Otoacoustic emissions from normal-hearing and hearing-impaired subjects: distortion product responses

Distortion product otoacoustic emission test of sensorineural hearing loss: performance regarding sensitivity, specificity and receiver operating characteristics

The performance of distortion product otoacoustic emissions (DPOEs) as a frequency-specific test of sensorineural hearing loss was evaluated in 142 ears of human adults with normal middle-ear function. The DPOE was measured with the stimulus levels of the two tones equal to 65 dB SPL (re 20 mu Pa) and the ratio between the two frequencies 1.2. In the DPOE test, the cochlear function of an ear at a test frequency was predicted to be normal or abnormal depending upon whether the DPOE level with the geometric mean of the two stimulus frequencies at the test frequency was greater or less than a criterion. The DPOE test outcomes were evaluated against the pure-tone hearing threshold as the standard. We found the sensitivity, specificity and predictive efficiency of the test to be 85-89% at 6000 and 4000 Hz, 82-83% at 2000 Hz and 78-79% at 1000 Hz, respectively. The performance was also evaluated using decision theory in terms of the area under the receiver operating characteristics. The latter was found to range from 0.90 (for 1000 Hz) to 0.94 (for 6000 Hz). These findings support the conclusion that the DPOEs can form a useful frequency-specific objective test of cochlear function.

Distortion product otoacoustic emissions in rhesus (Macaca mulatta) monkey ears: normative findings

Distortion product otoacoustic emissions (DPOAEs) in rhesus monkeys were characterized and the optimal parameters for their generation were determined. Robust DPOAEs were readily measurable from the ear canals of six rhesus monkeys (n = 12 ears). The nonmonotonic behavior of the f2/f1 ratio functions in rhesus monkeys was found to be similar to other animals and humans. The optimal mean f2/f1 ratio of 1.21 and the effect of the primary frequency and level on the optimal f2/f1 ratios were also similar to human measurements. The contour of the rhesus monkey DPOAE 'audiograms' and their behavior were also comparable to human measurements with slight differences in peak frequencies. The rhesus monkey DPOAE input/output (I/O) functions were generally monotonic with a slope approaching unity with increasing frequency. Therefore, our study shows that many basic DPOAE characteristics are remarkably similar in the two species and emphasizes the appropriateness of the rhesus monkey as a model for DPOAE research. Detailed studies of the behavior of DPOAEs can be carried out in a model that is phylogenetically close to human both in hearing and in the gross structure and histology of the inner ear.

A Neural Network Approach to the Prediction of Pure Tone Thresholds with Distortion Product Emissions

Distortion Product Emission (DPE) growth functions, demographic data, and pure tone thresholds were recorded in 229 normal-hearing and hearing-impaired ears. Half of the data set (115 ears) was used to train a set of neural networks to map DPE and demographic features to pure tone thresholds at six frequencies in the audiometric range. The six networks developed from this training process were then used to predict pure tone thresholds in the remaining 114-ear data set. When normal pure tone threshold was defined as a threshold less than 20 dB HL, frequency-specific prediction accuracy varied from 57% (correct classification of hearing impairment at 1025 Hz) to 100% (correct classification of hearing impairment at 2050 Hz). Overall prediction accuracy was 90% for impaired pure tone thresholds and 80% for normal pure tone thresholds. This neural network approach was found to be more accurate than discriminant analysis in the prediction of pure tone thresholds. It is concluded that a general strategy exists whereby DPE measures can accurately categorize pure tone thresholds as normal or impaired in large populations of ears with purely cochlear hearing dysfunction.

Distortion product otoacoustic emissions and sensorineural hearing loss

As other types of otoacoustic emissions, distortion product otoacoustic emissions (DPOAEs) allow the exploration of the active cochlear mechanisms known to take place in the outer hair cell system. Most authors consider that 2f1-f2 DPOAEs are generated in a cochlear region corresponding to the geometric mean (GM) of the primary frequencies. To verify the relevance of this hypothesis in clinical practice, DPOAEs were recorded at seven different frequencies, ranging from 0.5 to 4 kHz, in 81 hearing-impaired patients and in 24 normally hearing subjects. To test the hypothesis that DPOAEs reflect the hearing threshold at the frequency of the GM rather than at the 2f1-f2 frequency, this study compares the 2f1-f2 frequency and the GM of the primaries to the frequency of hearing loss. DPOAEs can be used to explore a large range of frequencies, especially at high frequencies, but responses at low frequencies are less reliable due to noise contamination. Secondly, DPOAEs can be recorded in ears that have a hearing threshold as high as 65 dB HL at the frequency corresponding to the GM of the primaries. Finally, DPOAE recordings show frequency specificity: i.e., hearing loss at a specific frequency correlates best with DPOAEs whose GM of primary frequencies corresponds to the frequency of the hearing loss. However, this frequency specificity is still unsatisfactory and decreases as the levels of primaries increase above 60 dB SPL. Moreover, DPOAE amplitude is too variable to predict hearing loss at a particular frequency, whereas DPOAE threshold allows a correct prediction of abnormal auditory threshold in more than 80% of the cases at frequencies above 1 kHz.

Sensitivity of distortion-product otoacoustic emissions in humans to tonal over-exposure: time course of recovery and effects of lowering L2

An important concern of industrial hearing-conservation programs is detecting the onset of noise-induced hearing loss. If it can be shown that otoacoustic emissions are sufficiently sensitive to reliably detect auditory fatigue and the permanent hearing loss that eventually develops, they could become an important part of the hearing-conservation test battery. The present study in humans was designed to examine the influence of overall primary-tone level and the effects of lowering the f2 primary on the sensitivity of distortion-product otoacoustic emissions (DPOAEs) to acoustic overstimulation. One ear from each of 14 subjects with normal hearing was exposed to a 105-dB SPL pure tone at 2.8 kHz for 3 min using a protocol consisting of distinct pre-exposure, exposure, and post-exposure periods. As a quantitative index of the functional status of the outer hair cells, 2f1-f2 DPOAEs were monitored systematically over time using four stimulus-test conditions consisting of either one of two levels of equilevel primary tones, or one of two levels of offset primaries, with L2 set 25 dB lower than L1. The overall finding was that the DPOAE protocol incorporating both the lowest level of stimulation and an f2-primary tone that was 25 dB below the level of the f1 stimulus [i.e., L1 (55 dB SPL) - L2 (30 dB SPL) = 25 dB] was most sensitive to the exposure effects. The results establish that DPOAEs elicited with unequal, in contrast to equal-level primaries, have comparable signal-to-noise ratios, but are considerably more sensitive to reductions in emission levels induced by exposure to short-lasting, moderately intense tones. The recovery of DPOAE amplitudes over the first 15 min post-exposure appeared to be roughly linear in log time and, in many cases, could be closely approximated by fitting a logarithmic curve to the post-exposure data. From these functions, the initial amount of loss (y-intercept) and the slope of recovery were identified as potential measures of vulnerability to acoustic exposure in that these variables appeared to be related to the susceptibility of some of the subjects, who also participated in a subsequent experiment on the behavioral effects of the exposure stimulus. Finally, compared to behaviorally measured temporary threshold shift (TTS), the time course of the recovery for DPOAEs was very similar, suggesting that, with the appropriate parameters, DPOAEs can be as sensitive to TTS as routine pure-tone audiometry.

Toluene exposure affects the functional activity of the outer hair cells

Rats were exposed to toluene by inhalation (1400 ppm, 16 h/d, 8 days) and the auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOEs) were used as measures of the auditory sensitivity. These measurements were made before the exposure to toluene, 3 and 5 days after the start of the exposure and 4 days after the end of the exposure. To quantify the repeated DPOE data the area under the curve of the DPOE amplitudes versus the stimuli levels was calculated and used for statistical analysis. Results demonstrate that 3 days of toluene exposure tended to lower DPOE amplitudes and elevate ABR thresholds. Similarly after 5 days of exposure significantly lower DPOE amplitudes were observed at most frequencies along with elevated ABR thresholds. At 4 days post-exposure DPOE amplitudes were greatly diminished at all frequencies and the ABR thresholds were raised by about 40 dB between 1.6 and 20 kHz. These results show a parallel shift between ABR thresholds and DPOE amplitudes during toluene exposure. Furthermore, the results from the DPOE measurements indicate that mainly the outer hair cells are adversely affected by toluene exposure.