Anomalies in White Matter Tracts Sustaining Phonological Processing Predate Reading

Cause or consequence of dyslexia? Anomalies in white matter tracts sustaining phonological processing predate reading

Vandermosten Maaike1, 2, Vanderauwera Jolijn1, 2, Theys Catherine3, Sunaert Stefan3, Wouters Jan2, & Ghesquière Pol1.

1 Parenting and Special Education Research Unit, KU Leuven (Belgium) 2 Experimental ORL, Dept. Neuroscience, KU Leuven (Belgium) 3 Department of Radiology, University Hospital Leuven (Belgium)

Abstract Converging neuroimaging research indicates that dyslexic readers exhibit a left-lateralized neural deficit in dorsal (i.e. temporoparietal) and ventral (i.e. occipitotemporal) grey matter regions as well as in dorsal white matter connections (i.e. arcuate fasciculus). The standard neuroanatomical model of dyslexia localizes the primary phonological decoding deficit in left dorsal regions, with a secondary deficit in building up orthographic word representations located in left ventral regions. However, this model is based on neuroimaging studies that generally involve older children and adults, hence evidence is lacking on whether a dorsal deficit indeed predates a ventral deficit and to what extent these neural deficits are a consequence or cause of reading difficulties. To fill this gap, we have set up a longitudinal study in which we aim to investigate how the dorsal (arcuate fasciculus, AF) and ventral (inferior-fronto-occipital fasciculus, IFOF) white matter tract develop during the different stages of reading acquisition and whether distorted brain connectivity in dyslexics is present already prior to reading, hence suggesting a causal influence. Using diffusion tensor imaging, we currently scanned 36 pre-readers with a family risk for dyslexia (FRD+) and 34 individually-matched pre-readers without a family risk for dyslexia (FRD -). The bilateral AF, split up in its anterior, posterior and long segment, as well as the bilateral IFOF were individually delineated. Mixed model analyses, taking into account that pairs of children attended the same class, showed abnormal white matter organization for the FRD+-group in left IFOF (F(1,26) = 9.87, p = .004) and the posterior segment of left AF (F(1, 26) = 5.14, p = .032). In addition, white matter organization in bilateral IFOF correlated with phonological awareness (left: r = .35, p = .003; right r = .37, p = .002), lexical retrieval (right: r = .31, p = .009) and letter knowledge (left: r = .26, p = .031; right r = .26, p = .027), which are known to be the most significant predictors of later reading skills. In addition, phoneme awareness correlated with the posterior (r = .29, p = .016) and long (r = .29, p = .016) segment of left AF. These relations with bilateral IFOF and left AF seem to be mainly driven by phoneme awareness as this was the only cognitive variable that could predict unique variance in white matter organization above the others. Although analyses should be reran when participants can later be classified as dyslexic, this DTI-study suggests that white matter alternations are fundamental to dyslexia and cannot solely be the result of reading failure itself. In contrast to the standard neuroanatomcial model on dyslexia and DTI-results in adults (Vandermosten et al., Brain, 2012), the primary deficit seem not to be predominantly located in dorsal connections. Furthermore, the ventral and dorsal tracts have not yet developed a specialized function for phonological versus orthographic aspects of reading. Both tracts probably closely interact with each other to achieve the complex process of reading acquisition.