The angular gyrus in developmental dyslexia: task-specific differences in functional connectivity within posterior cortex.
ABSTRACT Converging evidence from neuroimaging studies of developmental dyslexia reveals dysfunction at posterior brain regions centered in and around the angular gyrus in the left hemisphere. We examined functional connectivity (covariance) between the angular gyrus and related occipital and temporal lobe sites, across a series of print tasks that systematically varied demands on phonological assembly. Results indicate that for dyslexic readers a disruption in functional connectivity in the language-dominant left hemisphere is confined to those tasks that make explicit demands on assembly. In contrast, on print tasks that do not require phonological assembly, functional connectivity is strong for both dyslexic and nonimpaired readers. The findings support the view that neurobiological anomalies in developmental dyslexia are largely confined to the phonological-processing domain. In addition, the findings suggest that right-hemisphere posterior regions serve a compensatory role in mediating phonological performance in dyslexic readers.
Article: Low-frequency signal changes reflect differences in functional connectivity between good readers and dyslexics during continuous phoneme mapping.[show abstract] [hide abstract]
ABSTRACT: The current fMRI study investigated correlations of low-frequency signal changes in the left inferior frontal gyrus, right inferior frontal gyrus and cerebellum in 13 adult dyslexic and 10 normal readers to examine functional networks associated with these regions. The extent of these networks to regions associated with phonological processing (frontal gyrus, occipital gyrus, angular gyrus, inferior temporal gyrus, fusiform gyrus, supramarginal gyrus and cerebellum) was compared between good and dyslexic readers. Analysis of correlations in low-frequency range showed that regions known to activate during an "on-off" phoneme-mapping task exhibit synchronous signal changes when the task is administered continuously (without any "off" periods). Results showed that three functional networks, which were defined on the basis of documented structural deficits in dyslexics and included regions associated with phonological processing, differed significantly in spatial extent between good readers and dyslexics. The methodological, theoretical and clinical significance of the findings for advancing fMRI research and knowledge of dyslexia are discussed.Magnetic Resonance Imaging 05/2006; 24(3):217-29. · 1.99 Impact Factor
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ABSTRACT: In neuroimaging, connectivity refers to the correlations between signals in different brain regions. Although fMRI measures of connectivity have been widely explored, the methods used have varied. This complicates the interpretation of existing literature in cases when different techniques have been used with fMRI data to measure the single concept of "connectivity." Additionally the optimum choice of method for future analyses is often unclear. In this study, measures of functional and effective connectivity in the motor system were calculated based on three sources of variation: inter-subject variation in task activation level; within-subject variation in task-related responses; and within-subject residual variation after removal of task effects. Two task conditions were compared. The methods yielded different inter-regional correlation coefficients. However, all three approaches produced similar results, qualitatively and sometimes quantitatively, for condition differences in connectivity. While these results are specific to the motor regions studied, they do suggest that within-subject and across-subject results may be usefully compared. Also, the presence of task-specific correlations in residual time series supports arguments that residuals may not substitute for resting-state data, but rather may reflect the same underlying variations present during steady-state performance.PLoS ONE 02/2008; 3(11):e3708. · 4.09 Impact Factor
Article: Children with reading disability show brain differences in effective connectivity for visual, but not auditory word comprehension.[show abstract] [hide abstract]
ABSTRACT: Previous literature suggests that those with reading disability (RD) have more pronounced deficits during semantic processing in reading as compared to listening comprehension. This discrepancy has been supported by recent neuroimaging studies showing abnormal activity in RD during semantic processing in the visual but not in the auditory modality. Whether effective connectivity between brain regions in RD could also show this pattern of discrepancy has not been investigated. Children (8- to 14-year-olds) were given a semantic task in the visual and auditory modality that required an association judgment as to whether two sequentially presented words were associated. Effective connectivity was investigated using Dynamic Causal Modeling (DCM) on functional magnetic resonance imaging (fMRI) data. Bayesian Model Selection (BMS) was used separately for each modality to find a winning family of DCM models separately for typically developing (TD) and RD children. BMS yielded the same winning family with modulatory effects on bottom-up connections from the input regions to middle temporal gyrus (MTG) and inferior frontal gyrus(IFG) with inconclusive evidence regarding top-down modulations. Bayesian Model Averaging (BMA) was thus conducted across models in this winning family and compared across groups. The bottom-up effect from the fusiform gyrus (FG) to MTG rather than the top-down effect from IFG to MTG was stronger in TD compared to RD for the visual modality. The stronger bottom-up influence in TD was only evident for related word pairs but not for unrelated pairs. No group differences were noted in the auditory modality. This study revealed a modality-specific deficit for children with RD in bottom-up effective connectivity from orthographic to semantic processing regions. There were no group differences in connectivity from frontal regions, suggesting that the core deficit in RD is not in top-down modulation.PLoS ONE 01/2010; 5(10):e13492. · 4.09 Impact Factor