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Brief report: the cochlear microphonic as an indication of outer hair cell function
1Auditory Physiology Laboratory, Northwestern University, Evanston, Illinois, USA.
Abstract:
The extra-cellular cochlear microphonic is believed to be generated predominantly by outer hair cells and therefore it would seem reasonable to assume that the presence of a cochlear microphonic excludes outer hair cell dysfunction. Indeed, a diagnosis of auditory neuropathy might be, and has been, made on the basis of a cochlear microphonic present with an abnormal auditory brainstem response. Animal studies, however, have shown that the cochlear microphonic recorded from the round window is dominated by cellular generators located in the base of the cochlea. Primarily on this basis, it is argued that the presence of a cochlear microphonic does not exclude outer hair cell pathology and so outer hair cell integrity should not necessarily be inferred from the presence of the cochlear microphonic alone. In contrast, the absence of an otoacoustic emission in such cases is consistent with outer hair cell dysfunction.
Insights
The presence of a cochlear microphonic (CM) does not rule out outer hair cell dysfunction. Auditory neuropathy diagnosis requires further testing beyond CM presence alone.
Area of Science:
- Otoacoustic Emissions
- Auditory Neurophysiology
Background:
- The extra-cellular cochlear microphonic (CM) is traditionally linked to outer hair cell (OHC) function.
- A present CM with an abnormal auditory brainstem response (ABR) has led to auditory neuropathy diagnoses.
Purpose of the Study:
- To re-evaluate the assumption that a present CM excludes OHC dysfunction.
- To investigate the generators of the CM recorded from the round window.
Main Methods:
- Review of animal studies on CM generation.
- Analysis of the relationship between CM, OHC pathology, and otoacoustic emissions (OAEs).
Main Results:
- Animal studies indicate CMs recorded from the round window are primarily generated by basal cochlear cells.
- The presence of a CM does not exclude OHC pathology.
Conclusions:
- OHC integrity cannot be solely inferred from the presence of a CM.
- Absence of otoacoustic emissions in cases with CM is consistent with OHC dysfunction.