Lip movements enhance speech representations and effective connectivity in speech dorsal stream and its relationship with neurite architecture

Lip movements facilitate speech comprehension, especially under adverse listening conditions, but the neural mechanisms of this perceptual benefit at the phonemic and feature levels remain unclear. This fMRI study addresses this question by quantifying regional multivariate representation and network organization underlying audiovisual speech-in-noise perception. We found that valid lip movements enhanced neural representations of phoneme, place of articulation, or voicing feature of speech differentially in dorsal stream regions, including frontal speech motor areas and supramarginal gyrus. Such local changes were accompanied by strengthened dorsal stream effective connectivity. Moreover, the neurite orientation dispersion of left arcuate fasciculus, a structural basis of speech dorsal stream, predicted the visual enhancements of neural representations and effective connectivity. Our findings provide novel insight to speech science that lip movements promote both local phonemic and feature encoding and network connectivity in speech dorsal pathway and the functional enhancement is mediated by the microstructural architecture of the circuit.