Frequency responses of two- and three-tone distortion product otoacoustic emissions in Mongolian gerbils

The frequency responses of distortion product otoacoustic emission (DPOAEs) were investigated in adult Mongolian gerbils. The main goal was to investigate in this species the extent to which DPOAE measurements might be useful in estimating cochlear frequency-tuning characteristics. Specifically, this study investigated the parameter space for generation of DPOAEs to determine these regions, if any, where the emission responses gave "simple" frequency responses, i.e., responses similar in form to typical neural responses. At the same time, it was desired to determine in this species the existence, extent, and nature of the more complex three-tone emission frequency responses as observed in some other species [e.g., Martin er at., Hearing Res. 136, 105-123 (1999)]. In the present work, two-tone frequency response curves (f(2)/f(1) ratio functions) were obtained by varying the lower frequency, f(1), while holding the f(2) frequency and both amplitudes (L(1), L(2)) constant. Only for frequencies, f(2), near 8 kHz did the response at the emission frequency, 2 f(1)-f(2), form a simple, relatively broad peak. At all lower frequencies, the two-tone frequency response curve was typically complex and composed of multiple peaks. In comparison, three-tone frequency responses were constructed by fixing the primary stimulus pair (f(1), f(2)) and varying a third tone widely in frequency (f(3)) and intensity (L(3)) Points in f(3) and L(3) which caused a criterion reduction in primary emission amplitude (at 2 f(1)-f(2)) were used to construct emission suppression tuning curves (STCs). Only for primary frequencies, f(2), at 8 kHz and above were the emission STCs found to be simple, with shapes similar to neural frequency-tuning curves. At lower primary frequencies, particularly for relatively low primary frequency ratios (low f(2)/f(1)), three-tone responses were very complex. This complex response usually included a region of anomalous suppression in which very low suppression levels (L(3)) could result in significant decreases in the primary emission amplitude, often exceeding 12 dB. Regions of such anomalous suppression were typically observed under the following conditions: (1) for all f(3) frequencies from 0.5 to 4 kHz; (2) for f(3) frequencies between 1.4 and 8 kHz; (3) i.e., for f(3) frequencies 1-3 octaves above the primary frequency, f(2); (4) at L(3) levels often 10 dB lower or mon than the usual "best frequency" threshold, i.e., even lower than the relative minimum threshold found near the primary stimulus frequencies; (5) exhibiting sharp amplitude decreases often accompanied by emission phase shifts of about 180 deg; (6) present in both cubic emissions (2 f(1)-f(2) and 2 f(2)-f(1)); (7) to be less extreme at larger primary stimulus frequency ratios (larger f(2)/f(1)); and (8) less extreme at larger intensity ratios (larger L(1)/L(2)). Because of the anomalous behavior at f(2) frequencies below 8 kHz, "simple" emission STCs were typically only obtainable, if at all, near the extreme boundaries of the parameter space giving measurable emission amplitudes. (C) 2000 Acoustical Society of America. [S0001-4966(00)04104-7].