The Summating Potential (SP) in an Animal Model and in Human Cochlear Implant (CI) Subjects

Background: The SP is an often studied cochlear response to sound whose origins have historically been poorly understood. We used pharmacology to separate contributions from different sources, and examined the SP in human CI subjects in light of the results. The hypothesis was that the variability seen in the human SPs can be explained based on the information derived from animal studies.

Objectives: To identify the sources of the SP in an animal model to better understand the variability seen in CI subjects

Methods: Responses to tones (2-8 kHz) were collected from the round window (RW) of gerbils. Neural contributions were removed with TTX and kainic acid. Some gerbils were normal-hearing while others had outer hair cells (OHCs) removed with systemic application of furosemide/kanamycin. By subtracting responses within and across animals the contributions of inner hair cells (IHCs), OHCs, and neural sources were isolated. In human CI subjects responses to tones (0.25-4 kHz) were collected intraoperatively from the RW.

Results: The SP in gerbils was produced from IHCs, OHCs, and both spiking and dendritic components of neural responses. Similarly, the SP in CI subjects showed strong effects of neural and hair cell contributions that could be deduced from cases where neural responses were present or absent. The morphology of the SP in CI subjects varied by etiology of hearing loss.

Conclusions: The SP is derived from the sum of contributions from OHCs, IHCs and neural sources. This complexity is also observed in the SP from CI subjects.