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Description
Researchers and clinicians measured the gross action potentials from the round window or the scalp to investigate the physiological state of the auditory nerve. However, the generators underlying these responses are not fully understood. By combining intraoperative auditory nerve recordings with computational modeling, we show that the unitary action potential recorded directly on the nerve is markedly shorter than the responses measured at the round window or tympanic membrane. This likely reflects the heavier myelination of auditory nerve fibers (ANFs) near their exit from the internal auditory meatus, which shortens the extracellular action potential waveform and produces a spectral peak around 1300 Hz. Our findings are consistent with previous work demonstrating that sound activates a broad population of ANFs, whereas the compound action potential (CAP) reflects only a small, highly synchronized subset of these fibers. The onset response is dominated by high CF fibers—even when the stimulus frequency is lower—because basal fibers respond first and with the greatest synchrony. In contrast, offset responses arise primarily from fibers with lower characteristic frequencies, confirming that they are not simply the mirror image of onset responses. We also show that the human auditory nerve exhibits robust neurophonic phase locking to acoustic frequencies up to at least 2–3 kHz. Because the simulated single fiber action potentials closely match the simulated neurophonic responses, the human auditory nerve neurophonic appears to be a relevant physiological marker for defining the upper limits of phase locking in the human auditory nerve.