Volume 104, Issue 2, August 1998
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
On the existence of an age/threshold/frequency interaction in distortion product otoacoustic emissions104(1998); http://dx.doi.org/10.1121/1.423339View Description Hide Description
Interactions among age, threshold, and frequency in relation to distortion product otoacoustic emissions (DPOAE) have yet to be resolved. The effects of these variables were explored by analyzing DPOAEs in ears with thresholds not exceeding 20 dB HL. Multivariate regression analyses were performed in two different ways. For data to be included in the first analysis,audiometric threshold had to be 20 dB HL or better only at the particular frequency under study, but might exceed 20 dB HL at other half-octave frequencies. Significant main effects were found for age, threshold, and frequency. There was also an age-by-frequency interaction, but a significant age-by-threshold interaction was not observed. DPOAE amplitudes decreased as either age, frequency, or threshold increased. In the second analysis, when a more stringent inclusion criterion was applied (normal thresholds at all frequencies), the main effects for age, threshold, and frequency were not significant. The significant age-by-frequency interaction remained, whereby DPOAE amplitudes decreased as age and frequency increased, but the age-by-threshold interaction again was not significant. The magnitude of DPOAE amplitude change across age, threshold, and frequency and for the age-by-frequency interaction was small but similar for both groups of subjects. Age in association with threshold did not account for observed changes in DPOAE amplitudes for either group. Importantly, the lack of a significant age-by-threshold interaction indicates that there may be processes intrinsic to aging alone that act on DPOAE generation.
Effects of loop diuretics on the suppression tuning of distortion-product otoacoustic emissions in rabbits104(1998); http://dx.doi.org/10.1121/1.423340View Description Hide Description
The suppression tuning of distortion-product otoacoustic emissions (DPOAEs) is commonly assumed to measure frequency selectivity, because the dominant features of suppression-tuning curves (STCs) are similar to the principal properties of the neural-tuning curves (NTCs) of single auditory-nerve fibers. In the present study, several common loop diuretics were used to affect the DPOAE-generation process to determine if reversible ototoxicity could adversely modify the characteristics of STCs, in a manner similar to that shown previously for NTCs. Contour plots of DPOAE level in the presence of a series of variable-level suppressor tones were obtained before and after administering diuretic drugs that reversibly reduced or eliminated DPOAEs. Primary-tone pairs were centered at 2.8 or 4 kHz, with or From the resulting plots, STC parameters including tip frequency, threshold at the tip frequency, and measures of tuning were extracted for four suppression criteria of 3, 6, 9, and 12 dB. In the pre-drug nonototoxic state, suppression tuning depended on both primary-tone level and the relative levels of the primaries with tuning being sharper for lower- than for higher-level equilevel primaries, and sharpest for offset-level primary tones. Following drug injection, the expected decrease in sharpness of tuning evidenced by changes in as well as the dramatically elevated tip thresholds normally seen for NTCs under similar conditions, were not observed. Overall, increased or decreased more or less randomly, with a slight tendency for STCs to become sharper than prior to drug dosing, for the two highest suppression criteria. The STC-tip frequencies demonstrated significant decreases following diuretic administration that were weakly correlated with the associated decreases in DPOAE amplitude. The most consistent changes in response to the drug-induced reduction in DPOAE level were increases in the STC-tip thresholds. However, these changes were relatively small and rarely exceeded 10 dB. In the absence of notable changes in overall STC shape, a major finding was a change in the effectiveness of suppression following ototoxic insult. However, when the amount of suppression was expressed as a percentage of the DPOAE remaining, the effects of diuretic dosing were often almost completely obscured. Overall, the results demonstrated that when the generation of DPOAEs was interfered with by the introduction of a suppressor tone to produce STCs that resemble NTCs, STCs behaved quite differently following reversible cochlear insult than their previously documented neural counterparts. These findings imply that STCs do not assess the frequency-selective aspects of the cochlear amplification process in a manner similar to NTCs.