Distinguishing cochlear pathophysiology in 4-aminopyridine and furosemide treated ears using a nonlinear systems identification technique.

To test the adequacy of physiologic indices derived from a third-order polynomial model quantifying cochlear mechano-electric transduction (MET), 24 Mongolian gerbils were exposed to either 250-mM glucose (control), 150-mM 4-aminopyridine (4-AP), or 30-mM furosemide solutions applied to the round window (RW) membrane. The cochlear microphonic (CM) was recorded from the RW in response to 68- and 88-dB SPL Gaussian noise. A nonlinear systems identification technique (NLID) provided the frequency-domain parameters and physiologic indices of the polynomial model of MET. The control group showed no change in both compound action potential (CAP) thresholds and CM. Exposure to 4-AP and furosemide resulted in a similar elevation in CAP thresholds and a reduction in CM. However, the polynomial model of MET showed different changes. The operating point, slope, and symmetry of the MET function, the polynomial model parameters, and related nonlinear coherences differed between the experimental groups. It is concluded that the NLID technique is sensitive and specific to alterations in the cochlear physiology.