Dyslexia is a severe and persistent reading and spelling disorder caused by impairment to manipulate speech sounds. Here, we combine functional magnetic resonance brain imaging with multi-voxel pattern analysis and functional and structural connectivity analysis to disentangle whether dyslexics' phonological deficits are caused by poor quality of the phonetic representations or by difficulties in accessing intact phonetic representations. We show that phonetic representations are hosted bilaterally in primary and secondary auditory cortices, and that their neural quality is intact in adults with dyslexia. However, the functional and structural connectivity between bilateral auditory cortices and left inferior frontal gyrus (a region involved in higher-level phonological processing) is significantly hampered in dyslexics, suggesting deficient access to otherwise intact phonetic representations.Speech perception involves the mapping of spectrally complex and rapidly changing acoustic signals onto discrete and abstract phonetic sound categories or phonemes (1). Developmental dyslexia is a hereditary neurological disorder characterized by severe and persistent reading and/or spelling impairments (2). Individuals with dyslexia perform poorly on tasks that require phonological awareness, verbal short-term memory and lexical access. Performance on these phonological tasks predicts reading acquisition in both normal and dyslexic readers (3). One view is that success on these tasks reflects the quality of underlying phonological (phonetic) representations (4) and that these representations of speech sounds are distorted or less well specified in individuals with dyslexia (5). An alternative view holds that representations are intact, but access to the representations is problematic in people with dyslexia (6, 7). Here, we combine functional magnetic resonance imaging (fMRI) with multi-voxel pattern analysis (MVPA) (8-10) and functional and structural connectivity analysis to disentangle whether dyslexia is caused by poor quality of the phonetic representation or by difficulties in accessing an intact phonetic representation.We collected whole-brain functional images in 23 adults with a diagnosis of dyslexia and 22 matched normal readers (Table S1, 11-13), while they listened to different versions of four sublexical speech sounds (Fig. S1) and performed an easy phoneme discrimination task. The selection of stimuli allowed us to investigate both vowel and stop consonant discrimination, which relies on spectral versus spectrotemporal acoustic feature processing, respectively. If dyslexia is related to a deficit in the quality of phonetic representations, then we expect that the neural representations would be less robust and distinct in individuals with dyslexia than in normal readers. Given dyslexics' particular problems processing temporal cues, such as those involved in consonant discrimination (11), we expected the most prominent group differences for neural patterns distinguishing between consonants.We analyzed...

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Diffusion tensor imaging tractography is a structural magnetic resonance imaging technique allowing reconstruction and assessment of the integrity of three dimensional white matter tracts, as indexed by their fractional anisotropy. It is assumed that the left arcuate fasciculus plays a crucial role for reading development, as it connects two regions of the reading network, the left temporoparietal region and the left inferior frontal gyrus, for which atypical functional activation and lower fractional anisotropy values have been reported in dyslexic readers. In addition, we explored the potential role of the left inferior fronto-occipital fasciculus, which might connect a third region of the reading network, the left ventral occipitotemporal region with the left inferior frontal gyrus. In the present study, 20 adults with dyslexia and 20 typical reading adults were scanned using diffusion tensor imaging, and the bilateral arcuate fasciculus and the left inferior fronto-occipital fasciculus were delineated. Group comparisons show a significantly reduced fractional anisotropy in the left arcuate fasciculus of adults with dyslexia, in particular in the segment that directly connects posterior temporal and frontal areas. This fractional anisotropy reduction might reflect a lower degree of myelination in the dyslexic sample, as it co-occurred with a group difference in radial diffusivity. In contrast, no significant group differences in fractional anisotropy were found in the right arcuate fasciculus or in the left inferior fronto-occipital fasciculus. Correlational analyses (controlled for reading status) demonstrated a specific relation between performance on phoneme awareness and speech perception and the integrity of left arcuate fasciculus as indexed by fractional anisotropy, and between orthographic processing and fractional anisotropy values in left inferior fronto-occipital fasciculus. The present study reveals structural anomalies in the left arcuate fasciculus in adults with dyslexia. This finding corroborates current hypotheses of dyslexia as a disorder of network connections. In addition, our study demonstrates a correlational double dissociation, which might reflect neuroanatomical correlates of the dual route reading model: the left arcuate fasciculus seems to sustain the dorsal phonological route underlying grapheme-phoneme decoding, while the left inferior fronto-occipital fasciculus seems to sustain the ventral orthographic route underlying reading by direct word access.

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In adults and school-aged children, phonological aspects of reading seem to be sustained by left dorsal regions, while ventral regions seem to be involved in orthographic word recognition. Yet, given that the brain reorganises during reading acquisition, it is unknown when and how these reading routes emerge and whether neural deficits in dyslexia predate reading onset. Using diffusion MRI in 36 pre-readers with a family risk for dyslexia (FRD(+)) and 35 well matched pre-readers without a family risk (FRD(-)), our results show that phonological predictors of reading are sustained bilaterally by both ventral and dorsal tracts. This suggests that a dorsal and left-hemispheric specialisation for phonological aspects of reading, as observed in adults, is presumably gradually formed throughout reading development. Second, our results indicate that FRD(+) pre-readers display mainly white matter differences in left ventral tracts. This suggests that atypical white matter organisation previously found in dyslexic adults may be causal rather than resulting from a lifetime of reading difficulties, and that the location of such a deficit may vary throughout development. While this study forms an important starting point, longitudinal follow-up of these children will allow further investigation of the dynamics between emerging literacy development and white matter connections.