Volume 125, Issue 3, March 2009
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
Comparing two proposed measures of cochlear mechanical filter bandwidth based on stimulus frequency otoacoustic emissionsa)125(2009); http://dx.doi.org/10.1121/1.3068452View Description Hide Description
It has been hypothesized that the sharpness of the cochlear mechanical filter is related to two measures based on stimulus frequency otoacoustic emissions (SFOAEs). The first is the group delay of the SFOAE; the second is the bandwidth of the SFOAE two-tone suppression isoinput tuning characteristic. A corollary of this is that natural variability in cochlear mechanical bandwidth within a population would lead to a positive correlation between these two SFOAE-based measures of tuning within that population. To test this prediction, SFOAE group delay and SFOAE two-tone suppression isoinput tuning characteristics were measured in a sample of 16 audiometrically normal subjects. Contrary to the prediction, no statistically significant correlation was found between the two bandwidth measures. Cochlear model simulations were used to aid the interpretation of this result. These suggested that a positive correlation between the two measures is expected, but that it may well be too weak to detect with the given sample size, due to the influence on the SFOAE measures of random inhomogeneities in basilar membrane impedance.
125(2009); http://dx.doi.org/10.1121/1.3068446View Description Hide Description
In the mammalian auditory brainstem, two types of coincidence detector cells are involved in binaural localization: excitatory-excitatory (EE) and excitatory-inhibitory (EI). Using statistics derived from EE and EI spike trains, binaural discrimination abilities of single tones were predicted. The minimum audible angle (MAA), as well as the just noticeable difference of interaural time delay (ITD) and interaural level difference (ILD) were analytically derived for both EE and EI cells on the basis of two possible neural coding patterns, rate coding that ignores a spike’s timing information and all-information coding (AIN), which considers a spike’s timing occurrences. Simulation results for levels below saturation were qualitatively compared to experimental data, which yielded the following conclusions: (1) ITD is primarily estimated by EEcells with AIN coding when the ipsilateral auditory input exhibits phase delay between 40° and 65°. (2) In ILD, both AIN and rate coding provide identical performances. It is most likely that ILD is primarily estimated by EI cells according to rate coding, and for ILD the information derived from the spikes’ timing is redundant. (3) For MAA estimation, the derivation should take into account ambiguous directions of a source signal in addition to its true value.
Considering distortion product otoacoustic emission fine structure in measurements of the medial olivocochlear reflex125(2009); http://dx.doi.org/10.1121/1.3068442View Description Hide Description
In humans, when the medial olivocochlear (MOC) pathway is activated by noise in the opposite ear, changes in distortion product otoacoustic emission (DPOAE) level, i.e., the MOC reflex, can be recorded in the test ear. Recent evidence suggests that DPOAE frequency influences the direction (suppression/enhancement) of the reflex. In this study, DPOAEs were recorded at fine frequency intervals from 500 to 2500 Hz, with and without contralateral acoustic stimulation (CAS) in a group of 15 adults. The MOC reflex was calculated only at DPOAE frequencies corresponding to peaks in the fine structure. Additionally, inverse fast-Fourier transform was conducted to evaluate MOC effects on individual DPOAE components. Results show the following: (1) When considering peaks only, the mean MOC reflex was and 97% of observations reflected suppression, (2) CAS reduced distortion characteristic frequency component levels more than overlap component levels, and (3) CAS produced an upward shift in fine structure peak frequency. Results indicate that when the MOC reflex is recorded at DPOAE frequencies corresponding to fine structure maxima (i.e., when DPOAE components are constructive and in phase), suppression is reliably observed and level enhancement, which probably reflects component mixing in the ear canal rather than strength of the MOC reflex, is eliminated.
Use of stimulus-frequency otoacoustic emissions to investigate efferent and cochlear contributions to temporal overshoota)125(2009); http://dx.doi.org/10.1121/1.3068443View Description Hide Description
Behavioral threshold for a tone burst presented in a long-duration noise masker decreases as the onset of the tone burst is delayed relative to masker onset. The threshold difference between detection of early- and late-onset tone bursts is called overshoot. Although the underlying mechanisms are unclear, one hypothesis is that overshoot occurs due to efferent suppression of cochlear nonlinearity [von Klitzing, R., and Kohlrausch, A. (1994). J. Acoust. Soc. Am.95, 2192–2201]. This hypothesis was tested by using overshoot conditions to elicit stimulus-frequency otoacoustic emissions (SFOAEs), which provide a physiological measure of cochlear nonlinearity. SFOAE and behavioral thresholds were estimated using a modified maximum-likelihood yes-no procedure. The masker was a 400-ms “frozen” notched noise. The signal was a 20-ms, 4-kHz tone burst presented at 1 or 200 ms after the noise onset. Behavioral overshoot results replicated previous studies, but no overshoot was observed in SFOAE thresholds. This suggests that either efferent suppression of cochlear nonlinearity is not involved in overshoot, or a SFOAE threshold estimation procedure based on stimuli similar to those used to study behavioral overshoot is not sensitive enough to measure the effect.
125(2009); http://dx.doi.org/10.1121/1.3075551View Description Hide Description
Quantifying how the sound delivered to the ear canal relates to hearing threshold has historically relied on acoustic calibration in physical assemblies with an input impedance intended to match the human ear (e.g., a Zwislocki coupler). The variation in the input impedance of the human ear makes such a method of calibration questionable. It is preferable to calibrate the acoustic signal in each ear individually. By using a calibrated sound source and microphone, the acoustic input impedance of the ear can be determined, and the sound delivered to the ear calibrated in terms of either (i) the incident sound pressurewave or (ii) that portion of the incident sound pressurewave transmitted to the middle ear and cochlea. Hearing thresholds expressed in terms of these quantities are reported, these in situcalibrations not being confounded by ear canal standing waves. Either would serve as a suitable replacement for the current practice of hearing thresholds expressed in terms of sound pressure level calibrated in a 6cc or coupler.