Cortical tracking of speakers' spectral changes predicts selective listening

A social scene is particularly informative when people are distinguishable. To understand somebody amid a "cocktail party" chatter, we automatically index their voice. This ability is underpinned by parallel processing of vocal spectral contours from speech sounds, but it has not yet been established how this occurs in the brain's cortex. We investigate single-trial neural tracking of slow frequency modulations in speech using electroencephalography. Participants briefly listened to unfamiliar single speakers, and in addition, they performed a cocktail party comprehension task. Quantified through stimulus reconstruction methods, robust tracking was found in neural responses to slow (delta-theta range) modulations of frequency contours in the fourth and fifth formant band, equivalent to the 3.5-5 KHz audible range. The spectral spacing between neighboring instantaneous frequency contours (ΔF), which also yields indexical information from the vocal tract, was similarly decodable. Moreover, EEG evidence of listeners' spectral tracking abilities predicted their chances of succeeding at selective listening when faced with two-speaker speech mixtures. In summary, the results indicate that the communicating brain can rely on locking of cortical rhythms to major changes led by upper resonances of the vocal tract. Their corresponding articulatory mechanics hence continuously issue a fundamental credential for listeners to target in real time.