Area 55b forms a cortical transition zone for sensorimotor-semantic transformation in human language

Human language requires seamless transformations between amodal concepts and modality-specific sensorimotor systems, yet the mediating cortical interface remains elusive. Using individualized functional mapping across word comprehension (listening and reading) and production (naming and writing) tasks, we evaluated the modality-invariant lexical network against five stringent, a priori criteria. We identified left area 55b as the exclusive sensorimotor–semantic interface. Area 55b robustly distinguished comprehension from production, exhibited production-enhanced semantic representations, and dynamically reconfigured its effective connectivity to motor-effector systems during production. Crucially, it revealed an internal anterior-posterior functional gradient: an anterior language-network component carried stronger semantic information, whereas a posterior somato-cognitive action-network component carried stronger articulatory information. This organization was supported by white matter connectivity and positioning along macroscale cortical hierarchies. Together, these findings reveal area 55b as a cortical transition zone for sensorimotor–semantic transformation, providing a mechanistic account of how language bidirectionally bridges meaning with perception and action.