurobiological differences between different subtypes of dyslexic children with vs. without additional deficits in phonological awareness and to assess commonalities between dyslexia subtypes
The present study found increased activation for phonological dyslexics compared to non-phonological dyslexics in the right anterior dorsal insula, whereas better phonological awareness corresponded to lower activation intensity in this area
Summary
The present paper investigates the neurobiological underpinnings of reading problems in developmental dyslexia. The fMRI study reported here sought to elucidate the neurobiological differences between different subtypes of dyslexic children with vs without additional deficits in phonological awareness and to assess commonalities between dyslexia subtypes. We first give an introduction to cognitive models of dyslexia and recent findings on dyslexia subtypes. We review the literature on the overall neurobiological foundation of dyslexia, with a particular focus on the heterogeneity of findings. It will be argued that this heterogeneity may be the consequence of mixing different dyslexia subtypes in neuroimaging studies and of using paradigms which are more or less directly related to the deficits of.
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