Adaptive plasticity of loudness induced by chronic attenuation and enhancement of the acoustic background.
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Acoustic Stimulation↗
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Testing the concept of softness imperception: loudness near threshold for hearing-impaired ears.
Buus and Florentine [J. Assoc. Res. Otolaryngol. 3, 120-139 (2002)] have proposed that loudness recruitment in cases of cochlear hearing loss is caused partly by an abnormally large loudness at absolute threshold. This has been called "softness imperception." To evaluate this idea, loudness-matching functions were obtained using tones at very low sensation levels. For subjects with asymmetrical hearing loss, matches were obtained for a single frequency across ears. For subjects with sloping hearing loss, matches were obtained between tones at two frequencies, one where the absolute threshold was nearly normal and one where there was a moderate hearing loss. Loudness matching was possible for sensation levels (SLs) as low as 2 dB. When the fixed tone was presented at a very low SL in an ear (or at a frequency) where there was hearing impairment, it was matched by a tone with approximately the same SL in an ear (or at a frequency) where hearing was normal (e.g., 2 dB SL matched 2 dB SL). This relationship held for SLs up to 4-10 dB, depending on the subject. These results are not consistent with the concept of softness imperception.
Adult↗
The effects of neural synchronization and peripheral compression on the acoustic-reflex threshold.
This study investigates the acoustic reflex threshold (ART) dependency on stimulus phase utilizing low-level reflex audiometry [Neumann et al., Audiol. Neuro-Otol. 1, 359-369 (1996)]. The goal is to obtain optimal broadband stimuli for elicitation of the acoustic reflex and to obtain objective determinations of cochlear hearing loss. Three types of tone complexes with different phase characteristics were investigated: A stimulus that compensates for basilar-membrane dispersion, thus causing a large overall neural synchrony (basilar-membrane tone complex-BMTC), the temporally inversed stimulus (iBMTC), and random-phase tone complexes (rTC). The ARTs were measured in eight normal-hearing and six hearing-impaired subjects. Five different conditions of peak amplitude and stimulus repetition rate were used for each stimulus type. The results of the present study suggest that the ART is influenced by at least two different factors: (a) the degree of synchrony of neural activity across frequency, and (b) the fast-acting compression mechanism in the cochlea that is reduced in the case of a sensorineural hearing loss. The results allow a clear distinction of the two subjects groups based on the different ART for the utilized types and conditions of the stimuli. These differences might be useful for objective recruitment detection in clinical diagnostics.
Adult↗
Narrow-band AP latencies in normal and recruiting human ears.
Derived narrow-band action potential latencies increase monotonically with decreasing central frequency, and can be interpreted as reflecting the traveling wave delay in the cochlea. It was found that, for recruiting human ears with average flat hearing losses around 40 dB, this accumulating latency increase was smaller than for normal ears. A comparison of 15 normal ears and 37 recruiting ears showed, however, that in only half of the recruiting ears this difference was significant. These recruiting ears were therefore divided in two groups based on the waveform of the narrow-band action potential AP, which correlated well with the subdivision according to latency. The findings have been explained on the basis that latency of the narrow-band APs is not determined solely by the mechanical traveling-wave delay, but also by the response time of the (second?) cochlear filter. When this filter broadens, one expects a decrease in its impulse response time. Since this impulse response time. Since this impulse response depends on the sum of the high- and low-frequency slope values of the cochlear filter, one expects only a latency decrease when the steep high-frequency slope also becomes more shallow. A support for the influence of the response times of the cochlear filter is found in the narrow-band AP latencies for restricted cochlear losses (e.g., in a 4-kHz noise dip). It appears that the latency in that area actually is shorter than for the higher central frequencies, a fact which cannot be explained solely on the basis of a traveling wave phenomenon.
Acoustic Stimulation↗
Rate-versus-intensity functions and related AP responses in normal and pathological guinea pig and human cochleas.
Cochlear fiber discharge rate-versus-intensity functions, across frequency, have been measured in pathological guinea pig cochleas (ototoxic antibiotic poisoning) and compared with the normal animal. The frequency dependence of the slopes of these functions is reduced in cochlear pathology which results in minimum threshold elevations of more than 50 dB, i.e., there is a reduction in this frequency dependent nonlinearity. The rate functions at characteristic frequency (CF) become abnormally steep (e.g., 4--5 spikes/s/dB compared with a normal 1--2 spikes/s/dB), and comparable to those of the low-frequency tail region of normal cochlear fibers. The CF dynamic range is reduced from 30--40 to 10--15 dB in cochlear pathology. The fiber study has been confirmed by (and has confirmed) a method of indirect measurement of rate functions from AP suppression-versus-intensity functions using a pure-tone forward masking paradigm [Abbas and Gorga, J. Acoust. Soc. Am. 69, 492--499 (1981)]. This method has been used in normal and pathological guinea pigs, and the results parallel the single fiber study. In addition, AP suppression functions, across frequency, have been obtained in human subjects with (near) normal hearing thresholds, and in patients with sensorineural hearing loss of cochlear origin, during transtympanic electrocochleography. The AP suppression curves in pathology indicate, as for the animal studies, a loss of the frequency dependency of the rate function slopes, and predict steep rate functions at CF (and thus reduced dynamic range) in cochlear deafness. The findings are related to loudness recruitment.
Acoustics↗
Effects of flanking noise bands on the rate of growth of loudness of tones in normal and recruiting ears.
Five subjects with unilateral cochlear hearing impairments and three normally hearing subjects made loudness matches between tones presented alternately to two ears, as a function of the intensity of the tone in the impaired ear (or the left ear of the normal subjects). The impaired ears showed recruitment; the rate of growth of loudness with increasing intensity was more rapid in the impaired ear than the normal ear. Presenting the tone in the impaired ear with two noise bands on either side of the tone frequency, at a fixed signal-to-noise ratio, did not abolish the recruitment. This suggests that recruitment is not caused by an abnormally rapid spread of excitation in the peripheral auditory system. At low signal-to-noise ratios, a continuous background noise reduced the loudness of the tone more than a noise gated with the tone, suggesting that the continuous noise induces adaptation to the tone. The noise had a greater effect on the loudness of the tone in normal ears than in impaired ears. It is possible that the loudness reduction of the tone in noise is mediated by suppression; suppression is weak or absent in impaired ears, and so the loudness reduction is smaller.
Aged↗
On the relations of intensity jnd's to loudness and neural noise.
It is shown experimentally that, in contradiction of the fundamental concept of Fechner's law, the intensity jnd for auditory sinusoidal signals follows loudness, rather than its derivative with respect to sound intensity. The evidence is obtained by comparing the jnd's of a population with normal hearing to those of a population with hearing loss accompanied by loudness recruitment. Although the recruitment increases the slope of the loudness function, the jnd's of both populations were found to be practically equal when the loudness were equal. The phenomenon is accounted for mathematically by assuming that psychophysically relevant neural noise depends not only on the magnitude of loudness, but also on its derivative with respect to sound intensity. A related derivation accounts for the near miss to Weber's law.
Adult↗
Spike-rate intensity functions of cat cortical neurons studied with combined tone-noise stimuli.
In the auditory cortex of anesthetized cats, single neurons were studied for their sensitivity to tones presented against backgrounds of continuous wide-spectrum noise. Tone pulses and noise stimuli were mixed acoustically and presented using calibrated, sealed stimulating systems. Data collected were spike-rate intensity functions for tones delivered alone and in the presence of noise. In most neurons, noise of any given intensity induced tone sensitivity shifts that were greatest for frequencies to which the neurons were most sensitive. When the sensitivity loss was in excess of about 15 dB, continuous noise usually caused a steepening of the slope of the tone intensity function. These data suggest that the excitatory response area of a cortical neuron is shaped by multiple, incompletely overlapping inputs of varying sensitivities. In the presence of a continuous noise mask, the disparate thresholds of these inputs may be brought into closer register, resulting in a steepened rate intensity function. These observations may be germane to the neural genesis of the "recruitment" seen in the loudness judgments of normal listeners for masked tones.
Acoustic Stimulation↗
The negative effect of amplitude compression in multichannel hearing aids in the light of the modulation-transfer function.
The article deals with the question of why multichannel amplitude compression appears to have a negative rather than a positive effect on speech intelligibility by hearing-impaired listeners. It is argued that the small time constants of amplitude compression diminish the temporal as well as the spectral contrasts in the speech signal. According to the modulation-transfer function concept, this results in reduced intelligibility scores. Experimental evidence is reviewed indicating that the following two arguments in favor of amplitude compression in case of sensorineural hearing loss are not valid: (1) to compensate for the effects of loudness recruitment and (2) to get weak consonants above threshold. The author concludes that, in multichannel hearing aids, automatic gain control with time constants of 0.25-0.5 s should be given preference to amplitude compression.
Adult↗
Villchur revisited: another look at automatic gain control simulation of recruiting hearing loss.
An algorithm to simulate the effects of sensorineural hearing impairment on speech reception was investigated. Like that described by Villchur [J. Acoust. Soc. Am. 62, 665-674 (1977)], this simulation employs automatic gain control in independent frequency bands to reproduce the elevated audibility thresholds and loudness recruitment that are characteristic of this type of loss. In the present implementation, band gains are controlled in an effort to simulate loudness recruitment directly, using recruitment functions that depend only on the magnitude of hearing loss in the band. In a preliminary evaluation, two normal-hearing subjects listened to the simulation matched to hearing losses studied previously [Zurek and Delhorne, J. Acoust. Soc. Am. 82, 1548-1559 (1987)] with noise-masking simulations. This evaluation indicated that the present automatic gain control simulation yielded scores roughly similar to those of both the hearing-impaired listeners and the masked-normal listeners. In the more-detailed evaluation, the performance of three listeners with severe sensorineural hearing loss on several speech intelligibility tests was compared to that of normal-hearing subjects listening to the output of the simulation. These tests included consonant-vowel syllable identification and sentence keyword identification for several combinations of speech-to-noise ratio, frequency-gain characteristic, and overall level. Generally, the simulation algorithm reproduced speech intelligibility well, though there was a clear trend for the simulation to result in better intelligibility than observed for impaired listeners when high-frequency emphasis placed more of the speech spectrum above threshold at higher frequencies. Also, the hearing-impaired listener with the greatest loss showed the largest discrepancies with the simulation. Overall, however, the simulation provides a very good approximation to speech reception by hearing-impaired listeners. The results of this study, together with previous studies of noise-making simulation, suggest that threshold elevation and recruitment, which are necessary features of a simulation of cochlear hearing loss, can also be largely sufficient for simulating the speech-reception performance of listeners with moderate to severe hearing impairments.
Adult↗
Simulation of the effect of threshold elevation and loudness recruitment combined with reduced frequency selectivity on the intelligibility of speech in noise.
The effect of loudness recruitment and threshold elevation together with reduced frequency selectivity have been simulated to examine the combined effect of the two major consequences of cochlear hearing loss on the intelligibility of speech in speech-shaped noise. In experiment 1, four conditions were simulated: a moderate flat loss with auditory filters broadened by a factor of three (B3R2); a moderate-to-severe sloping loss with auditory filters broadened by a constant factor of three (B3RX); and these conditions with linear amplification applied prior to the simulation processing (B3R2+, B3RX+). For conditions B3R2 and B3RX, performance was markedly worse than for a control condition (normal hearing, condition R1) tested in a previous study. For conditions B3R2+ and B3RX+, linear amplification improved performance considerably. However, performance remained below that for condition R1 by between 5% and 19%. In experiment 2 the broadening of the auditory filters was made more realistic by making it a function of the absolute threshold at the center frequency of the auditory filter. Three different hearing losses were simulated: a moderate-to-severe sloping loss with variable broadening of the auditory filters (BXRX); the same moderate-to-severe sloping loss with linear amplification (BXRX+); and the same broadening of the auditory filters but without the simulation of loudness recruitment and threshold elevation (BX). For condition BXRX, performance was markedly worse than in condition R1, while performance in condition BX was somewhat worse than for condition R1. For condition BXRX+, linear amplification according to the NAL procedure improved performance to a large extent but it remained worse than for condition R1. The results are consistent with previous evidence indicating that only part of the decrease of performance produced by actual cochlear hearing loss can be compensated by conventional linear hearing aids.
Auditory Threshold↗
Effects of amplitude nonlinearity on phoneme recognition by cochlear implant users and normal-hearing listeners.
It is widely assumed that the proper transformation of acoustic amplitude to electric amplitude is a critical factor affecting speech recognition in cochlear implant users and normal-hearing listeners. A four-channel noise-band speech processor was implemented, reducing spectral information to four bands. A power-law transformation was applied to the amplitude mapping stage in the speech processor design, and the exponent of the power function varied from a strongly compressive (p = 0.05) to a weakly compressive (p = 0.75) for implant listeners and from 0.3 to 3.0 for acoustic listeners. Results for implants showed that the best performance was achieved with an exponent of about 0.2, and performance gradually deteriorated when either more compressive or less compressive exponents were applied. The loudness growth functions of the four activated electrodes in each subject were measured and those data were well fit by a power function with a mean exponent of 2.72. The results indicated that the best performance was achieved when the normal loudness growth was restored. For acoustic listeners, results were similar to those observed with cochlear implant listeners, except that best performance was achieved with no amplitude nonlinearity (p = 1.0). The similarity of results in both acoustic and electric stimulation indicated that the performance deterioration observed for extreme nonlinearity was due to similar perceptual effects. The function relating amplitude mapping exponent and performance was relatively flat, indicating that phoneme recognition was only mildly affected by amplitude nonlinearity.
Adult↗
Current perspectives on tinnitus.
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Child↗
Audition: some relations between normal and pathological hearing.
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Animals↗
Tinnitus retraining therapy: a different view on tinnitus.
Tinnitus retraining therapy (TRT) is a method for treating tinnitus and decreased sound tolerance, based on the neurophysiological model of tinnitus. This model postulates involvement of the limbic and autonomic nervous systems in all cases of clinically significant tinnitus and points out the importance of both conscious and subconscious connections, which are governed by principles of conditioned reflexes. The treatments for tinnitus and misophonia are based on the concept of extinction of these reflexes, labeled as habituation. TRT aims at inducing changes in the mechanisms responsible for transferring signal (i.e., tinnitus, or external sound in the case of misophonia) from the auditory system to the limbic and autonomic nervous systems, and through this, remove signal-induced reactions without attempting to directly attenuate the tinnitus source or tinnitus/misophonia-evoked reactions. As such, TRT is effective for any type of tinnitus regardless of its etiology. TRT consists of: (1) counseling based on the neurophysiological model of tinnitus, and (2) sound therapy (with or without instrumentation). The main role of counseling is to reclassify tinnitus into the category of neutral stimuli. The role of sound therapy is to decrease the strength of the tinnitus signal. It is crucial to assess and treat tinnitus, decreased sound tolerance, and hearing loss simultaneously. Results from various groups have shown that TRT can be an effective method of treatment.
Acoustic Stimulation↗
On the differential diagnosis, by Fowler's "loudness recruitment" test, of changes in the cochlear nerve, with reference to the "loudness shock test".
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Cochlear Nerve↗
Nonlinearity of mechanoelectrical transduction of outer hair cells as the source of nonlinear basilar-membrane motion and loudness recruitment.
The sound-induced travelling wave in the mammalian cochlea is believed to be enhanced and sharpened by a positive-feedback mechanism, causing the passive, linear growth function of the basilar membrane (BM) to become nonlinear. Based on direct measurements of the receptor potential of isolated outer hair cells, it is shown here how nonlinear BM motion might be due predominantly to the nonlinear growth function of the receptor potential. Since intensity coding in the inner ear is supposed to depend on an interaction of nonlinear BM motion with afferent fibres of different synaptic thresholds, intensity coding is expected to be directly dependent on the mechanoelectrical transduction of outer hair cells (OHC). According to the present experimental data and the feedback concept of outer hair cell action, disruption of the mechanoelectrical transduction of OHC leads to both a reduction of gain and linearizing of the response; that is, to both hearing loss and loudness recruitment.
Animals↗
Audiological considerations in Ménière's disease.
The audiological findings in Ménière's disease are reviewed particularly in respect of loudness recruitment and speech discrimination loss. Although recruitment is pathognomonic of all cochlear lesions, certain distinguishing features are apparent in Ménière's disease which differ from those in other non-metabolic disorders of the cochlea. With mild degrees of hearing loss, recruitment appears to aid speech discrimination supporting the suggestion that it represents the ears adjustment to failing hearing. With more profound degrees of hearing loss, there is a linear inverse relationship between maximum discrimination score and hearing level. Actual hearing aid gains preferred by patients correlate well with those predicted from a consideration of recruitment curves.
Hearing↗