Two reading routes, two white matter tracts: Evidence from dyslexia following brain tumors

Cognitive models propose that reading aloud relies on two routes, a lexical route for familiar word recognition and a sublexical route for graphene-to-phoneme conversion. Selective disruption of these routes results in surface or phonological dyslexia, respectively. Although the dual route model is well supported behaviorally, its white matter substrates remain incompletely characterized. This study examined the anatomical correlates of acquired lexical and sublexical reading impairments in patients with high-grade glioma in the dominant hemisphere. Thirty-seven patients underwent diffusion tensor imaging (DTI) and a comprehensive reading battery. Reading aloud of regular and irregular words, and pseudowords was used to classify patients as having intact reading, surface dyslexia or phonological dyslexia, based on normative criteria. Six language-related white matter tracts were reconstructed individually for each patient using subject-specific tractography. Each tract was then systematically segmented along its longitudinal axis, allowing diffusion parameters to be extracted from anatomically defined subsegments. Lesion-tract overlap measures were computed to quantify focal tract involvement. Fourteen patients exhibited impaired sublexical reading, 12 impaired lexical reading, and 11 intact reading. Deficits in the sublexical route were more frequent among patients with arcuate fasciculus lesion overlap, and sublexical error rates correlated with global tract integrity. Deficits in the lexical route were associated with the posterior temporal segment of the inferior longitudinal fasciculus, where surface dyslexia errors correlated with reduced fractional anisotropy. In contrast, uncinate fasciculus involvement was more frequent among patients with intact reading, indicating it does not contribute critically to either reading route. These findings provide a fine-grained structure-function mapping of the dual route reading model onto white matter pathways.