Human frequency-following response to tonal sweeps in normal hearing sujects

Recent studies examining auditory nerve single-unit responses to steady-state and time-variant complex speech- like sounds have implicated neural phase-locking as the basis for the neural encoding of those stimuli. Further, others have shown that the scalp-recorded frequency- following response (FFR) uses the same mechanism to encode steady-state complex sounds at the level of the brainstem. This study proposed to find whether the FFR encodes time- varying sounds at the level of the brainstem. Eight otologically normal subjects listened to tones gliding in frequency from low to high or from high to low. FFRS were recorded from scalp electrodes placed at high forehead (Fpz), left mastoid (Al), and the back of the neck at the seventh cervical vertebra (C7). Characteristics of FFRs to tonal sweeps were consistent with the nature of neural excitation patterns shown in previous studies. The change of frequency in the stimulus and the rate of that change were well preserved in the FFR as indicated by calculated slope values. These results indicate the human FFR is sufficiently dynamic for the encoding of time-varying low frequency stimuli.