Temporal Envelope Coding of the Human Auditory Nerve Inferred from Electrocochleography: Comparison with Envelope Following Responses.

Neural coding of the slow amplitude fluctuations of sound (i.e., temporal envelope) is thought to be essential for speech understanding; however, such coding by the human auditory nerve is poorly understood. Here, neural coding of the temporal envelope by the human auditory nerve is inferred from measurements of the compound action potential in response to an amplitude modulated carrier (CAP_ENV) for modulation frequencies ranging from 20 to 1000 Hz. The envelope following response (EFR) was measured simultaneously with CAP_ENV from active electrodes placed on the high forehead and tympanic membrane, respectively. Results support the hypothesis that phase locking to higher modulation frequencies (> 80 Hz) will be stronger for CAP_ENV, compared to EFR, consistent with the upper-frequency limits of phase locking for auditory nerve fibers compared to auditory brainstem/cortex neurons. Future work is needed to determine the extent to which (1) CAP_ENV is a useful tool for studying how temporal processing of the auditory nerve is affected by aging, hearing loss, and noise-induced cochlear synaptopathy and (2) CAP_ENV reveals the relationship between auditory nerve temporal processing and perception of the temporal envelope.