Relationship between distortion product otoacoustic emissions and frequency discrimination in normal-hearing and hearing-impaired ears

Abstract

Distortion product otoacoustic emissions (DPOAEs) and frequency difference limens (FDLs) were recorded in each of 10 ears with a sensorineural hearing loss of the dip-type and with sharp-cut high frequency hearing loss. DPOAEs were recorded with an Otodynamic Analyser ILO92 (ver. 1.35) at a f2/f1 frequency ratio 1.2. The distortion-product level (DP level) at 2f1-f2 was measured at 11 points at f2 frequencies between 696 Hz and 6348 Hz and at 26 points at f2 frequencies between 708 Hz and 6165 Hz in the dip-typed and the sharp-cut high frequency hearing loss patients, respectively. The stimulus intensity of primaries was fixed at 70 dBSPL at L and 60 dBSPL at L2. Graphs of DP levels versus f2 frequency (DP-gram) were plotted. FDLs were measured to use "same" or "different" method. Estimates of FDL were obtained at the same frequencies at 11 points from 696 Hz to 6348 Hz as f2 of DPOAE and at sensation levels from 25 dB to 35 dB. As a control, DPOAEs and FDLs were measured in 30 and 50 normal young adults, respectively. The following results ane conclusions were obtained: 1. The DP-gram obtained from 30 normal-hearing ears showed two peaks, at around 1.5 kHz and 5 kHz. 2. DP-grams were similar to Békésy audiograms in frequency specificity in 20 hearing-impaired ears. 3. DP-grams with a f2 abscissa were more similar in shape to Békésy audiograms than DP-grams with a (f1 x f2)1/2 abscissa. 4. The FDLs obtained in 50 normal-hearing ears showed an approximately linear increase in log delta F versus logarithmic frequency of stimuli up to 5 kHz. 5. FDLs at the DPOAE frequencies below the noise level in 20 hearing-impaired ears were greater than two standard deviations from the value for normal-hearing ears. 6. A significant correlation between DPOAEs and hearing threshold levels (HTLs) was found in the 20 ears of the hearing impaired. 7. Significant correlations between DPOAEs and FDL ratios (FDLs in hearing-impaired ears divided by FDLs in normal-hearing ears) were found in these 20 ears, although the correlation coefficient was not as high in the 10 ears with a sharp-cut high frequency hearing loss as in the 10 ears with the dip-typed hearing loss. These results suggested that DPOAEs might serve as an objective indicator of frequency discrimination. Care should be taken, however not to generalize this suggestion to all kinds of hearing loss patients.