Cortical speech envelope tracking reflects lesion-symptom profiles in post-stroke aphasia

Comprehending connected speech is critical for human interaction and is vulnerable in post-stroke aphasia. Understanding the neural mechanisms underlying impaired speech listening is necessary for accurate and effective assessment and treatment. Neural speech tracking methods offer a window into naturalistic speech processing and may reveal causal contributions to comprehension. EEG was recorded during story listening in 15 people with aphasia with left temporal lesions (temporal group), 14 people with aphasia with left frontal lesions (frontal group) and 15 age- and hearing-matched controls (control group). All participants listened to clear and unintelligible stories (∼12 min per condition). Controls additionally listened to low-intelligibility stories that equated comprehension success to the temporal group. Envelope tracking was measured at syllable- (theta) and multi-syllable- (delta) rates. Neural decoding and encoding analyses measured global (whole-brain) and local (sensor-wise) speech tracking, respectively. Group comparisons used linear mixed-effects regression. Linear and quadratic relationships assessed associations between neural tracking and behavioural measures of comprehension while accounting for covariates. Behavioural measures of comprehension showed the temporal group were significantly impaired compared to both frontal and control groups. Comparison of intelligible and unintelligible speech found that intelligibility affected delta but not theta tracking-suggesting that delta tracking is linked to higher-order linguistic processing and theta tracking reflects sensory responses. Theta and delta envelope decoding reflected group/comprehension status: tracking was reduced in the temporal group compared to the control group, but tracking was not significantly different when control comprehension was behaviour-matched via speech degradation. Delta encoding produced a similarly behaviour-linked pattern, but theta encoding was lesion-rather than behaviour-sensitive, in that reduced tracking was observed in temporo-parietal sensors in both aphasia groups. Theta tracking correlated with comprehension in a lesion-specific manner. Positive correlations between theta tracking and comprehension were found in the temporal group and negative correlations in the frontal and control groups. This produced a quadratic (inverted-U) relationship between theta tracking and comprehension success across all participants. The control-like, negative theta tracking-comprehension correlations in the frontal group are consistent with listening effort effects in which greater task difficulty result in greater tracking. In the temporal group, where comprehension was impaired, better envelope tracking at syllable rates may help build a stable representation of the speech stream supporting phonological and lexical analysis and comprehension. These results indicate that envelope tracking reflects a combination of lesion and symptom profiles and is a viable method for investigating the mechanisms of speech comprehension in aphasia.