Human frequency-following responses to two-tone approximations of steady-state vowels.

Auditory nerve single-unit population studies have demonstrated that phase-locking plays a dominant role in the neural encoding of the spectrum of speech sounds. Since the scalp-recorded human frequency-following response (FFR) reflects synchronous, phase-locked activity in a population of neurons in the rostral auditory brainstem, it was reasoned that the human FFR might preserve information about certain acoustic features of speech sounds. FFRs to three different two-tone approximations of vowels (symbols, see text) were obtained from 10 normal-hearing human adults at 85, 75, 65 and 55 dB nHL. Spectrum analyses of the FFRs revealed distinct peaks at frequencies corresponding to the first and the second formants across all levels suggesting that phase-locked activity among two distinct populations of neurons are indeed preserved in the FFR. Also, the FFR spectrum for vowels (symbols, see text) revealed a robust component at 2F1-F2 frequency suggesting that the human FFR contains a neural representation of cochlear nonlinearity. Finally, comparison of FFRs to the vowel approximations and the individual components at F1 and F2 revealed effects that may be suggestive of two-tone synchrony suppression and/or lateral inhibition. These results suggest that the scalp-recorded FFR may be used to evaluate not only neural encoding of speech sounds but also processes associated with cochlear nonlinearity.