Identifying the Cellular Sources of the Low-Frequency Cochlear Response

Abstract

Medical and technologic treatments for hearing loss are quickly outpacing current clinical diagnostic techniques. In order to knowledgeably treat patients and accurately predict treatment outcomes, we need diagnostic tools that can identify the anatomic damage or dysfunction underlying the loss of hearing. The cochlear microphonic –a reflection of current flow through outer hair cells –in conjunction with high-pass noise or suppressing tones, shows promise as a method of assessing the health of outer hair cells at specific locations along the cochlear partition in rodent models. In this study, we propose that the electrical potential recorded from the round window in gerbils to low-frequency tones contains responses from a number of cellular sources in addition to and, in some high-pass noise conditions, more substantial than the outer hair cell response. A model is created from data found in existing literature and our previous studies, which provides evidence for identifying each cellular source contributing to this low-frequency round window potential, termed the cochlear response (CR). The CR was recorded via an electrode placed in the round window niche of 16 Mongolian gerbils and elicited with a 45 Hz tone burst embedded in 18 high-pass filtered noise conditions, in order to target responses from increasing apical-to-basal regions along the cochlear partition. Independent component analysis and filtering of the response recovered several contributing sources whose spectral and temporal content provided clues to their identification. These suspected sources were modeled using previously-published hair cell and auditory nerve response data, and then weighted and combined using linear regression to produce a model response that fits closely to the mean CR waveform. We conclude that the low-frequency CR contains contributions from several cellular sources, for which the model provides evidence of the most significant contributors being outer hair cells, inner hair cells, and apical auditory nerve fibers. Therefore, the CR shows the capacity to be developed into a diagnostic tool that can assess the health of multiple structures in the cochlea objectively, simultaneously, and independently.