Adversarial relationship between combined medial olivocochlear (MOC) and middle-ear-muscle (MEM) reflexes and alarm-in-noise detection thresholds under negative signal-to-noise ratios (SNRs).

The role of auditory efferent feedback from the medial olivocochlear system (MOCS) and the middle-ear-muscle (MEM) reflex in tonal detection tasks for humans in the presence of noise is not clearly understood. Past studies have yielded inconsistent results on the relationship between efferent feedback and tonal detection thresholds. This study attempts to address this inconsistency. Fifteen human subjects with normal hearing participated in an experiment where they were asked to identify an alarm signal in the presence of 80 dBA background (pink) noise. Masked detection thresholds were estimated using the method of two-interval forced choice (2IFC). Contralateral suppression of transient-evoked otoacoustic emissions (TEOAEs) was measured to estimate the strength of auditory efferent feedback. Subsequent correlation analysis revealed that the contralateral suppression of TEOAEs was significantly negatively correlated (r = -0.526, n = 15, p = 0.0438) with alarm-in-noise (AIN) detection thresholds under negative signal-to-noise conditions. The result implies that the stronger the auditory efferent feedback, the worse the detection thresholds and thus the poorer the tonal detection performance in the presence of loud noise.

Exploration of a physiologically-inspired hearing-aid algorithm using a computer model mimicking impaired hearing.

OBJECTIVE: To use a computer model of impaired hearing to explore the effects of a physiologically-inspired hearing-aid algorithm on a range of psychoacoustic measures. DESIGN: A computer model of a hypothetical impaired listener's hearing was constructed by adjusting parameters of a computer model of normal hearing. Absolute thresholds, estimates of compression, and frequency selectivity (summarized to a hearing profile) were assessed using this model with and without pre-processing the stimuli by a hearing-aid algorithm. The influence of different settings of the algorithm on the impaired profile was investigated. To validate the model predictions, the effect of the algorithm on hearing profiles of human impaired listeners was measured. STUDY SAMPLE: A computer model simulating impaired hearing (total absence of basilar membrane compression) was used, and three hearing-impaired listeners participated. RESULTS: The hearing profiles of the model and the listeners showed substantial changes when the test stimuli were pre-processed by the hearing-aid algorithm. These changes consisted of lower absolute thresholds, steeper temporal masking curves, and sharper psychophysical tuning curves. CONCLUSION: The hearing-aid algorithm affected the impaired hearing profile of the model to approximate a normal hearing profile. Qualitatively similar results were found with the impaired listeners' hearing profiles.

Acquisition of auditory profiles for good and impaired hearing.

OBJECTIVE: The aim of this study was to develop a user-friendly way of measuring patients' threshold and supra-threshold hearing, with potential for application in clinical research. The end-product of these tests is a graphical profile summarizing absolute threshold, frequency selectivity, and compression characteristics across a spectrum of frequencies (0.25-6 kHz). DESIGN: A battery of three psychophysical hearing tests consisted of measures of absolute threshold, frequency selectivity, and compression. An automated, cued, single-interval, adaptive tracking procedure was employed. The tests results were collated and used to generate a readily visualized 'profile' for each listener. STUDY SAMPLE: Participants were 83 adults (57 impaired-hearing and 26 good-hearing, age 20-75 years). RESULTS: Listeners tolerated the tests well. Single-ear profiles were obtained in an average of 74 minutes testing time (range 46-120 minutes). The variability of individual measurements was low. Substantial differences between normal and impaired listeners and also among the impaired listeners were observed. Qualitative differences in compression and frequency-selectivity were seen that could not be predicted by threshold measurements alone. CONCLUSIONS: The hearing profiles are informative with respect to supra-threshold hearing performance and the information is easily accessible through the graphical display. Further development is required for routine use in a clinical context.

The psychophysics of absolute threshold and signal duration: a probabilistic approach.

The absolute threshold for a tone depends on its duration; longer tones have lower thresholds. This effect has traditionally been explained in terms of "temporal integration" involving the summation of energy or perceptual information over time. An alternative probabilistic explanation of the process is formulated in terms of simple equations that predict not only the time ∕ duration dependence but also the shape of the psychometric function at absolute threshold. It also predicts a tight relationship between these two functions. Measurements made using listeners with either normal or impaired hearing show that the probabilistic equations adequately fit observed threshold-duration functions and psychometric functions. The mathematical formulation implies that absolute threshold can be construed as a two-valued function: (a) gain and (b) sensory threshold, and both parameters can be estimated from threshold-duration data. Sensorineural hearing impairment is sometimes associated with a smaller threshold ∕ duration effect and sometimes with steeper psychometric functions. The equations explain why these two effects are expected to be linked. The probabilistic approach has the potential to discriminate between hearing deficits involving gain reduction and those resulting from a raised sensory threshold.

Further studies on the dual-resonance nonlinear filter model of cochlear frequency selectivity: responses to tones.

A number of phenomenological models that simulate the response of the basilar membrane motion can reproduce a range of complex features observed in animal measurements over different sites along its cochlea. The present report shows a detailed analysis of the responses to tones of an improved model based on a dual-resonance nonlinear filter. The improvement consists in adding a third path formed by a linear gain and an all-pass filter. This improvement allows the model to reproduce the gain and phase plateaus observed empirically at frequencies above the best frequency. The middle ear was simulated by using a digital filter based on the empirical impulse response of the chinchilla stapes. The improved algorithm is evaluated against observations of basilar membrane responses to tones at seven different sites along the chinchilla cochlear partition. This is the first time that a whole set of animal observations using the same technique has been available in one species for modeling. The resulting model was able to simulate amplitude and phase responses to tones from basal to apical sites. Linear regression across the optimized parameters for seven different sites was used to generate a complete filterbank.

A computer model of the auditory-nerve response to forward-masking stimuli.

A computer model of the auditory periphery is used to study the involvement of auditory-nerve (AN) adaptation in forward-masking effects. An existing model is shown to simulate published AN recovery functions both qualitatively and quantitatively after appropriate parameter adjustments. It also simulates published data showing only small threshold shifts when a psychophysical forward-masking paradigm is applied to AN responses. The model is extended to simulate a simple but physiologically plausible mechanism for making threshold decisions based on coincidental firing of a number of AN fibers. When this is used, much larger threshold shifts are observed of a size consistent with published psychophysical observations. The problem of how stimulus-driven firing can be distinguished from spontaneous activity near threshold is also addressed by the same decision mechanism. Overall, the modeling results suggest that poststimulatory reductions in AN activity can make a substantial contribution to the raised thresholds observed in many psychophysical studies of forward masking.