Task-dependent modulation of the visual sensory thalamus assists visual-speech recognition

TL;DR

This study demonstrates that top-down modulation of the visual thalamus (LGN) enhances visual-speech recognition by selectively increasing responses to fast-varying articulatory movements, highlighting a subcortical role in communication.

Contribution

It reveals that the visual sensory thalamus is modulated during speech recognition tasks, a novel finding linking subcortical sensory processing to complex human behavior.

Findings

LGN response increased during visual-speech processing

LGN activity correlated with speech recognition scores

Modulation was specific to speech-related movements

Abstract

The cerebral cortex modulates early sensory processing via feed-back connections to sensory pathway nuclei. The functions of this top-down modulation for human behavior are poorly understood. Here, we show that top-down modulation of the visual sensory thalamus (the lateral geniculate body, LGN) is involved in visual-speech recognition. In two independent functional magnetic resonance imaging (fMRI) studies, LGN response increased when participants processed fast-varying features of articulatory movements required for visual-speech recognition, as compared to temporally more stable features required for face identification with the same stimulus material. The LGN response during the visual-speech task correlated positively with the visual-speech recognition scores across participants. In addition, the task-dependent modulation was present for speech movements and did not occur for control conditions involving non-speech biological movements. In face-to-face communication, visual speech recognition is used to enhance or even enable understanding what is said. Speech recognition is commonly explained in frameworks focusing on cerebral cortex areas. Our findings suggest that task-dependent modulation at subcortical sensory stages has an important role for communication: Together with similar findings in the auditory modality the findings imply that task-dependent modulation of the sensory thalami is a general mechanism to optimize speech recognition.