Analysis of compressive properties of the BioAid hearing aid algorithm.

OBJECTIVE: This technical paper describes a biologically inspired hearing aid algorithm based on a computer model of the peripheral auditory system simulating basilar membrane compression, reflexive efferent feedback and its resulting properties. DESIGN: Two evaluations were conducted on the core part of the algorithm, which is an instantaneous compression sandwiched between the attenuation and envelope extraction processes of a relatively slow feedback compressor. STUDY SAMPLE: The algorithm's input/output (I/O) function was analysed for different stationary (ambient) sound levels, and the algorithm's response to transient sinusoidal tone complexes was analysed and contrasted to that of a reference dynamic compressor. RESULTS: The algorithm's emergent properties are: (1) the I/O function adapts to the average sound level such that processing is linear for levels close to the ambient sound level and (2) onsets of transient signals are marked across time and frequency. CONCLUSION: Adaptive linearisation and onset marking, as inherent compressive features of the algorithm, provide potentially beneficial features to hearing-impaired listeners with a relatively simple circuit. The algorithm offers a new, biological perspective on hearing aid amplification.

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.

Hearing dummies: individualized computer models of hearing impairment.

UNLABELLED: Objective: Our aim was to explore the usage of individualized computer models to simulate hearing loss based on detailed psychophysical assessment and to offer hypothetical diagnoses of the underlying pathology. DESIGN: Individualized computer models of normal and impaired hearing were constructed and evaluated using the psychophysical data obtained from human listeners. Computer models of impaired hearing were generated to reflect the hypothesized underlying pathology (e.g. dead regions, outer hair cell dysfunction, or reductions in endocochlear potential). These models were evaluated in terms of their ability to replicate the original patient data. STUDY SAMPLE: Auditory profiles were measured for two normal and five hearing-impaired listeners using a battery of three psychophysical tests (absolute thresholds, frequency selectivity, and compression). RESULTS: The individualized computer models were found to match the data. Useful fits to the impaired profiles could be obtained by changing only a single parameter in the model of normal hearing. Sometimes, however, it was necessary to include an additional dead region. CONCLUSION: The creation of individualized computer models of hearing loss can be used to simulate auditory profiles of impaired listeners and suggest hypotheses concerning the underlying peripheral pathology.

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.

A computer model of the auditory periphery and its application to the study of hearing.

Computer models of the auditory periphery provide a tool for -formulating theories concerning the relationship between the physiology of the auditory system and the perception of sounds both in normal and impaired hearing. However, the time-consuming nature of their construction constitutes a major impediment to their use, and it is important that transparent models be available on an 'off-the-shelf' basis to researchers. The MATLAB Auditory Periphery (MAP) model aims to meet these requirements and be freely available. The model can be used to simulate simple psychophysical tasks such as absolute threshold, pitch matching and forward masking and those used to measure compression and frequency selectivity. It can be used as a front end to automatic speech recognisers for the study of speech in quiet and in noise. The model can also simulate theories of hearing impairment and be used to make predictions about the efficacy of hearing aids. The use of the software will be described along with illustrations of its application in the study of the psychology of hearing.

Tinnitus and patterns of hearing loss.

Tinnitus is strongly linked with the presence of damaged hearing. However, it is not known why tinnitus afflicts only some, and not all, hearing-impaired listeners. One possibility is that tinnitus patients have specific inner ear damage that triggers tinnitus. In this study, differences in cochlear function inferred from psychophysical measures were measured between hearing-impaired listeners with tinnitus and hearing-impaired listeners without tinnitus. Despite having similar average hearing loss, tinnitus patients were observed to have better frequency selectivity and compression than those without tinnitus. The results suggest that the presence of subjective tinnitus may not be strongly associated to outer hair cell impairment, at least where hearing impairment is evident. The results also show a different average pattern of hearing impairment amongst the tinnitus patients, consistent with the suggestion that inner hair cell dysfunction with subsequent reduced auditory innervation is a possible trigger of tinnitus.

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.

A simple single-interval adaptive procedure for estimating thresholds in normal and impaired listeners.

This report presents a single-interval adaptive procedure for measuring thresholds in untrained normal and impaired listeners. The accuracy of the procedure is evaluated using Monte Carlo methods and human data allowing a method to be proposed for deciding in advance the number of trials required to achieve a specified level of accuracy. The number of trials depends on the slope of the psychometric function. The slope of the psychometric function is evaluated in normal and impaired listeners, and is found to give a useful guide to the required number of trials. The single-interval up/down procedure is subsequently compared with two other popular traditional methods (two-interval forced-choice, two-down/one-up and maximum-likelihood procedures) and is shown to yield similar thresholds and be more efficient.