Interpretation of distortion product otoacoustic emission measurements. II. Estimating tuning characteristics using three stimulus tones

摘要

The simple model introduced in Part I [J. Acoust. Soc. Am. 102, 413-429 (1997)] is used to simulate the response of the cochlea to three stimulus tones. The focus is on "emission suppression tuning curves" constructed using a third tone to suppress the cubic distortion tone emission (CDT, 2f_1 - f_2) generated by two primary tones at frequencies f_1 and f_2 (intensities L_1 and L_2). A criterion decrease (here, 5 dB) of the CDT emission amplitude defines the 2f_1 - f_2 emission suppression tuning curve. Applying traditional tuning curve measures to emission suppression tuning curves appears ineffective in determining the underlying cochlear amplifier characteristics. However, it is shown that there are three characteristics of emission suppression tuning curves which are particularly useful: (1) the "f_2 threshold" which is the level of the third tone, L_3, required for the criterion CDT amplitude decrease, under the condition that the third tone frequency, f-3, is approximately equal to f_2; (2) the "shoulder threshold" similarly defined for f_3 f_2; and (3) the "tuning width," w_(40). The tuning width is defined to be the distance (in octaves) from the frequency f_2 to the upper f_3 frequency for which there is a criterion CDT decrease, in this case using the L_3 level which is 40 dB above the f_2 threshold. Model calculations appropriate to gerbils show that these measures are most accurately related to the underlying cochlear amplifier characteristics for parameters where the primary stimulus amplitudes satisfy L_1/L_2 > 20 dB and for which L_1 is 25 dB or more below the sharp "notch" seen in the two tone input-output function. In this parameter region, the cochlear amplifier characteristics are related to measured quantities by the relationships w_r approx= w_(40) and G_a approx= T_E + w_(40)g_p. Here, G_a is the gain (dB) of the cochlear amplifier, defined as the total increase in cochlear response over the passive response, w_r is the distance (octaves) over which the active cochlear response rises to a maximum, and g_p is the passive increase (dB/octave) of the traveling wave along the basilar membrane. The measured quantities are T_E, the difference (dB) between the shoulder threshold and the f_2 threshold, and the tuning width, w_(40) (octaves), defined above. Model predictions are confirmed by measurements in adult gerbils.