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Callosal pathways associated with reading and phonological awareness in children

RF Dougherty, GK Deutsch, M Ben-Shachar, P Potanina, R Bammer and BA Wandell
Stanford Institute for Reading and Learning

Introduction: Several reports have identified white matter differences related to reading ability in adults (Klingberg 2000) and children (Deutsch 2005; Beaulieu 2005; Niogi 2005) using diffusion tensor imaging (DTI). The locations of these differences- in the internal capsule and corona radiata- are puzzling because these structures are not thought to include important language pathways. We proposed that these differences may be an indirect consequence of differences in callosal pathways implicated in acquired alexia (Dougherty 2003). Here we use a region-of-interest approach to test this hypothesis.
Methods: We used DTI to measure fractional anisotropy (FA) in 54 children aged 7 to 12. We also measured reading, phonological awareness (PA), and Full-Scale IQ. These scores were compared to FA in four callosal segments: the splenium, isthmus, body and genu/rostrum. These segments were created by manual selection of the corpus callosum in the mid-sagittal plane followed by automated fiber tracking to identify callosal tracts up to 10mm from the mid-sagittal plane.
Results: Of the four callosal segments, only the splenium showed a significant correlation between FA and behavioral measures. Mean FA in the splenium was negatively correlated with all the reading measures and with PA, but not with IQ. Of the reading measures, passage comprehension (r=-0.35, p<0.01) and PA (r=-0.46, p<0.001) were the most strongly correlated with FA. The correlations were strongest near the mid-sagittal plane, but also present in the splenial pathways 10mm to either side.
Conclusion: FA increases with myelination, axon density, axon size and directional coherence. While we cannot disentangle these causes, the results suggest increased hemispheric connectivity between posterior cortical regions in poor readers, perhaps due to a failure to prune these long-range connections. Such an anatomical difference is consistent with imaging results that suggest a less lateralized language system in poor readers.
Support Contributed By: NIH EY-015000

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