Cortical Speech Envelope Tracking Reflects Lesion-Symptom Profiles in Post-Stroke Aphasia

Background: Comprehending connected speech is critical for human interaction and is vulnerable in post-stroke aphasia. However, the neural mechanisms responsible for impaired speech listening remain largely unknown. Neural speech tracking methods offer a window into naturalistic speech processing and may reveal causal contributions to comprehension. Methods: EEG was recorded during story listening in 15 PWA with left temporal lesions, 14 PWA with left frontal lesions, and 15 age- and hearing-matched controls. All participants listened to clear and unintelligible stories. 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 band) 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 (age, hearing, lesion volume, attention/alertness).Results: The temporal group showed significantly impaired comprehension 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 equalised 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 temporoparietal 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 participants.Conclusions: Envelope tracking reflects a combination of lesion and symptom-profiles and is a viable method for investigating the neural basis of speech comprehension impairments in aphasia. PWA with frontal lesions displayed relatively retained comprehension and speech envelope tracking and produced a control-like, negative theta tracking-comprehension correlation, implying increased listening effort with greater task difficulty. In PWA with comprehension impairments and left temporal lesions, envelope tracking at syllable-rates may support the extraction of lexical-semantic information and speech understanding.