A Study On The Lateralization Defect Of Brain Structure In Children With Developmental Dyslexia

Reading activity requires activation in multiple brain regions and efficient communication among them.The brain mechanism of developmental dyslexia has been investigated for a long term.Specifically,anomalies in gray matter morphology and connectivity of white matter pathways have been demonstrated as brain phenotypes of developmental dyslexia.Studies of brain function and structural image have confirmed that there are differences in reading-related white matter between normal and dyslexic readers.However,cortical thickness,which is a sensitive metric of dynamic brain changes throughout development stemming from both maturational pruning and experiential neuroplasticity,did not show consistence among findings about dyslexia.In addition,structural asymmetry has been seen as an alternative biomarker for dyslexia,as reading activity depends more on the cooperative work of the left hemisphere including temporo-parietal and occipito-parietal regions.Studies have found that dyslexic children showed a reduced leftward asymmetry of the inferior fronto-occipital fasciculus(IFOF)and an increased rightward asymmetry of the second branch of the SLF(SLF?)compared with age-matched controls.The lateralization pattern of IFOF and SLF ? also accounts for individual differences in dyslexic children's reading abilities.Until now,no studies have systematically investigated atypical pattern of reading-related white matter pathways and cortical thickness of grey matter regions which are connected by those white matter pathways.Moreover,the relationship between asymmetry patterns in grey and white matter,as well as their association with reading abilities and between modality,have not been well explored yet.In terms of these questions,the present study selected five regions of interests,including inferior frontal gyrus(IFG),middle frontal gyrus(MFG),inferior parietal gyrus(IPG),superior temporal gyrus(STG),and fusiform gyrus for their cortical thickness,and two white matter pathways connecting those five regions:sagittal stratum(SS)and superior longitudinal fasciculus(SLF)for their fractional anisotropy(FA).We investigated asymmetry patterns in these regions and pathways in dyslexic children.First,the FA value of the white matter pathway and cortical thickness of 57 participants were calculated using tract-based spatial statistics(TBSS).A 2*2(group vs hemisphere)design was performed to analyze the differences between groups and hemisphere for these target regions and pathways.The asymmetry pattern of those regions and pathways in dyslexic and control group were then explored separately.Also,group differences of those asymmetry patterns based on lateralization indexes were tested.Finally,partial Pearson correlation was applied to examine the association between lateralization indexes of gray and white matter with reading performance in dyslexic group.Pearson correlation was also used to test relationship between lateralization indexes of white and grey matter.We have found that the group by hemisphere interaction was significant on IFG,MFG and sagittal stratum.IPG,MFG and IFG exhibited significant leftward lateralization only in control group.All white matter pathways showed leftward lateralization both in control and dyslexic group.That is,the differences between inter-hemisphere in control more than dyslexia.In addition,the lateralization index between two groups differ significantly in IFG,MFG and sagittal stratum.Specifically,dyslexic children showed decreased leftward lateralization in IFG,MFG and sagittal stratum.Correlation analyses showed that in dyslexic group,the lateralization index of sagittal stratum was negatively correlated with word reading accuracy and word spelling accuracy,and the lateralization index of IFG was positively correlated with phonological ability.The lateralization index of superior longitudinal fasciculus was positively correlated with the lateralization indexes of IFG,MFG and STG.The present study first demonstrates atypical asymmetry pattern of cortical thickness in frontal lobe in developmental dyslexia,accordingly identifying a new brain marker for diagnosis of dyslexia.Second,the present study replicated previous finding on altered asymmetry pattern in white matter pathways with the same data sample using a different calculation method,confirming the abnormal white matter pathway lateralization in dyslexia.Results from the present study also provide a new lateralization physiological indicator for reading disabilities.