Volume 126, Issue 2, August 2009
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
Distortion-product otoacoustic emission input/output characteristics in normal-hearing and hearing-impaired human ears126(2009); http://dx.doi.org/10.1121/1.3158859View Description Hide Description
Distortion-product otoacoustic emission (DPOAE) input/output (I/O) functions were measured in 322 ears of 176 subjects at as many as 8 frequencies per ear for a total of 1779 I/O functions. The frequencies ranged from 0.7 to 8 kHz in half-octave steps. Behavioral thresholds (BTs) at the frequencies ranged from −5 to 60 dB hearing loss (HL). Both linear-pressure and nonlinear, two-slope functions were fitted to the data. The two-slope function describes I/O compression as output-controlled self-suppression. Most I/O functions (96%) were better fitted by the two-slope method. DPOAE thresholds based on each method were used to predict BTs. Compared to estimates based on linear-pressure functions, individual BTs predicted from DPOAE thresholds based on the two-slope model had lower residual error and accounted for more variance. Another advantage of the two-slope method is that it provides an estimate of response growth rate (RGR) that is not tied to threshold. At all frequencies, the median low-level RGR (across I/O functions of the same and BT) usually increased as BT increased, while high-level compression decreased. The observed characteristics of DPOAE I/O functions are consistent with the loss of cochlear compression that is typically associated with mild-to-moderate HL.
126(2009); http://dx.doi.org/10.1121/1.3158861View Description Hide Description
It is somewhat surprising that linear analysis can account for so many features of the cochlea when it is inherently nonlinear. For example, the commonly detected spacing between adjacent spontaneous otoacoustic emissions (SOAEs) is often explained by a linear theory of “coherent reflection” [Zweig and Shera (Year: 1995). J. Acoust. Soc. Am.98, 2018–2047]. The nonlinear saturation of the cochlear amplifier is, however, believed to be responsible for stabilizing the amplitude of a SOAE. In this investigation, a state space model is used to first predict the linear instabilities that arise, given distributions of cochlear inhomogeneities, and then subsequently to simulate the time-varying spectra of the nonlinear models. By comparing nonlinear simulation results to linear predictions, it is demonstrated that nonlinear effects can have a strong impact on the steady-state response of an unstable cochlear model. Sharply tuned components that decay away exponentially within 100 ms are shown to be due to linearly resonant modes of the model generated by the cochlear inhomogeneities. Some oscillations at linearly unstable frequencies are suppressed over a longer time scale, whereas those that persist are due to linear instabilities and their distortion products.
126(2009); http://dx.doi.org/10.1121/1.3158919View Description Hide Description
A nonlinear piezoelectric circuit is proposed to model electromechanical properties of the outer hair cell (OHC) in mammalian cochleae. The circuit model predicts (a) that the nonlinear capacitance decreases as the stiffness of the load increases, and (b) that the axial compliance of the cell reaches a maximum at the same membrane potential for peak capacitance. The model was also designed to be integrated into macro-mechanical models to simulate cochlear wave propagation. Analytic expressions of the cochlear-partition shunt admittance and the wave propagation function are derived in terms of OHC electro-mechanical parameters. Small-signal analyses indicate that, to achieve cochlear amplification, (1) nonlinear capacitance must be sufficiently high and (2) the OHC receptor current must be sensitive to the velocity of the reticular lamina.
126(2009); http://dx.doi.org/10.1121/1.3158855View Description Hide Description
Gender identification of human voices was studied in a juvenile population of cochlear implant(CI) users exposed to naturalistic speech stimuli from 20 male and 20 female speakers using two different voice gender perception tasks. Stimulus output patterns were recorded from each individual CI for each stimulus, and features related to voice fundamental frequency and spectral envelope were extracted from these electrical output signals to evaluate the relationship between implant output and behavioral performance. In spite of the fact that temporal and place cues of similar quality were produced by all CI devices, only about half of the subjects were able to label male and female voices correctly. Participants showed evidence of using available temporal cues, but showed no evidence of using place cues. The implants produced a consistent and novel cue to voice gender that participants did not appear to utilize. A subgroup of participants could discriminate male and female voices when two contrasting voices were presented in succession, but were unable to identify gender when voices were singly presented. It is suggested that the nature of long-term auditory categorical memories needs to be studied in more detail in these individuals.