Abstract
The present study investigated whether children with a typical dyslexia profile and children with isolated spelling deficits show a distinct pattern of white matter alteration compared with typically developing peers. Relevant studies on the topic are scarce, rely on small samples, and often suffer from the limitations of conventional tensor‐based methods. The present Constrained Spherical Deconvolution study includes 27 children with typical reading and spelling skills, 21 children with dyslexia and 21 children with isolated spelling deficits. Group differences along major white matter tracts were quantified utilizing the Automated Fiber Quantification software and a lateralization index was calculated in order to investigate the structural asymmetry of the tracts. The two deficit groups mostly displayed different patterns of white matter alterations, located in the bilateral inferior longitudinal fasciculi, right superior longitudinal fasciculus, and cingulum for the group with dyslexia and in the left arcuate fasciculus for the group with isolated spelling deficits. The two deficit groups differed also with respect to structural asymmetry. Children with dyslexia did not show the typical leftward asymmetry of the arcuate fasciculus, whereas the group with isolated spelling deficits showed absent rightward asymmetry of the inferior fronto‐occipital fasciculus. This study adds evidence to the notion that different profiles of combined or isolated reading and spelling deficits are associated with different neural signatures.
Highlights
Cognitive models of reading (Coltheart, Rastle, Perry, Langdon, & Ziegler, 2001; Perry, Ziegler, & Zorzi, 2007) describe two main reading routes: The sublexical route, based on grapheme-phoneme conversion rules for decoding unknown words and pseudowords and the lexical route, accessing the orthographic long-term memory to read known regular or irregular words
Based on the anatomical location of the two routes, Vandermosten, Boets, Wouters, and Ghesquière (2012) hypothesized that the dorsal route might involve the arcuate fasciculus (AF) and the superior longitudinal fasciculus (SLF), while the ventral route might overlap with the inferior fronto-occipital fasciculus (IFOF) and the inferior longitudinal fasciculus (ILF)
Among other tensor-based parameters, we focused on fractional anisotropy (FA), which was calculated on 100 nodes in each delineated tract: right and left thalamic radiations, forceps major and minor of corpus callosum, right and left inferior fronto-occipital, inferior longitudinal, superior longitudinal, arcuate and uncinate fasciculi, corticospinal tract, and cingulum
Summary
Cognitive models of reading (Coltheart, Rastle, Perry, Langdon, & Ziegler, 2001; Perry, Ziegler, & Zorzi, 2007) describe two main reading routes: The sublexical route, based on grapheme-phoneme conversion rules for decoding unknown words and pseudowords and the lexical route, accessing the orthographic long-term memory to read known regular or irregular words. Enzinger, et al (2012) found no structural white matter differences compared with a typically developing control group, while Gebauer, Fink, Filippini, et al (2012) identified several clusters of reduced FA in the right hemisphere They were interpreted as less efficient connectivity in right white matter pathways, most likely related to functional overactivity in right-hemisphere regions, mirroring inefficient cognitive compensatory strategies (Gebauer et al, 2012). The present study investigated alterations in reading-related white matter tracts in reasonably large and carefully screened children with dyslexia and isolated spelling deficits. The degree of structural white matter asymmetry was analyzed in order to investigate whether altered tract lateralization is a specific feature related to dyslexia or whether it characterizes white matter organization of children with isolated spelling disorder as well
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