Brain morphology of Chinese developmental dyslexia

Abstract

Developmental dyslexia is characterized by inaccurate, slow and effortful reading in people who have normal intelligence and schooling. In functional neuroimaging studies of alphabetic reading, developmental dyslexia is found to be associated with reduced neural activity in left temporoparietal regions, which perform phonemic analysis and conversion of written symbols to phonological units of speech (grapheme-to-phoneme conversion). The reduced neural activity in left temporoparietal regions is likely a consequence of reduced grey matter density in dyslexics relative to normal controls (Hoeft et al., 2007). This pivotal role of left temporoparietal regions in reading, however, was not observed in Chinese reading and dyslexia. Chinese as a logographic writing system presents a sharp contrast with alphabetic writing systems. Chinese characters map onto the morpheme (meaning) and a monosyllable and cannot be pronounced by recourse to grapheme-phoneme correspondence rules. Thus, a Chinese character has a more direct association with its meaning than a written word in English does (Wang, 1973). Past brain mapping research has indicated that reading Chinese is served by a distinct neural system involving the left middle frontal gyrus (LMFG) and that functional disruption of the LMFG is associated with impaired reading of the Chinese language (Siok et al., 2004). The LMFG functions as a center for fluent Chinese reading that coordinates and integrates various information about written characters in verbal and spatial working memory. It is not certain, however, if Chinese dyslexics exhibit abnormal brain morphology in the LMFG. In the current study, we measured brain activation by functional MRI during visual rhyme judgment compared with font-size judgment, and we measured grey matter morphology by voxel-based morphometry in 10 Chinese dyslexic children (mean age=11 year 0 month) and 10 Chinese normal-reading children (mean age=10 year 11 months). Functional imaging results showed that dyslexic children exhibited weaker neural activity in the LMFG relative to normal-reading children. Voxel-based morphometry revealed that grey-matter density in the same region was smaller in dyslexics than in normal-reading children. This pattern of findings confirms the central importance of the LMFG in Chinese reading development and dyslexia.