IDENTIFICATION OF THE NONLINEARITY GOVERNING EVEN-ORDER DISTORTION PRODUCTS IN COCHLEAR POTENTIALS

In order to characterize the cochlear transducer nonlinearities which are involved in the generation of the summating potential (SP), we inv estigated the effect of a change in the electrical operating point of the cochlear transducer on the SP. The electrical operating point of t he cochlear transducer was affected by suppressing reversibly the endo cochlear potential (EP). This was realized by intravenous injection of furosemide in guinea pig. A differential recording technique was used in the basal turn of the cochlea to measure locally generated even-or der distortion products: the SP and the second harmonic component (2F( 0)) of the cochlear microphonics (CM). These potentials were evoked by 2 and 8 kHz stimuli presented at 60 dB SPL. Following furosemide inje ction, the SP changed polarity twice over time. The zero crossings of the SP coincided with a minimum in the amplitude of 2F(0). Concomitant ly, the phase of 2F(0) shifted about 120 degrees. The changes in the e lectrical even-order products were comparable to the changes that occu rred in a mechanical even-order intermodulation distortion product (th e difference tone F-2-F-1 otoacoustic emission) after furosemide appli cation (Mills et al., J. Acoust. Sec. Am. 94 (1993) 2108-2122). The co mbined results suggest that only one sigmoidal transfer function may a ccount for the SP, 2F(0), and the emission of the difference tone F-2- F-1, and that shifts in the operating point of the transfer function w ould be the major cause behind the furosemide-induced changes in the e ven-order distortion products. The sigmoidal transfer function is like ly associated with the mechano-electrical transducer channel at the ap ical pole of the outer hair cell. (C) 1997 Elsevier Science B.V.