Bridging Continuous and Discrete Models of the Anterior Temporal Lobe via Cortical Gradients

Description: The anterior temporal lobe (ATL) is critical for semantic cognition, yet its functional organisation remains debated. Some evidence supports a patchwork of discrete, domain-specific parcels, whereas the graded hub hypothesis proposes gradual convergence across modalities, producing smooth transitions from unimodal to heteromodal cortex. Using a spline along the dorsal-to-medial curvature of the ATL, we projected structural and functional connectivity into macroscale cortical gradients, which capture low-dimensional axes of whole-brain organisation. We tested whether parcels along the spline had connectivity profiles that varied systematically or discontinuously along these gradients. On gradient 1, spanning unimodal to heteromodal cortex, the structural connectivity of ATL parcels generally followed a U-shaped pattern, with middle parcels showing the strongest heteromodal connectivity, consistent with graded hub predictions; however, medial parcels deviated from this pattern. Gradient 2 showed partial dorsal-to-medial shifts from auditory–motor to visual connectivity, but parcels at both ends of the spline did not follow this trend. Gradient 3, separating default mode from cognitive control networks, revealed largely systematic dorsal-to-medial connectivity shifts, though left anterior superior temporal gyrus exhibited stronger connectivity to control regions than predicted. Replication using functional connectivity confirmed the U-shaped pattern in gradient 1. Functionally, individual differences in the structural connectivity of anterior superior temporal gyrus predicted task activation. Overall, these results support a largely graded hub model in which visual and auditory information converges in heteromodal regions, while also revealing functional variation that is not systematic that may enable flexible semantic processing.

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