A cortical gradient in area 55b links language and somato-cognitive action networks.

How the cortex converts high-level cognition into action remains unresolved. We identify left frontal area 55b as a graded transition between the canonical language network and the somato-cognitive action network (SCAN). Across individualized fMRI mapping of word listening, reading, speaking and writing, resting-state connectivity, diffusion tractography and independent datasets, 55b contained a reproducible anterior-posterior gradient. Its anterior end aligned and structurally connected with the language network and carried stronger semantic-category information, whereas its posterior end aligned and connected with SCAN and preferentially encoded production and articulatory features. Production further reweighted model-estimated 55b coupling toward frontal language and speech- and hand-motor regions. Area 55b also occupied convergent transitions across multiple cortical hierarchies. These findings show how a macroscale relationship between cognitive and action networks can be implemented within a small cortical territory, providing a general mechanism for flexible cognition-to-action transformation.