Deriving a Cochlear Transducer Function from Low Frequency Modulated Cochlear Microphonic

Choia, Chul-Hee; Bianb, Lin; EChertoff, Mark; Choia, Chul-Hee; Bianb, Lin; EChertoff, Mark

ORIGINAL ARTICLE

Commun Sci Disord. 2010;15(2): 232-250.

Chul-Hee Choia^` , Lin Bianb^` , and Mark EChertoff^`

최철희(Chul-Hee Choia)| Lin Bianb(린 비안)| Mark E. Chertoff(마크 체토프)

ABSTRACT

Background & Objectives
Recently, a new method was developed to construct a cochlear transducer function from the summating potential using a low-frequency bias tone. In this study, another method to derive a cochlear transducer function (fTr) from the cochlear microphonic (CM) using the same low frequency modulation technique was introduced and compared to the fTrobtained from the summating potential (SP). Before real experiment, mathematical exploration and computer simulation suggested that the CM magnitude is essentially proportional to the first derivative [fTr(1)]of the fTr.
Methods
Two probe tones (6 or 12 kHz) ranging from 70 to 90 dB SPL in 10 dB step and a 25 Hz bias tone with a high level (130 dB SPL) were simultaneously presented to record the CM from an electrode placed on the round window at gerbils. After band-pass filtering to remove the low frequency CM responses, the modulation envelope of the CM was obtained.
Results
Results showed that the CM modulation envelope as a function of the bias levels has a shape similar to the fTr(1)as shown in the simulation. Furthermore, when the cochlear transducer function derived from the CM were compared to those obtained from the SP previously described by Choi et al.(2004), there were significant differences between the Boltzmann functions fitted to the CM and SP responses.
Discussion & Conclusion
This study showed another method to derive a cochlear transducer function from the cochlear microphonic using a low-frequency bias tone. However, there were differences between cochlear transducer functions derived from CM and SP. The most significant difference was the dynamic range in the cochlear transducer function from the CM greater than those from SP. The cochlear transducer function obtained from the CM was more symmetric than those from the SP. These features in the cochlear transducer function from the CM may reflect the origin of outer hair cells (OHCs). Therefore, the major difference between the cochlear transducer functions obtained from the CM and the SP may result from different contribution of the inner hair cells (IHCs) and OHCs.