The concept of susceptibility to hearing loss

Loud Music and Leisure Noise Is a Common Cause of Chronic Hearing Loss, Tinnitus and Hyperacusis

High sound levels capable of permanently damaging the ear are experienced not only in factories and war zones but in concert halls, nightclubs, sports stadiums, and many other leisure environments. This review summarizes evidence that loud music and other forms of "leisure noise" are common causes of noise-induced hearing loss, tinnitus, and hyperacusis, even if audiometric thresholds initially remain within clinically normal limits. Given the huge global burden of preventable noise-induced hearing loss, noise limits should be adopted in a much broader range of settings, and education to promote hearing conservation should be a higher public health priority.

Utility of otoacoustic emissions and olivocochlear reflex in predicting vulnerability to noise-induced inner ear damage

Aim:

The aim of the present study was to explore the possible utility of otoacoustic emissions (OAEs) and efferent system strength to determine vulnerability to noise exposure in a clinical setting.

Materials and Methods:

The study group comprised 344 volunteers who had just begun mandatory basic training as Hellenic Corps Officers Military Academy cadets. Pure-tone audiograms were obtained on both ears. Participants were also subjected to diagnostic transient-evoked otoacoustic emissions (TEOAEs). Finally, they were all tested for efferent function through the suppression of TEOAEs with contralateral noise. Following baseline evaluation, all cadets fired 10 rounds using a 7.62 mm Heckler & Koch G3A3 assault rifle while lying down in prone position. Immediately after exposure to gunfire noise and no later than 10 h, all participants completed an identical protocol for a second time, which was then repeated a third time, 30 days later.

Results:

The data showed that after the firing drill, 280 participants suffered a temporary threshold shift (TTS) (468 ears), while in the third evaluation conducted 30 days after exposure, 142 of these ears still presented a threshold shift compared to the baseline evaluation [permanent threshold shift (PTS) ears]. A receiver operating characteristics curve analysis showed that OAEs amplitude is predictive of future TTS and PTS. The results were slightly different for the suppression of OAEs showing only a slight trend toward significance. The curves were used to determine cut points to evaluate the likelihood of TTS/PTS for OAEs amplitude in the baseline evaluation. Decision limits yielding 71.6% sensitivity were 12.45 dB SPL with 63.8% specificity for PTS, and 50% sensitivity were 12.35 dB SPL with 68.2% specificity for TTS.

Conclusions:

Interestingly, the above data yielded tentative evidence to suggest that OAEs amplitude is both sensitive and specific enough to efficiently identify participants who are particularly susceptible to hearing loss caused by impulse noise generated by firearms. Hearing conservation programs may therefore want to consider including such tests in their routine. As far as efferent strength is concerned, we feel that further research is due, before implementing the suppression of OAEs in hearing conservations programs in a similar manner.

Effects of noise overexposure on tone detection in noise in nonhuman primates

This report explores the consequences of acoustic overexposures on hearing in noisy environments for two macaque monkeys trained to perform a reaction time detection task using a Go/No-Go lever release paradigm. Behavioral and non-invasive physiological assessments were obtained before and after narrowband noise exposure. Physiological measurements showed elevated auditory brainstem response (ABR) thresholds and absent distortion product otoacoustic emissions (DPOAEs) post-exposure relative to pre-exposure. Audiograms revealed frequency specific increases in tone detection thresholds, with the greatest increases at the exposure band frequency and higher. Masked detection was affected in a similar frequency specific manner: threshold shift rates (change of masked threshold per dB increase in noise level) were lower than pre-exposure values at frequencies higher than the exposure band. Detection thresholds in sinusoidally amplitude modulated (SAM) noise post-exposure showed no difference from those in unmodulated noise, whereas pre-exposure masked detection thresholds were lower in the presence of SAM noise compared to unmodulated noise. These frequency-dependent results were correlated with cochlear histopathological changes in monkeys that underwent similar noise exposure. These results reveal that behavioral and physiological effects of noise exposure in macaques are similar to those seen in humans and provide preliminary information on the relationship between noise exposure, cochlear pathology and perceptual changes in hearing within individual subjects.

Musicianship enhances ipsilateral and contralateral efferent gain control to the cochlea

Human hearing sensitivity is easily compromised with overexposure to excessively loud sounds, leading to permanent hearing damage. Consequently, finding activities and/or experiential factors that distinguish "tender" from "tough" ears (i.e., acoustic vulnerability) would be important for identifying people at higher risk for hearing damage. To regulate sound transmission and protect the inner ear against acoustic trauma, the auditory system modulates gain control to the cochlea via biological feedback of the medial olivocochlear (MOC) efferents, a neuronal pathway linking the lower brainstem and cochlear outer hair cells. We hypothesized that a salient form of auditory experience shown to have pervasive neuroplastic benefits, namely musical training, might act to fortify hearing through tonic engagement of these reflexive pathways. By measuring MOC efferent feedback via otoacoustic emissions (cochlear emitted sounds), we show that dynamic ipsilateral and contralateral cochlear gain control is enhanced in musically-trained individuals. Across all participants, MOC strength was correlated with the years of listeners' training suggested that efferent gain control is experience dependent. Our data provide new evidence that intensive listening experience(s) (e.g., musicianship) can strengthen the ipsi/contralateral MOC efferent system and sound regulation to the inner ear. Implications for reducing acoustic vulnerability to damaging sounds are discussed.

A Role of Medial Olivocochlear Reflex as a Protection Mechanism from Noise-Induced Hearing Loss Revealed in Short-Practicing Violinists

Previous studies have indicated that extended exposure to a high level of sound might increase the risk of hearing loss among professional symphony orchestra musicians. One of the major problems associated with musicians' hearing loss is difficulty in estimating its risk simply on the basis of the physical amount of exposure, i.e. the exposure level and duration. The aim of this study was to examine whether the measurement of the medial olivocochlear reflex (MOCR), which is assumed to protect the cochlear from acoustic damage, could enable us to assess the risk of hearing loss among musicians. To test this, we compared the MOCR strength and the hearing deterioration caused by one-hour instrument practice. The participants in the study were music university students who are majoring in the violin, whose left ear is exposed to intense violin sounds (broadband sounds containing a significant number of high-frequency components) during their regular instrument practice. Audiogram and click-evoked otoacoustic emissions (CEOAEs) were measured before and after a one-hour violin practice. There was a larger exposure to the left ear than to the right ear, and we observed a left-ear specific temporary threshold shift (TTS) after the violin practice. Left-ear CEOAEs decreased proportionally to the TTS. The exposure level, however, could not entirely explain the inter-individual variation in the TTS and the decrease in CEOAE. On the other hand, the MOCR strength could predict the size of the TTS and CEOAE decrease. Our findings imply that, among other factors, the MOCR is a promising measure for assessing the risk of hearing loss among musicians.

Assessment of the noise-protective action of the olivocochlear efferents in humans

It has been demonstrated in different mammals that the medial olivocochlear efferents (MOC) exert a noise-protective effect on the cochlea. In humans such an effect has not unambiguously been shown as of yet. The objective of this study was to assess the relationship between MOC activity and susceptibility of the cochlea to noise-induced hearing loss in humans. In 40 normally hearing human subjects, we measured the following: (1) magnitude of temporary threshold shift (TTS) after exposure to 60 min broadband noise of 94 dB SPL and (2) contralateral suppression (CS) of distortion product otoacoustic emissions (which reflects MOC activity) using two different measurement paradigms. CS was measured in duplicate on 2 measurement days. The relationship between TTS and CS was assessed. Individual TTS in the most affected frequencies (4 > 3 > 8 kHz) ranged from 9 to 28 dB HL, with an average maximum TTS of 18.4 dB HL. The amount of CS ranged between 0.3 and 3 dB. The repeatability of CS, evaluated by Cronbach's α value, ranged from 0.76 (acceptable repeatability) to 0.86 (good repeatability). One of the two different measurement paradigms showed a statistically significant inverse correlation between CS magnitude and amount of TTS, which was hypothesized. This is the first study on the relationship between TTS and CS in humans employing TTS induced under controlled laboratory conditions and two different MOC paradigms. The findings are compatible with the hypothesis that MOC activity is noise protective in humans. Future perspectives include modified CS paradigms, longitudinal cohort studies or efforts to also monitor lateral efferent effects in humans.

Low-level otoacoustic emissions may predict susceptibility to noise-induced hearing loss

In a longitudinal study with 338 volunteers, audiometric thresholds and otoacoustic emissions were measured before and after 6 months of noise exposure on an aircraft carrier. While the average amplitudes of the otoacoustic emissions decreased significantly, the average audiometric thresholds did not change. Furthermore, there were no significant correlations between changes in audiometric thresholds and changes in otoacoustic emissions. Changes in transient-evoked otoacoustic emissions and distortion-product otoacoustic emissions were moderately correlated. Eighteen ears acquired permanent audiometric threshold shifts. Only one-third of those ears showed significant otoacoustic emission shifts that mirrored their permanent threshold shifts. A Bayesian analysis indicated that permanent threshold shift status following a deployment was predicted by baseline low-level or absent otoacoustic emissions. The best predictor was transient-evoked otoacoustic emission amplitude in the 4-kHz half-octave frequency band, with risk increasing more than sixfold from approximately 3% to 20% as the emission amplitude decreased. It is possible that the otoacoustic emissions indicated noise-induced changes in the inner ear, undetected by audiometric tests. Otoacoustic emissions may therefore be a diagnostic predictor for noise-induced-hearing-loss risk.

Olivocochlear activity and temporary threshold shift-susceptibility in humans

STUDY OBJECTIVES

Animal studies (guinea pig, cat, chinchilla) have shown that activity of the medial olivocochlear efferents can exert noise-protective effects on the cochlea. It is not yet known whether such effects are also existent in humans. Olivocochlear activity can be estimated indirectly by contralateral suppression (CS) of otoacoustic emissions (OAE).

MATERIAL AND METHODS

We measured Input/Output functions of distortion products of OAE (DPOAE), with and without contralateral acoustic stimulation by white noise, in 94 normal hearing young male subjects. Seven stimuli with L2 between 20 and 60 dB SPL and L1 = 39 dB + 0.4 L2 ("scissor paradigm") were used at f2 = 2, 3, 4, 5, and 6 kHz. The measurement was repeated 2 weeks later. In 83 subjects of the same group, pure tone audiometry was registered before and 6 minutes after shooting exercises to evaluate individual susceptibility to develop a temporary threshold shift (TTS).

RESULTS

Test-retest repeatability of CS was generally good. CS averaged 0.98 dB SPL (SD 1.19 dB, median 0.56 dB). As expected, CS was greatest at low stimulus levels (median 1.06 dB at L2 = 20 dB, as compared with 0.33 dB at L2 = 60 dB). The smallest average CS was found at 4 kHz, and the greatest CS appeared at 2 kHz. A TTS occurred in 7 of 83 (8.5%) subjects. Statistical analysis did not reveal any correlation between the amount of CS and individual TTS susceptibility.

CONCLUSIONS AND OUTLOOK

1) Measurement of CS of DPOAE using an extensive measurement paradigm revealed good test-retest repeatability, confirming the reliability of this audiologic tool. 2) CS of DPOAE does not predict individual susceptibility to mild TTS induced by impulse noise in humans. Possible explanations for the missing association are discussed. Future perspectives include longitudinal studies to further elucidate the association between medial olivocochlear bundle-activity and permanent threshold shift in humans. The goal is to develop a diagnostic tool for the prediction of individual noise vulnerability in humans, thereby preventing noise-induced hearing loss.

[Diagnostics of the cochlear amplifier by means of DPOAE growth functions]

Extrapolated DPOAE growth functions can be applied in ENT diagnostics for a specific assessment of cochlear dysfunction. In screening newborn hearing, they are able to detect transitory sound conductive hearing loss and thus help to reduce the rate of false positive TEOAE responses in the early postnatal period. Since DPOAE growth functions are correlated with loudness functions, DPOAEs offer the potential for basic hearing aid adjustment, especially in children. Extrapolated DPOAE I/O-functions provide a tool for a fast, automated frequency-specific and quantitative evaluation of hearing loss. However, DPOAE diagnostics is limited to a hearing loss of 50 dB HL. Thus, a combined measurement of DPOAE and AMFR would be useful.

Predicting vulnerability to acoustic injury with a noninvasive assay of olivocochlear reflex strength

Permanent noise-induced damage to the inner ear is a major cause of hearing impairment, arising from exposures occurring during both work- and pleasure-related activities. Vulnerability to noise-induced hearing loss is highly variable: some have tough, whereas others have tender ears. This report documents, in an animal model, the efficacy of a simple nontraumatic assay of normal ear function in predicting vulnerability to acoustic injury. The assay measures the strength of a sound-evoked neuronal feedback pathway to the inner ear, the olivocochlear efferents, by examining otoacoustic emissions created by the normal ear, which can be measured with a microphone in the external ear. Reflex strength was inversely correlated with the degree of hearing loss after subsequent noise exposure. These data suggest that one function of the olivocochlear efferent system is to protect the ear from acoustic injury. This assay, or a simple modification of it, could be applied to human populations to screen for individuals most at risk in noisy environments.

Safety and clinical performance of acoustic reflex tests

OBJECTIVE

Safety and effectiveness of acoustic reflex tests are important issues because these tests are widely applied to screen for retrocochlear pathology. Previous studies have reported moderately high sensitivity and specificity for detection of acoustic neuroma. However, there have been reports of possible iatrogenic hearing loss resulting from acoustic reflex threshold (ART) and decay (ARD) tests. This study assessed safety and clinical performance of ART tests for detection of acoustic neuroma.

DESIGN

We report a case in which ARD testing resulted in a significant bilateral permanent threshold shift. This case was the impetus for us to investigate the clinical utility of ART and ARD tests. We analyzed sensitivity and specificity of ART, as well as asymmetry in pure-tone thresholds (PTT) for detection of acoustic neuroma in 56 tumor and 108 non-tumor ears.

RESULTS AND CONCLUSIONS

Sensitivity and specificity were higher for PTT asymmetry than for ART. Ipsilateral ART at 1000 Hz had poor sensitivity and specificity for detection of acoustic neuroma, and involves some potential risk to residual hearing for presentation levels higher than 115 dB SPL. Approximately half of the acoustic neuroma group had ipsilateral ARTs that would require administration of ARD tests at levels exceeding 115 dB SPL. Therefore, we conclude that PTT asymmetry is a more effective test for detection of acoustic neuroma, and involves no risk to residual hearing. Future studies of contralateral reflex threshold and ARD in combination with PTT asymmetry are recommended.

Distortion-product emissions in rabbit: II. Prediction of chronic-noise effects by brief pure-tone exposures

In an attempt to predict the susceptibility of the cochlea to the harmful influences of excessive sound, the effects of initially exposing the same rabbits to brief pure-tones were related to the subsequent effects of octave-band noise (OBN) exposure using measures of distortion-product otoacoustic emissions (DPOAEs). The pure-tone exposure paradigm consisted of determining the rate at which a 100-dB SPL, low-frequency tone reduced the amplitude of a 1.5-kHz DPOAE, elicited by 50-dB SPL primaries. To establish the stability of the rate-reduction index, the tonal-exposure protocol was repeated on three separate occasions for each subject. Subsequently, the same rabbits were exposed chronically to a 95-dB SPL OBN, centered at 1-kHz, until DPOAE amplitudes between 1-5 kHz were diminished to noise-floor levels, i.e., by 10-30 dB, in response to 45-dB SPL primaries. The results revealed a visually apparent relation between the slope of the tonal-induced DPOAE-loss functions and the number of days required to reach the criterion decrement in emission level during chronic exposure to noise. Analysis of the frequency extent of the noise-induced changes revealed significant correlations between the previously measured rate of pure-tone induced reductions of DPOAE amplitude and the subsequent amount of decrement produced by OBN exposure. Thus, rabbits exhibiting slow rates of tonal-induced decrements in low-frequency DPOAEs were resistant to the amplitude-reducing effects of a subsequent chronic OBN exposure in that it typically took more than four days to achieve the targeted amount of DPOAE loss.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of eye colour on susceptibility to TTS in humans

In order to investigate the function of the inner ear melanin, noise-induced temporary hearing loss (temporary threshold shift, TTS) was studied in humans with either blue or brown iris colour. Sixty-eight normally hearing teenage boys participated in this study. Hearing thresholds before and after exposure were established with a computerized sweep frequency audiometer in the frequency range 0.8-8 kHz. The noise exposure consisted of a 1/3 octave band-filtered noise with centre frequency 2 kHz at 105 dB SPL for 10 min. The mean TTS in the frequency range 2-8 kHz showed a significant difference with the brown-eyed subjects developing least TTS, and the blue-eyed subjects most TTS.

The effect of 'conditioning' exposures on hearing loss from traumatic exposure

The role of 'conditioning' exposures as moderators of hearing loss produced by exposure to a higher level noise was explored using chinchillas. Monaural chinchillas were exposed to an octave band of noise centered at 0.5 kHz at 95 dB for six hours/day for ten days. The subjects were allowed to recover to pre-exposure sensitivity and at five days after the last exposure they were re-exposed to the same noise at 106 dB. Thresholds recorded at various time intervals following the second exposure were compared with those recorded in a control group exposed only to the higher level noise. The experimental animals were found to have less threshold shift at all stages of recovery. Results are discussed in the light of results of other related studies and possible mechanisms involved are hypothesized.

Hearing acuity and susceptibility to noise-induced hearing loss

Above a critical level (CL) of fatiguing tone, temporary threshold shift (TTS) rises rapidly with intensity. Using a fatiguing tone of 1000 Hz, threshold shift following 1 min stimulation at varying intensities was measured at the same frequency in 55 normal subjects and the CL determined for each. In a second and more limited study, TTS was measured at a frequency half an octave above that of the fatiguing tone. A significant inverse correlation was established between hearing level (HL) and CL at the same frequency with a fall of 2.5 dB in the latter for every 10 dB rise in the former. CLs measured half an octave above the fatiguing tone frequency were appreciably lower by 5 to 11 dB. No correlation was apparent between TTS and HTL, implying that TTS and CL are subserved by differing physiological mechanisms. The findings have relevance to the topic of susceptibility to noise-induced hearing loss and the formulation of damage risk criteria.

Study on an evaluation index for noise susceptibility. II. Reduction of [ART1k-ARTWN] and critical bandwidth in acoustic reflex

Effects of noise exposure on the difference between ART1k (acoustic reflex threshold) and ARTWN [( ART1k-ARTWN]), and on the critical bandwidth (CBW) in acoustic reflex (AR) were investigated. One hundred and eleven workers using hand-held vibrating tools (exposed group) served as subjects. The results were compared with the values obtained in the control group reported by the authors previously (Miyakita and Miura 1985). The exposed group was classified into four groups according to their hearing levels (HL) at 4 and 8 kHz; i.e. Group A (HL(4k + 8k)/2 less than 25 dB), Group B (25 dB less than or equal to HL (4k + 8k)/2 less than 50 dB), Group C (50 dB less than or equal to HL (4k + 8k)/2 less than 75 dB), and Group D (HL (4k + 8k)/2 greater than or equal to 75 dB). [ART1k-ARTWN] was reduced significantly (p less than 0.001) in the exposed groups; i.e. 12.4 dB in the control group, 6.7 dB in Group A, 2.3 dB in Group B, -0.3 dB in Group C, and -1.3 dB in Group D. On the other hand, a significant widening of the CBW was observed in the exposed group compared to that of the control group (p less than 0.05); i.e. 1138 Hz in the control group, 1410 Hz in Group A, 3287 Hz in Group B, and greater than or equal to 4000 Hz in Groups C and D. In addition, we confirmed that the reduction of [ART1k-ARTWN] might be the result of the widening of the CBW in AR.(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory-evoked potential correlates of susceptibility to noise-induced hearing loss

Correlations between changes in cochlear microphonics (CM) and auditory brainstem-evoked potentials (ABEP) resulting from transitory-threshold-shift (TTS)-inducing noise, in normally hearing subjects, and the eventual permanent threshold shift (PTS) which the same subjects developed after 9-14 months of well-quantified occupational noise were evaluated. In addition, the predictive value of pigmentation, as an indicator of eventual PTS, was assessed. Eleven CM and ABEP indices which showed significant correlation with the eventual hearing loss were identified. Eight of these indices reflect the effect of increased stimulus rate and experimental TTS-inducing noise. These results show that the difference in ABEP latencies as a result of increased stimulus rate is smaller in persons with large eventual PTS (high susceptibility to noise-induced hearing loss). The effect is larger in persons that eventually developed a small PTS (low susceptibility). Pigmentation, as reflected by iris and skin color, was found to have a negative correlation with susceptibility to noise, i.e. the more pigment the less PTS developed. The above indices may be useful to determine personal susceptibility to noise in normally hearing subjects.

Aspirin can potentiate the temporary hearing loss induced by intense sounds

Aspirin is known to produce a reversible loss of hearing that can be as great as 40 dB, depending upon the dose and the individual subject. Here we show that aspirin-induced losses exacerbate the temporary hearing loss induced by exposure to intense sound. EXPOSUREs that ordinarily produce about 14 dB of temporary threshold shift (TTS) will produce about 18-27 dB of TTS if the listener has been taking 3.9 g of aspirin for the past two days or more. A lesser dose or a shorter duration of use produces a smaller, or no, increment in temporary hearing loss. This greater TTS, and an apparent prolongation of recovery from exposure, make chronic aspirin use ill-advised for people routinely exposed to intense sounds.

EXPOSURE

2500 Hz, 10 min, varying intensity. TTS frequency: 3550 Hz. Psychophysical method: 2IFC, adaptive.

Temporary threshold shift after exposure to predicted and unpredicted noise

In order to investigate if anticipation of noise could influence individual susceptibility to temporary threshold shift (TTS), an experimental study was carried out comprising 10 voluntary normal-hearing subjects. The noise stimulus consisted of noise bursts centered at 4000 Hz with a duration of one second. Initially, each subject was exposed in a test situation where the noise was presented instantly upon the subject's response (producer). The subject was instructed to respond by pressing a button as fast as possible after an optical stimulation. The rate was approximately 1 noise burst/6 sec and the total number of bursts were 100. The intervals between bursts were randomized. Each individual's exposure time pattern was recorded and used for the second test situation. Here the first exposure was reproduced, but each noise burst presented without notice (consumer). Each subject participated in 5 producer sessions and in 5 consumer sessions. Evaluation of TTS measurements revealed a maximum TTS at 6000 Hz. The mean TTS for all subjects in the two different situations, showed no significant difference and only one subject demonstrated a tendency toward a greater TTS susceptibility in the 'consumer' sessions.

Noise-induced hearing loss in shipyard workers with unilateral conductive hearing loss

In a study of 6500 workers in the shipbuilding industry, 8 subjects were found to have unilateral conductive hearing impairment established before the noise exposure period and without recurrent attacks of acute or chronic infection or clinical diagnosis of otosclerosis. All subjects demonstrated a more pronounced sensorineural hearing loss at 4.0 kHz in the ear with normal middle ear function. The results show the value of even a small permanent conductive hearing loss for protection against noise-induced hearing loss. The observations are discussed in relation to the role of individual variations in sound transmission, the value of the acoustic reflex in noise-induced hearing loss and the efficiency of continuous use of hearing protectors.

Review of environmental factors affecting hearing

The major nongenetic causes of sensorineural hearing loss are exposure to noise, aging, ototoxic drugs, viral and bacterial infections, and interactions between these factors. Regarding exposure to continuous noise, the data base from laboratory and field studies indicates that a risk of hearing loss is present when noise levels exceed 75-80 dBA. As noise level, duration and number of exposures increase so does risk. The data base for other forms of noise (intermittent, impact) is not as established. Risk of hearing loss due to impulse noise increases as the peak SPL exceeds 145-155 dB and as the duration of the impulse, the number of impulses and the number of exposures increase. High-level acoustic impulses can cause severe, permanent hearing loss. Interaction between some steady-state noises and some acoustic impulses can be synergistic, producing extensive injuries to the organ of Corti. Noise can also interact synergistically with some aminoglycoside antibiotics to produce severe injuries in the inner ear. These antibiotics are also capable of producing hearing loss and indeed may do so in up to 55% of the one million persons who receive aminoglycoside antibiotics during the course of treatment for tuberculosis or severe gram-negative infections. Bacterial and viral infections may also produce mild to severe hearing loss. With the development of rubella vaccine and Rhogam, cytomegalovirus may have become the most common cause of congenital deafness. Aging is also a major cause of hearing loss. Exposure to occupational and environmental noise, certain diseases and life styles (diet, stress, drugs) may interact with the specific effects of aging. The result is moderate to severe hearing loss in a majority of older persons.

The relations of eye colour and smoking to noise-induced permanent threshold shift

The hearing of 118 otologically normal soldiers 40 years of age or less with at least 8 years service was tested. Their eye colours were also recorded. Each soldier was questioned concerning his medical history, history of noise exposure and smoking habits. Left ear average hearing levels at 3 kHz were poorer in soldiers with eye colours indicating no melanin pigmentation of the iris than those with iris pigmentation (p < 0.05). Effects of smoking were statistically significant at one frequency but difficult to interpret in terms of current theories.

Eye colour and susceptibility to noise-induced permanent threshold shift

257 3rd-year apprentices were given ear, nose and throat examination, electroacoustic impedance tests and audiometry. Their eye colour was also recorded. Average hearing levels of otologically normal left ears were poorer at 4 kHz (p less than 0.05) for apprentices with eye colours indicating no melanin pigmentation of the iris than for apprentices with melanin iris pigmentation.

Temporary threshold shift for masked pure tones

The present investigation examines the dependence of temporary threshold shift (TTS) on pre-exposure hearing level within the same group of normal hearers by simulating varying degrees of hearing loss via the addition of masking noise. The results confirm those of earlier studies and indicate that TTS decreases as hearing level increases. A model is developed which can account for the data of the present study as well as the results of previous investigations which utilized hearing-impaired subjects.

A test battery approach to the investigation of susceptibility to temporary threshold shift

This study examined individual differences among various psychophysical measures that have been suggested previously as predictors of susceptibility to noise-induced temporary threshold shift (TTS). The test battery was administered to a group of five normal-hearing young adults and consisted of the following procedures: (1) critical intensity (CI) at 2 kHz; (2) loudness discomfort level (LDL) at 0.5, 1, 2 and 4 kHz; (3) aural overload thresholds at 0.5, 1 and 2 kHz. Three measures of TTS, one employing a broad-band noise as the exposure stimulus and two others using tonal fatiguers, were utilized to assess susceptibility to TTS. Results revealed that the test battery delineated tough from tender ears. Furthermore, the aural overload test was found to be a highly accurate predictor of TTS.