Evaluating Human Auditory Nerve (AN) and Brainstem Function Through Level-Dependent Chirps and Predicted AN Firing Distributions.

The amplitudes of the human compound action potential (CAP) and auditory brainstem response (ABR) are influenced by the cochlear traveling wave. To synchronize auditory nerve (AN) firing, short, rising-frequency chirps have been used to compensate for traveling wave delays; however, for high presentation levels, basal spread of excitation may disrupt AN synchrony. Further, unintended spread of excitation over the time course of the chirp may adapt basal AN fibers, thereby reducing CAP and ABR amplitudes-opposite of the objective of using chirps. Previous work has altered the rate of frequency change as a function of chirp level to maximize AN synchrony. Here, we evaluated, in normal-hearing adults (N = 12, six males), an alternative approach of attenuating low-frequency chirp energy and hypothesized this manipulation would increase CAP and ABR amplitudes relative to the original, flat-spectrum chirp for high stimulus levels. Low-frequency-attenuated chirps produced larger CAP and ABR wave amplitudes than clicks and the original chirp for nearly all stimulus levels. To explore mechanisms underlying these results, we derived AN firing distributions for chirps and clicks. The distributions for low-frequency-attenuated chirps were consistent with greater AN synchrony compared to those for clicks and the original chirp. ABR waveforms were reasonably simulated by convolving the derived AN firing distributions with an impulse response waveform associated with the ABR electrode montage. These findings suggest that low-frequency-attenuated chirps and predicted AN firing distributions may be useful in evaluating human AN function, including the effects of AN fiber loss, and other disease states.