Estimates of the single fiber action potential and probability of neuronal discharge obtained from human compound action potentials

Abstract

Sensory neural hearing loss results from damage to various cochlear structures and/or auditory nerve fibers. However, current clinical diagnostic tests cannot differentiate between cochlear and neuronal pathology. The effectiveness of future treatments such as cochlear hair cell regeneration and stem cell transplantation in the inner ear requires information about the site of pathology. The long-term objective of this research is to develop clinical identifiers of anatomic structures that have been compromised in cases of sensory neural hearing loss. Our current approach is to utilize the compound action potential (CAP), which is a stimulus-evoked electrical response of numerous auditory nerve fibers. The CAP is conceptualized as the combination of the probability of neuronal discharge across all auditory nerve fibers [P(t)] with the profile of an action potential from a single auditory nerve fiber [U(t)]. In vivo estimates of P(t) and U(t) can be obtained by combining functional descriptors of P(t) and U(t) to provide an analytic equation of the CAP, which is fit to physiologic CAPs. Here we seek to translate this technique previously developed in normal hearing and hearing-impaired gerbils to human application by characterizing P(t) and U(t) from normal human ears. P(t) and U(t) from CAPs were examined at multiple stimulus levels. Results illustrate that it is possible to apply to human CAPs the analytic treatment that was developed in gerbil.