Cognitive load weakens neural speech tracking without altering response timing

Everyday speech perception often occurs under concurrent cognitive demands, but it remains unclear how speech-extrinsic load influences the neural processing of continuous speech. Here, electroencephalography (EEG) recordings from 29 healthy adults are analyzed with temporal response functions (TRFs), artificial neural network (ANN) decoding, and phase-lag index (PLI)-based connectivity measures during speech listening with or without concurrent mental arithmetic. Higher cognitive load reduces the amount of speech-envelope information recoverable from EEG, as reflected by lower delta-band TRF prediction accuracy and lower ANN-based decoding performance, whereas theta-band encoding accuracy did not differ significantly between conditions. In the delta-band TRF, cognitive load is associated with reduced response amplitude without an obvious shift in dominant peak latency. Higher load is also accompanied by selective changes in delta-band sensor-level phase coupling and a more clustered, more small-world-like topology in the thresholded delta-band PLI subnetwork. These findings suggest that speech-extrinsic cognitive load weakens neural representations of continuous speech and alters frontotemporal coupling patterns, even in the absence of speech degradation or competing talkers.