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Language Without Speech"Seed regions of interest for tractography", Figure from Fink et al. (https://doi.org/10.1093/cercor/bhz128) licensed under CC BY 4.0

Language Without Speech

Language is a fundamental part of human cognition. The question of whether language is processed independently of speech, however, is still heavily discussed. The absence of speech in deaf signers offers the opportunity to disentangle language from speech in the human brain. Using probabilistic tractography, we compared brain structural connectivity of adult deaf signers who had learned sign language early in life to that of matched hearing controls. Quantitative comparison of the connectivity profiles revealed that the core language tracts did not differ between signers and controls, confirming that language is independent of speech. In contrast, pathways involved in the production and perception of speech displayed lower connectivity in deaf signers compared to hearing controls. These differences were located in tracts towards the left pre-supplementary motor area and the thalamus when seeding in Broca’s area, and in ipsilateral parietal areas and the precuneus with seeds in left posterior temporal regions. Furthermore, the interhemispheric connectivity between the auditory cortices was lower in the deaf than in the hearing group, underlining the importance of the transcallosal connection for early auditory processes. The present results provide evidence for a functional segregation of the neural pathways for language and speech.

Tractograaphy results with seed in Brodmann area 44 (BA44) (left) and in Heschl’s Gyrus (right). Average tractograms of all participants are displayed on the standard T1 MNI-brain. Seed ROIs are marked with dashed lines. Significant differences in connectivity between groups are depicted in orange. Color coding in slices ranges from 0 (no connectivity with seed ROI) to 1 (maximal connectivity). Tracts are shown at a threshold of 0.2, which was also used for statistical testing. For purposes of clarity, the tracts in the 3D images are presented at a threshold of 0.3. 3D images and horizontal slices are viewed from above and coronal slices from behind with left in the pictures representing left in the brain. Boxplots indicate mean logarithmized connectivity values of controls (C) and deafs (D) in areas with significant connectivity differences; Cohen’s d was calculated post hoc. P-values are FWE-corrected at cluster level. All coordinates are given in MNI-space. Figure from Fink et al. (https://doi.org/10.1093/cercor/bhz128) licensed under CC BY 4.0
Tractograaphy results with seed in Brodmann area 44 (BA44) (left) and in Heschl’s Gyrus (right). Average tractograms of all participants are displayed on the standard T1 MNI-brain. Seed ROIs are marked with dashed lines. Significant differences in connectivity between groups are depicted in orange. Color coding in slices ranges from 0 (no connectivity with seed ROI) to 1 (maximal connectivity). Tracts are shown at a threshold of 0.2, which was also used for statistical testing. For purposes of clarity, the tracts in the 3D images are presented at a threshold of 0.3. 3D images and horizontal slices are viewed from above and coronal slices from behind with left in the pictures representing left in the brain. Boxplots indicate mean logarithmized connectivity values of controls (C) and deafs (D) in areas with significant connectivity differences; Cohen’s d was calculated post hoc. P-values are FWE-corrected at cluster level. All coordinates are given in MNI-space. Figure from Finkl et al. (https://doi.org/10.1093/cercor/bhz128) licensed under CC BY 4.0

Publications

  • Finkl, T., Hahne, A., Friederici, A.D., Gerber, J., Mürbe, D., & Anwander, A. (2020). Language without speech: Segregating distinct circuits in the human brain. Cerebral Cortex, 30, 812-823. https://doi.org/10.1093/cercor/bhz128

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Anja Hahne