Dissociated neural tracking as an index of speech understanding in background noise

Abstract

Human brain activity has been shown to track hierarchical linguistic units embedded in connected speech and these responses can be directly modulated by changes in speech intelligibility caused by spectral degradation or prior knowledge. In this study, we introduce a background noise and manipulate its level relative to the target speech to test the hypothesis that the tracking responses are modulated differently by the variations in Signal-to-Noise Ratio (SNR) and changes in speech intelligibility. Electroencephalography (EEG) responses to speech in quiet and in noise (multi-talker babble) at three different levels were measured from 19 normal hearing participants. Driven by the reduction in intelligibility, cortical coherence to “abstract” linguistic units with no accompanying acoustic cues was reduced relative to the “speech only” condition, and lateralized to single cerebral hemispheres. In contrast, brain responses coherent to words, aligned with acoustic onsets, were bilateral and reduced systematically as noise level increased. Strength of the tracking response correlated with subjective ratings from each participant on how much they can understand the speech sentences at all different linguistic levels. These results provide an objective and sensitive neural marker of speech intelligibility which can be further developed into clinical applications for objective assessment of speech-in-noise understanding.