Functional and morphometric brain dissociation between dyslexia and reading ability

Abstract

In functional neuroimaging studies, individuals with dyslexia frequently exhibit both hypoactivation, often in the left parietotemporal cortex, and hyperactivation, often in the left inferior frontal cortex, but there has been no evidence to suggest how to interpret the differential relations of hypoactivation and hyperactivation to dyslexia. To address this question, we measured brain activation by functional MRI during visual word rhyme judgment compared with visual cross-hair fixation rest, and we measured gray matter morphology by voxel-based morphometry in dyslexic adolescents in comparison with (i) an age-matched group, and (ii) a readingmatched group younger than the dyslexic group but equal to the dyslexic group in reading performance. Relative to the agematched group (n ‫؍‬ 19; mean 14.4 years), the dyslexic group (n ‫؍‬ 19; mean 14.4 years) exhibited hypoactivation in left parietal and bilateral fusiform cortices and hyperactivation in left inferior and middle frontal gyri, caudate, and thalamus. Relative to the readingmatched group (n ‫؍‬ 12; mean 9.8 years), the dyslexic group (n ‫؍‬ 12; mean 14.5 years) also exhibited hypoactivation in left parietal and fusiform regions but equal activation in all four areas that had exhibited hyperactivation relative to age-matched controls as well. In regions that exhibited atypical activation in the dyslexic group, only the left parietal region exhibited reduced gray matter volume relative to both control groups. Thus, areas of hyperactivation in dyslexia reflected processes related to the level of current reading ability independent of dyslexia. In contrast, areas of hypoactivation in dyslexia reflected functional atypicalities related to dyslexia itself, independent of current reading ability, and related to atypical brain morphology in dyslexia.inferior frontal region ͉ inferior parietal lobule ͉ voxel-based morphometry ͉ functional MRI ͉ compensation D yslexia is a developmental condition characterized by low reading achievement in people who otherwise have cognitive abilities, motivation, and education necessary for accurate and fluent reading (1). Dyslexia, estimated to affect 5-17% of children and 80% of all individuals with a learning disability (2, 3), is characterized by inaccurate and/or slow, effortful reading that typically originates with weakness in the phonological processing of language (4-8).The brain basis of dyslexia has been examined by functional and structural neuroimaging. Functional imaging studies regularly report hypoactivation in dyslexia, especially in the left parietotemporal region, which may support the mapping of phonology onto orthography, and in the left fusiform region, which may support skilled orthographic decoding (9-12). Hyperactivation in dyslexia has also been observed, most frequently in left inferior frontal gyrus (IFG) (13)(14)(15)(16)(17)(18)(19). Hyperactivation in left IFG, a region associated with articulation and naming (20), may reflect compensatory processes engaged by dyslexic individuals attempting to overcome...

Functional characteristics of developmental dyslexia in left-hemispheric posterior brain regions predate reading onset

“…Converging evidence from many neuroimaging studies point toward a characteristic hypoactivation of left-hemispheric temporoparietal (44,46,(48)(49)(50)(51) and occipitotemporal (44,(46)(47)(48)50) brain regions in children and adults with DD compared with typical reading controls. Furthermore, reduced gray-matter volume indices in temporoparietal and occipitotemporal brain regions have been reported in prereading children at risk for DD compared with their peers (62).…”

“…Crosssectional studies have demonstrated changes in these highly integrated reading networks depending on reading skill level (e.g., refs. 44 and 45) and converging evidence points toward a characteristic hypoactivation of temporoparietal as well as occipitotemporal brain areas in individuals with DD (44,(46)(47)(48)(49)(50)(51)(52)(53)(54)(55).…”

“…Furthermore, an increase in activation in left frontal and right lateralized anterior brain areas has been shown in individuals with DD (44,45,49,50,63). This hyperactivation seen in individuals with DD has been suggested to reflect a compensatory mechanism for the dysfunctional reading system (e.g., refs.…”

“…Voxel-based morphometry reveals differences in gray-matter volume indices in individuals with DD (compared with typical reading controls) in various areas of the brain, including left occipitotemporal and temporoparietal areas (44,(56)(57)(58)(59)(60)(61), bilateral fusiform (59), and lingual gyrus (LG) (58) as well as the cerebellum (56-58). Morphological abnormalities in these regions can be identified even before reading skills are present in children as young as 5 to 6 y of age, suggesting atypical early development or even a genetic basis for DD (62).…”

The practical and principled problems with educational neuroscience.

“…First consider an fMRI by Hoeft et al (2007). The authors tested children between 8 and 12 years of age identified by teachers at the start of a school year as EDUCATIONAL NEUROSCIENCE 30 being at risk for reading difficulty.…”