Rethinking language lateralisation

We are excited to share our latest preprint:

Manifold lateralisation and variability in the language connectome at 7T

by Lilit Dulyan, Cesare Bortolami, Eva Guzmán Chacón, Ahmad Beyh, Michel Thiebaut de Schotten, and Stephanie Forkel

📄 https://www.researchsquare.com/article/rs-9267400/v1

📜 Licensed under CC BY 4.0

Please note: this is a preprint and has not yet been peer reviewed.

Language has long been considered a left-dominant brain function. But what if this classical view is too simple?

Using ultra-high-field 7T diffusion MRI from 172 Human Connectome Project participants, this study revisits the structural basis of language lateralisation at an unprecedented level of detail. Rather than assuming fixed categories (left, right, bilateral), the work asks a more fundamental question: How does lateralisation actually vary across individuals? By combining advanced tractography, a newly developed Variability Index, Bayesian inference, and manifold learning, the study reveals a striking result: Language lateralisation is not discrete — it lies on a continuous manifold.

Instead of falling into neat groups, individuals distribute along a graded spectrum of structural asymmetry across the language connectome. The findings also show that lateralisation is not uniform across the network. For example Leftward asymmetry: Arcuate fasciculus (long segment), inferior longitudinal fasciculus, frontal aslant tract. Rightward asymmetry: Anterior arcuate segment, uncinate fasciculus

This highlights a network-level organisation, where different pathways contribute asymmetrically in distinct ways. Beyond average asymmetries, the study uncovers substantial inter-individual variability, including:

• Differences in microstructural dispersion between hemispheres

• Tract-specific patterns of variability

• A balance between shared anatomical scaffolds and individual flexibility

Crucially, Bayesian analyses show no relationship between structural lateralisation and handedness and language performance.

A new framework for language organisation

Together, these findings challenge traditional views of hemispheric dominance and instead propose: A graded, manifold-based organisation of the language connectome. This perspective aligns with emerging ideas in network neuroscience, where variability is not noise but a core organising principle of brain function.