Enhanced Delta Band Neural Tracking of Degraded Fundamental Frequency Speech in Noisy Environments

Pitch variation of the fundamental frequency (F0) is critical to speech understanding, especially in noisy environments. Degrading the F0 contour reduces behaviorally measured speech intelligibility, posing greater challenges for tonal languages like Mandarin Chinese where the F0 pattern determines semantic meaning. However, neural tracking of Mandarin speech with degraded F0 information in noisy environments remains unclear. This study investigated neural envelope tracking of continuous Mandarin speech with three F0-flattening levels (original, flat-tone, and flat-all) under various signal-to-noise ratios (0, -9, and -12 dB). F0 contours were flattened at the word level for flat-tone and at the sentence level for flat-all Mandarin speech. Electroencephalography responses were indexed by the temporal response function in the delta (<4 Hz) and theta (4-8 Hz) frequency bands. Results show that delta-band envelope tracking is modulated by the degree of F0 flattening in a nonmonotonic manner. Notably, flat-tone Mandarin speech elicited the strongest envelope tracking compared with both original and flat-all speech, despite reduced F0 information. In contrast, the theta band, which primarily encodes speech signal-to-noise level, was not affected by F0 changes. In addition, listeners with better pitch-related music skills exhibited more efficient neural envelope speech tracking, despite being musically naive. These findings indicate that neural envelope tracking in the delta (but not theta) band is highly specific to F0 pitch variation and highlight the role of intrinsic musical skills for speech-in-noise benefits.