Interpretation of distortion product otoacoustic emission measurements. I. Two stimulus tones.

The interpretation of common but poorly understood observed characteristics of distortion product emissions is assisted by the development of a simple model. This model essentially includes only saturation of the cochlear amplifier, with emissions arising naturally from the same nonlinear processes which cause the saturation. The model provides useful physical explanations of emission behaviour, particularly considered as a function of the stimulus intensities of the two primaries, i.e., behavior with fixed stimulus frequencies. It is assumed that emission generation consists of two main components which are always present in the total emission, but which most often have approximately opposite, i.e., canceling, phases. One component arises in a small region centered about the peak of the emission generation function, while the other arises from the region basal to this peak. At low stimulus levels with normal cochlear amplifier operation, the peak of the emission generation function is sharp, so the emission from the peak region dominates the total emission. This "peak" emission has typically been characterized as the "active" emission. At high stimulus levels where saturation is important, or at all levels when the gain of the cochlear amplifier is reduced, the summed "basal" component dominates the total emission. The characteristics of this basal emission are similar to, and continuous with, the characteristics of the truly "passive" emission, i.e., the emission observed when the cochlear amplifier gain is identically zero. Under circumstances when the emissions from the peak and basal components are approximately equal, there is seen a sharp "notch" characteristic of phase cancellation. The simple model produces emission distributions as a function of independent variation of the two stimulus amplitudes which are in good agreement with observation. It is shown that the furosemide assay provides a good estimate of cochlear amplifier gain when a correction factor of about 10 dB is added. However, when using two stimulus tones, neither absolute emission amplitudes, or emission input-output functions, or the furosemide assay can adequately distinguish between cases of moderate versus poor cochlear amplifier dysfunction when the cochlear amplifier gains are in the range from about half normal to zero.