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Description
Auditory nerve (AN) fibers transmit the output of cochlear hair cells to the brainstem. Fibers vary in their threshold, with some sensitive to lower-level sounds and others requiring higher levels to respond. While threshold-specific fiber responses can be directly recorded in animals, non-invasive measures in humans, i.e., auditory brainstem responses or the compound action potential (CAP), lack threshold-specific resolution. Here we designed a refractory-masking paradigm to differentiate the electrophysiological responses of AN fibers with different thresholds in humans. Stimuli consisted of five rapid click trains with one, two, three, four, or five clicks. The clicks in each train were spaced 0.25 ms apart, and each click was 15 dB higher in level than the one that preceded it. Preceding lower-level clicks within a train were designed to mask part of the response to the next click, since lower-threshold fibers would be in a refractory period when the following click was presented. Fiber-specific CAP responses to the final click of each train were isolated by subtracting the response to the train with one less click, canceling out neural responses to prior clicks. Consequently, we were able to isolate the responses from fibers with different threshold rates. Identifying fiber-specific response patterns not only helps to bridge the gap between human and animal research in auditory nerve function but may also hold potential as a proxy for peripheral neural degeneration.