Magnitudes and phases of human distortion-product otoacoustic emissions at 2f 1−f 2 against f 2/f 1: effects of an audiometric notch

The influence of the frequency ratio f(2)/f(1) of two pure-tone stimuli on the distortion-product otoacoustic emissions (DPOAEs) at 2f(1)-f(2) was assessed in 14 hearing-impaired human ears exhibiting a narrow audiometric notch at 4 kHz, whereas 11 normally hearing ears served as controls. A fixed-f(2) paradigm was used, with f(2) values varying from 2 to 8 kHz in 1-kHz steps. The intensities of the two stimuli were either 60 or 70 dB SPL. The magnitudes and phases of DPOAEs were extracted so that the bandpass filter-like profile of DPOAE magnitude against f(2)/f(1) (f(2)/f(1) magnitude function) could be characterized by the presence and position of its maximum, and DPOAE group delays were derived from the phase gradient of the DPOAE when f(1) varied. The main difference between normal and impaired ears occurred at 4 kHz in that, in contrast with normal ears, nine impaired ears out of 14 did not present any peak in their f(2)/f(1) magnitude function, while the remaining five ears only differed from normal ones by a slightly shifted maximum toward larger f(2)/f(1)s. Group delays were significantly shortened in all impaired ears, with a tendency to be shorter in the subset of ears with flat DPOAE magnitude profiles with f(2)/f(1). No clear effect of notch depth was observed, and with the chosen stimulus characteristics, no abnormal DPOAE result was observed whenever f(2) fell outside the audiometric notch. It is concluded that DPOAE group delays apparently provide useful objective clues of cochlear dysfunction, more sensitive than DPOAE magnitudes inasmuch as many of them remained near the normal range. Although a majority of f(2)/f(1) magnitude functions were clearly flattened when f(2) was impaired, this feature was less systematic.