Loudness Recruitment Research Articles

Effects of hearing loss, age, noise exposure, and listening skills on envelope regularity discrimination.

The envelope regularity discrimination (ERD) test assesses the ability to discriminate irregular from regular amplitude modulation (AM). The measured threshold is called the irregularity index (II). It was hypothesized that the II at threshold should be almost unaffected by the loudness recruitment that is associated with cochlear hearing loss because the effect of recruitment is similar to multiplying the AM depth by a certain factor, and II values depend on the amount of envelope irregularity relative to the baseline modulation depth. To test this hypothesis, the ERD test was administered to 60 older adults with varying degrees of hearing loss, using carrier frequencies of 1 and 4 kHz. The II values for the two carrier frequencies were highly correlated, indicating that the ERD test was measuring a consistent characteristic of each subject. The II values at 1 and 4 kHz were not significantly correlated with the audiometric thresholds at the corresponding frequencies, consistent with the hypothesis. The II values at 4 kHz were significantly positively correlated with age. There was an unexpected negative correlation between II values and a measure of noise exposure. This is argued to reflect the confounding effects of listening skills.

Machine Learning Based Dynamic Band Selection for Splitting Auditory Signals to Reduce Inner Ear Hearing Losses

Quality of hearing has been severely impacted due to signal losses occurs in the human inner ear explicitly in the region of cochlea. Loudness recruitment, degraded frequency selectivity and auditory masking are the major outward effects of inner ear hearing losses. Splitting auditory signals into frequency bands and presenting dichotically to both ears became a comprehensive solution to reduce inner ear hearing losses. However, these methods divide input signal into the fix number of frequency bands, this limits their applicability where signals have large variations in their spectral characteristics. To address this challenge, we have proposed machine learning based intelligent band selection algorithm to split auditory signals dynamically. Proposed algorithm analyze input speech signal based on spectral characteristics to determine the optimum number of bands required to effectively present major acoustic cues of the signal. Further, dynamic splitting algorithm efficiently divides signal for dichotic presentation. Proposed method has been examined on large number of subjects from different age groups and gender having cochlear hearing impairment. Qualitative and quantitative assessment shown significant improvement in the recognition score with substantial reduction in the response time.

A Prospective Study of Etiology and Auditory Profiles in Infants with Congenital Unilateral Sensorineural Hearing Loss.

Congenital unilateral sensorineural hearing loss (uSNHL) is associated with speech-language delays and academic difficulties. Yet, controversy exists in the choice of diagnosis and intervention methods. A cross-sectional prospective design was used to study hearing loss cause in twenty infants with congenital uSNHL consecutively recruited from a universal neonatal hearing-screening program. All normal-hearing ears showed ≤20 dB nHL auditory brainstem response (ABR) thresholds (ABRthrs). The impaired ear median ABRthr was 55 dB nHL, where 40% had no recordable ABRthr. None of the subjects tested positive for congenital cytomegalovirus (CMV) infection. Fourteen subjects agreed to participate in magnetic resonance imaging (MRI). Malformations were common for all degrees of uSNHL and found in 64% of all scans. Half of the MRIs demonstrated cochlear nerve aplasia or severe hypoplasia and 29% showed inner ear malformations. Impaired ear and normal-hearing ear ABR input/output functions on a group level for subjects with ABRthrs < 90 dB nHL were parallel shifted. A significant difference in interaural acoustic reflex thresholds (ARTs) existed. In congenital uSNHL, MRI is powerful in finding a possible hearing loss cause, while congenital CMV infection may be relatively uncommon. ABRs and ARTs indicated an absence of loudness recruitment, with implications for further research on hearing devices.

A Review of the Neurobiological Mechanisms that Distinguish Between Loudness Recruitment and Hyperacusis.

Loudness recruitment is a common symptom of hearing loss induced by cochlear lesions, which is defined as an abnormally fast growth of loudness perception of sound intensity. This is different from hyperacusis, which is defined as “abnormal intolerance to regular noises” or “extreme amplification of sounds that are comfortable to the average individual”. Although both are characterized by abnormally high sound amplification, the mechanisms of occurrence are distinct. Damage to the outer hair cells alters the nonlinear characteristics of the basilar membrane, resulting in aberrant auditory nerve responses that may be connected to loudness recruitment. In contrast, hyperacusis is an aberrant condition characterized by maladaptation of the central auditory system. Peripheral injury can produce fluctuations in loudness recruitment, but this is not always the source of hyperacusis. Hyperacusis can also be accompanied by aversion to sound and fear of sound stimuli, in which the limbic system may play a critical role. This brief review aims to present the current status of the neurobiological mechanisms that distinguish between loudness recruitment and hyperacusis.

Spectral weighting functions for localization of complex sound: Effects of sensorineural hearing loss

When localization cues are distributed across the spectrum of a complex sound, listeners with normal hearing (NH) utilize cues near the 600-800 Hz “ITD dominance region” [Bilsen and Raatgever, Acustica 28, 131–132 (1973)]. Recent work has quantified this pattern using spectral weighting functions (SWFs) [Folkerts and Stecker, ARO 42, 505 (2018)] in NH listeners. However, the salience of the ITD dominance region in listeners with sensorineural hearing loss (SNHL) remains unexamined. Folkerts and Stecker [ARO 45, 171 (2022)] used simulated SNHL in NH listeners to measure impacts of elevated hearing thresholds, loudness recruitment, and reduced high-frequency ITD sensitivity [Steinberg and Gardener, J Acoust. Soc Am 9, 11–23 (1937)] on SWFs. The current study measured SWFs in listeners with bilateral sloping high-frequency SNHL. Direct comparisons to SWFs measured with simulated SNHL test the hypothesis that reduced audibility of high-frequency components shifts the ITD dominance region to lower frequencies (i.e., 400–600 Hz). Individual SWFs measured in listeners with SNHL are compared to their headphone and free field thresholds to assess whether audibility can account for weighting changes. [Work supported by NIH R01-DC016643.]

Measurements From Ears With Endolymphatic Hydrops and 2-Hydroxypropyl-Beta-Cyclodextrin Provide Evidence That Loudness Recruitment Can Have a Cochlear Origin.

Background: Loudness recruitment is commonly experienced by patients with putative endolymphatic hydrops. Loudness recruitment is abnormal loudness growth with high-level sounds being perceived as having normal loudness even though hearing thresholds are elevated. The traditional interpretation of recruitment is that cochlear amplification has been reduced. Since the cochlear amplifier acts primarily at low sound levels, an ear with elevated thresholds from reduced cochlear amplification can have normal processing at high sound levels. In humans, recruitment can be studied using perceptual loudness but in animals physiological measurements are used. Recruitment in animal auditory-nerve responses has never been unequivocally demonstrated because the animals used had damage to sensory and neural cells, not solely a reduction of cochlear amplification. Investigators have thus looked for, and found, evidence of recruitment in the auditory central nervous system (CNS). While studies on CNS recruitment are informative, they cannot rule out the traditional interpretation of recruitment originating in the cochlea.Design: We used techniques that could assess hearing function throughout entire frequency- and dynamic-range of hearing. Measurements were made from two animal models: guinea-pig ears with endolymphatic-sac-ablation surgery to produce endolymphatic hydrops, and naïve guinea-pig ears with cochlear perfusions of 13 mM 2-Hydroxypropyl-Beta-Cyclodextrin (HPBCD) in artificial perilymph. Endolymphatic sac ablation caused low-frequency loss. Animals treated with HPBCD had hearing loss at all frequencies. None of these animals had loss of hair cells or synapses on auditory nerve fibers.Results: In ears with endolymphatic hydrops and those perfused with HPBCD, auditory-nerve based measurements at low frequencies showed recruitment compared to controls. Recruitment was not found at high frequencies (> 4 kHz) where hearing thresholds were normal in ears with endolymphatic hydrops and elevated in ears treated with HPBCD.Conclusions: We found compelling evidence of recruitment in auditory-nerve data. Such clear evidence has never been shown before. Our findings suggest that, in patients suspected of having endolymphatic hydrops, loudness recruitment may be a good indication that the associated low-frequency hearing loss originates from a reduction of cochlear amplification, and that measurements of recruitment could be used in differential diagnosis and treatment monitoring of Ménière's disease.

Using auditory reaction time to measure loudness growth in rats

Previous studies have demonstrated that auditory reaction time (RT) is a reliable surrogate of loudness perception in humans. Reaction time-intensity (RT-I) functions faithfully recapitulate equal loudness contours in humans while being easier to obtain than equal loudness judgments, especially in animals. In humans, loudness estimation not only depends on sound intensity, but on a variety of other acoustic factors. Stimulus duration and bandwidth are known to impact loudness perception. In addition, the presence of background noise mimics loudness recruitment; loudness growth is rapid near threshold, but growth becomes normal at suprathreshold levels. Therefore, to evaluate whether RT-I functions are a reliable measure of loudness growth in rats, we obtained auditory RTs across a range of stimulus intensities, durations, and bandwidths, in both quiet and in the presence of background/masking noise. We found that reaction time patterns across stimulus parameters were repeatable over several months in rats and generally consistent with human loudness perceptual data. Our results provide important building blocks for future animal model studies of loudness perception and loudness perceptual disorders.

A Unified Theory of Psychophysical Laws in Auditory Intensity Perception

Psychophysical laws quantitatively relate perceptual magnitude to stimulus intensity. While most people have accepted Stevens’s power function as the psychophysical law, few believe in Fechner’s original idea using just-noticeable-differences (jnd) as a constant perceptual unit to educe psychophysical laws. Here I present a unified theory in hearing, starting with a general form of Zwislocki’s loudness function (1965) to derive a general form of Brentano’s law. I will arrive at a general form of the loudness-jnd relationship that unifies previous loudness-jnd theories. Specifically, the “slope,” “proportional-jnd,” and “equal-loudness, equal-jnd” theories, are three additive terms in the new unified theory. I will also show that the unified theory is consistent with empirical data in both acoustic and electric hearing. Without any free parameters, the unified theory uses loudness balance functions to successfully predict the jnd function in a wide range of hearing situations. The situations include loudness recruitment and its jnd functions in sensorineural hearing loss and simultaneous masking, loudness enhancement and the midlevel hump in forward and backward masking, abnormal loudness and jnd functions in cochlear implant subjects. Predictions of these loudness-jnd functions were thought to be questionable at best in simultaneous masking or not possible at all in forward masking. The unified theory and its successful applications suggest that although the specific form of Fechner’s law needs to be revised, his original idea is valid in the wide range of hearing situations discussed here.

Effect of the number of amplitude-compression channels and compression speed on speech recognition by listeners with mild to moderate sensorineural hearing loss.

The use of a large number of amplitude-compression channels in hearing aids has potential advantages, such as the ability to compensate for variations in loudness recruitment across frequency and provide appropriate frequency-response shaping. However, sound quality and speech intelligibility could be adversely affected due to reduction of spectro-temporal contrast and distortion, especially when fast-acting compression is used. This study assessed the effect of the number of channels and compression speed on speech recognition when the multichannel processing was used solely to implement amplitude compression, and not for frequency-response shaping. Computer-simulated hearing aids were used. The frequency-dependent insertion gains for speech with a level of 65 dB sound pressure level were applied using a single filter before the signal was filtered into compression channels. Fast-acting (attack, 10 ms; release, 100 ms) or slow-acting (attack, 50 ms; release, 3000 ms) compression using 3, 6, 12, and 22 channels was applied subsequently. Using a sentence recognition task with speech in two- and eight-talker babble at three different signal-to-babble ratios (SBRs), 20 adults with sensorineural hearing loss were tested. The number of channels and compression speed had no significant effect on speech recognition, regardless of babble type or SBR.

Noise-Induced loudness recruitment and hyperacusis: Insufficient central gain in auditory cortex and amygdala

Noise-induced hearing loss generally induces loudness recruitment, but sometimes gives rise to hyperacusis, a debilitating condition in which moderate intensity sounds are perceived abnormally loud. In an attempt to develop an animal model of loudness hyperacusis, we exposed rats to a 16–20 kHz noise at 104 dB SPL for 12 weeks. Behavioral reaction time-intensity functions were used to assess loudness growth functions before, during and 2-months post-exposure. During the exposure, loudness recruitment (R) was present in the region of hearing loss, but subtle evidence of hyperacusis (H) started to emerge at the border of the hearing loss. Unexpectedly, robust evidence of hyperacusis appeared below and near the edge of the hearing loss 2-months post-exposure. To identify the neural correlates of hyperacusis and test the central gain model of hyperacusis, we recorded population neural responses from the cochlea, auditory cortex and lateral amygdala 2-months post-exposure. Compared to controls, the neural output of the cochlea was greatly reduced in the noise group. Consistent with central gain models, the gross neural responses from the auditory cortex and amygdala were proportionately much larger than those from the cochlea. However, despite central amplification, the population responses in the auditory cortex and amygdala were still below the level needed to fully account for hyperacusis and/or recruitment. Having developed procedures that can consistently induce hyperacusis in rats, our results set the stage for future studies that seek to identify the neurobiological events that give rise to hyperacusis and to develop new therapies to treat this debilitating condition.

Sliding-band dynamic range compression for use in hearing aids

Sensorineural hearing loss is associated with elevated hearing thresholds, reduced dynamic range, and loudness recruitment. Signal processing with dynamic range compression is used in hearing aids for presenting all the sounds comfortably within the limited dynamic range of the listener. A dynamic range compression technique, named as ‘sliding-band compression’, is proposed to overcome the shortcomings of the commonly used single-band and multiband compression techniques. The gain at each frequency index is calculated as a function of the short-time power in an auditory critical band centered at it. The technique avoids the attenuation of high-frequency components due to the presence of strong low-frequency components, which may occur in single-band compression. Further, it avoids distortions in the shape of spectral resonances and discontinuities during the resonance transitions, which may occur in multiband compression. The proposed technique was implemented for offline processing and compared with single-band compression and multiband compression using objective measures and different test inputs. For two-tone inputs, the single-band compression showed attenuation of the high-frequency tone with an increase in the level of the low-frequency tone. For swept-frequency single-tone input, the multiband compression showed errors in the tone amplitude of 1.5–2.5 dB for compression ratio of 2–10. Sliding-band compression did not show either type of error. The technique has been also implemented using a fixed-point processor for real-time processing with audio latency acceptable for face-to-face communication.

A history of loudness before 1950

Loudness is a perceptual correlate of the physical intensity of a sound, but the relationship between loudness and intensity is not simple. The derivation from loudness-matching data of equal-loudness contours for tones across frequency was described as early as 1927. However, the most influential work on loudness prior to 1950 was the formulation by Fletcher and Munson (1933) of a method for calculating the loudness of any steady sound from the intensities of its frequency components. In a subsequent publication, Fletcher and Munson (1937) described how the loudness of a tone changes in the presence of another tone, which is a phenomenon known as masking. Stevens (1936) made extensive contributions to the theory of loudness scaling and methods for loudness measurement. Miller (1947) discovered that “just detectable increments in intensity” were of the same order of magnitude for noise as for pure tones.” Dix et al. (1948) suggested a clinical use in differential diagnosis of hearing loss for the phenomenon known as loudness recruitment, which is defined as an abnormally-rapid loudness growth. In part because of its relevance to the remediation of hearing loss, the measurement, quantification, and prediction of loudness continue to be active areas of psychophysical research.

Contributions of Louis Braida to improved signal processing for hearing aids: Addressing the problem of reduced dynamic range in listeners with sensorineural hearing loss

Lou’s early work in the area of improved signal processing for hearing aids included his research on compression amplification to combat the effects of loudness recruitment in listeners with sensorineural hearing loss. Working with his doctoral students (including Rich Lippmann, Steve De Gennaro, and Diane Bustamante), Lou made major contributions towards an analytical understanding of the benefits and limitations of compression amplification as a component of hearing aids. Recently, Lou has been involved in work on a new signal-processing scheme which operates to equalize the energy in a speech signal over time. This energy-equalization (EEQ) scheme shares a similar goal with compression amplification in that they both attempt to match the range of speech levels into the reduced dynamic range of a listener with sensorineural loss. Their operation, however, is different: while compression amplification is based on the actual sound-pressure level of the signal, the EEQ scheme operates on relative energy ca...

A Sound Therapy-Based Intervention to Expand the Auditory Dynamic Range for Loudness among Persons with Sensorineural Hearing Losses: Case Evidence Showcasing Treatment Efficacy.

Case evidence is presented that highlights the clinical relevance and significance of a novel sound therapy-based treatment. This intervention has been shown to be efficacious in a randomized controlled trial for promoting expansion of the dynamic range for loudness and increased sound tolerance among persons with sensorineural hearing losses. Prior to treatment, these individuals were unable to use aided sound effectively because of their limited dynamic ranges. These promising treatment effects are shown in this article to be functionally significant, giving rise to improved speech understanding and enhanced hearing aid benefit and satisfaction, and, in turn, to enhanced quality of life posttreatment. These posttreatment sound therapy effects also are shown to be sustained, in whole or part, with aided environmental sound and to be dependent on specialized counseling to maximize treatment benefit. Importantly, the treatment appears to be efficacious for hearing-impaired persons with primary hyperacusis (i.e., abnormally reduced loudness discomfort levels [LDLs]) and for persons with loudness recruitment (i.e., LDLs within the typical range), which suggests the intervention should generalize across most individuals with reduced dynamic ranges owing to sensorineural hearing loss. An exception presented in this article is for a person describing the perceptual experience of pronounced loudness adaptation, which apparently rendered the sound therapy inaudible and ineffectual for this individual. Ultimately, these case examples showcase the enormous potential of a surprisingly simple sound therapy intervention, which has utility for virtually all audiologists to master and empower the adaptive plasticity of the auditory system to achieve remarkable treatment benefits for large numbers of individuals with sensorineural hearing losses.

Structured Counseling for Auditory Dynamic Range Expansion.

A structured counseling protocol is described that, when combined with low-level broadband sound therapy from bilateral sound generators, offers audiologists a new tool for facilitating the expansion of the auditory dynamic range (DR) for loudness. The protocol and its content are specifically designed to address and treat problems that impact hearing-impaired persons who, due to their reduced DRs, may be limited in the use and benefit of amplified sound from hearing aids. The reduced DRs may result from elevated audiometric thresholds and/or reduced sound tolerance as documented by lower-than-normal loudness discomfort levels (LDLs). Accordingly, the counseling protocol is appropriate for challenging and difficult-to-fit persons with sensorineural hearing losses who experience loudness recruitment or hyperacusis. Positive treatment outcomes for individuals with the former and latter conditions are highlighted in this issue by incremental shifts (improvements) in LDL and/or categorical loudness judgments, associated reduced complaints of sound intolerance, and functional improvements in daily communication, speech understanding, and quality of life leading to improved hearing aid benefit, satisfaction, and aided sound quality, posttreatment.

Discrimination of amplitude-modulation depth by subjects with normal and impaired hearing.

The loudness recruitment associated with cochlear hearing loss increases the perceived amount of amplitude modulation (AM), called "fluctuation strength." For normal-hearing (NH) subjects, fluctuation strength "saturates" when the AM depth is high. If such saturation occurs for hearing-impaired (HI) subjects, they may show poorer AM depth discrimination than NH subjects when the reference AM depth is high. To test this hypothesis, AM depth discrimination of a 4-kHz sinusoidal carrier, modulated at a rate of 4 or 16 Hz, was measured in a two-alternative forced-choice task for reference modulation depths, mref, of 0.5, 0.6, and 0.7. AM detection was assessed using mref = 0. Ten older HI subjects, and five young and five older NH subjects were tested. Psychometric functions were measured using five target modulation depths for each mref. For AM depth discrimination, the HI subjects performed more poorly than the NH subjects, both at 30 dB sensation level (SL) and 75 dB sound pressure level (SPL). However, for AM detection, the HI subjects performed better than the NH subjects at 30 dB SL; there was no significant difference between the HI and NH groups at 75 dB SPL. The results for the NH subjects were not affected by age.

Cochlear microphonics recording during ABR threshold testing in children

Objective: It is agreed that the cochlear microphonic (CM) arises mainly from the normal outer hair cells (OHCs) in cochlea. The aim of this research is to study the CM characteristics in different hearing profiles and reflect the usefulness of recording CM simultaneously during Auditory Brainstem Response (ABR) threshold testing in children.Methods: This is a retrospective study that included 33 cases comprised of children with autism spectral disorders (ASD), children with cochlear sensorineural hearing loss (SNHL), children with auditory neuropathy spectrum disorder (ANSD) and 41 normal hearing healthy children as controls. The children’s ages ranged from 0.5 to 96 months. Both the CM and ABR waves were simultaneously recorded using alternating split polarity ABR.Results: CM amplitudes and thresholds in normal hearing children with ASD did not significantly differ from normal hearing healthy children. CM were preserved in children with ANSD despite the absence of distortion product OAE responses, but in a significantly lower amplitude than those with DPOAE. There was no statistically significant correlation between the children’s ages and CM amplitude at any intensity level and in any of the different child groups, except in the controls where there was a statistically significant negative correlation between the children’s ages and CM amplitude at 70dBnHL, while there was a statistically significant positive correlation between the children’s ages and CM threshold. There were no differences in the CM amplitude between the two ears at any intensity level in all of the different groups of children.Conclusion: Children with ASD showed comparable outer hair cell function to normal hearing healthy children, reflecting absence of any peripheral hyperacusis due to loudness recruitment. CM should always be searched for when testing young children when there is absence of ABR response with absence or presence of otoacoustic emissions, to avoid any false negative results for ANSD. CM can be preserved in children with SNHL with loudness recruitment. This finding could be confused with ANSD, so CM should be traced down to its threshold for an appropriate diagnosis.

Outer Hair Cell Damage: A Completely Different Listening Experience

Outer Hair Cell Damage: A Completely Different Listening Experience

Cochlear function tests in estimation of speech dynamic range

Objectives: The loss of active cochlear mechanics causes elevated thresholds, loudness recruitment, and reduced frequency selectivity. The problems faced by hearing-impaired listeners are largely related with reduced dynamic range (DR). The aim of this study was to determine which index of the cochlear function tests correlates best with the DR to speech stimuli.Methods: Audiological data on 516 ears with pure tone average (PTA) of ≤55 dB and word recognition score of ≥70% were analyzed. PTA, speech recognition threshold (SRT), uncomfortable loudness (UCL), and distortion product otoacoustic emission (DPOAE) were explored as the indices of cochlear function. Audiometric configurations were classified. Correlation between each index and the DR was assessed and multiple regression analysis was done.Results: PTA and SRT demonstrated strong negative correlations with the DR (r = −0.788 and −0.860, respectively), while DPOAE sum was moderately correlated (r = 0.587). UCLs remained quite constant for the total range of the DR. The regression equation was Y (DR) = 75.238 − 0.719 × SRT (R2 = 0.721, p < 0.001). The other variables such as audiometric configurations and DPOAE sum were excluded from the final model.Conclusion: SRT was the most predictive of the DR among the indices of the cochlear function tests. A reduced DR in cochlear hearing loss was the product of an elevated audiometric threshold and a relatively constant UCL level. The results enable prediction of the DR from SRT and possibly PTA using the suggested regression equation.

Effects of Sound-Induced Hearing Loss and Hearing Aids on the Perception of Music

Exposure to high-level music produces several physiological changes in the auditory system that lead to a variety of perceptual effects. Damage to the outer hair cells within the cochlea leads to a loss of sensitivity to weak sounds, loudness recruitment (a more rapid than normal growth of loudness with increasing sound level), and reduced frequency selectivity. Damage to inner hair cells and/or synapses leads to degeneration of neurons in the auditory nerve and to a reduced flow of information to the brain. This leads to poorer auditory discrimination and may contribute to reduced sensitivity to the temporal fine structure of sounds and to poor pitch perception. Hearing aids compensate for the effects of threshold elevation and loudness recruitment via multichannel amplitude compression, but they do not compensate for reduced frequency selectivity or loss of inner hair cells/synapses/neurons. Multichannel compression can impair some aspects of the perception of music, such as the ability to hear out one instrument or voice from a mixture. The limited frequency range and irregular frequency response of most hearing aids is associated with poor sound quality for music. Finally, systems for reducing acoustic feedback can have undesirable side effects when listening to music.