Resumen

Introduction: Language processing in the adult neurotypical brain is subserved by several white-matter pathways which connect inferior frontal, temporal, and parietal language-relevant cortical regions. Here, we used diffusion-weighted MRI to compare the macro- and micro-structural properties of the core and extended language network in a group of deaf signers and a control group hearing non-signers. Methods: This analysis includes diffusion-weighted and structural data from 24 deaf signers (11 male; mean age = 32.25, SD = 8.69) who acquired a sign language already early in life (mean age of acquisition = 1.29, SD = 2.22) and primarily from their deaf parents (N = 19), as well as 24 hearing non-signers (11 male; mean age = 32.75, SD = 8.25). All data were pre-processed using the standardized workflow implemented in QSIPrep (Cieslak et al., 2021) which integrates advanced quality control measures. We then used pyAFQ (Garyfallidis et al., 2014; Kruper et al., 2021; Yeatman et al., 2012) to perform automatic fiber quantification for generating macro- and micro-structural information derived from diffusion data for the core and extended language network and its right-hemispheric homologues (i.e., bilateral arcuate fasciculus, superior longitudinal fasciculus, posterior arcuate fasciculus, inferior fronto-occipital fasciculus, uncinate fasciculus, and the inferior longitudinal fasciculus). The pyAFQ workflow constitutes a standardized multi-step process that produces robust axonal models and tract profiles. Macro-structural differences in language-related pathways between groups were quantified by extracting the cleaned streamline counts and fitting a linear mixed-effects model accounting for possible individual-level variation in R (R Core Team, 2024). Macro-structural asymmetry of language-related pathways in both groups of participants was determined by computing a lateralization index (LI) for streamline counts (S) for every pathway as follows: LI = (Sleft – Sright) / (Sleft + Sright). In addition, we also quantified for every tract micro-structural differences in fractional anisotropy (FA), mean diffusivity (MD), and radial diffusivity (RD) by fitting linear mixed-effects models for every pyAFQ tract segment and determining significant clusters using a permutation test procedure. Results: We observed no statistically significant macro-structural differences between deaf signers and hearing non-signers in the core language network (i.e., left arcuate fasciculus) and any of the language-related white-matter pathways in the left hemisphere, in line with earlier work (Finkl. et al., 2019; Cheng et al., 2019). However, we found a significant difference in the total number of streamlines between groups (higher in the group of deaf signers) for the right posterior arcuate fasciculus connecting parietal and posterior temporal regions. The lateralization analysis indicated similar lateralization profiles for all pathways across groups. We also observed the following micro-structural group differences: Left and right uncinate fasciculus differed in FA and MD respectively. Both right arcuate and right superior longitudinal fasciculus differed in FA as well as RD. Conclusion: Our data indicate the modality-independent and universal nature of the core left-hemispheric white-matter pathways subserving language processing. At the same time, the observed right-hemispheric macro- and micro-structural differences related to deafness and sign language acquisition constitute a neurobiological alteration that potentially reflects modality-specific processing demands of sign languages.