Long-term assessment of auditory changes resulting from a single noise exposure associated with non-occupational activities

We examined the long-term sequelae in both ears of 42 patients who reported the occurrence of auditory changes resulting from a single exposure to intense sound levels during non-occupational activities. We divided these patients into two groups, based upon noise exposures of either continuous duration or single high-energy impulse. Audiometric data were available for each of these subjects shortly after their noise-exposure events and follow-up examinations took place more than one year after the noise occurrence (range: 1–16 years). The initial median hearing loss for the continuous-type noise exposure group at 3–8 kHz was found to be 9 dB, relative to the age-appropriate norms, in the more affected ears, and hearing function was found to have returned to normal levels at follow-up. The same initial hearing loss was measured for the impulse-type noise group, but a residual hearing loss of 4?dB was measured at follow-up. Furthermore, the majority of the subjects from both groups reported tinnitus and hypersensitivity to sound at follow-up, but with minimal impact on their lives.

Sumario

Examinamos en ambos oídos las secuelas a largo plazo en 42 pacientes que reportaban cambios auditivos como resultado de una exposición única a ruido intenso durante actividades no laborales. Dividimos a los pacientes en dos grupos, dependiendo de la exposición a ruido, ya fuera de duración continua o debida a un impulso de gran energía. Los datos audiométricos estuvieron disponibles para cada uno de estos sujetos poco después de su evento de exposición sonora y el seguimiento se llevó a cabo durante más de un año después de la exposición a ruido (rango de 1 a 16 años). El promedio de hipoacusia inicial entre 3 y 8 kHz para el grupo de exposición continua fue de 9dB, con relación a las normas adecuadas por edad en el oído más afectado, y la función auditiva había regresado a los niveles normales en el seguimiento. La misma pérdida auditiva fue medida en el grupo de ruido tipo impulso, pero en el seguimiento se detectó una pérdida de 4dB. Incluso la mayoría de los sujetos de ambos grupos reportaron acúfeno e hipersensibilidad al sonido en el seguimiento, pero con un impacto mínimo en sus vidas.     

Hearing in nonprofessional pop/rock musicians

OBJECTIVE: The purpose of this study was to evaluate the hearing and subjective auditory symptoms in a group of nonprofessional pop/rock musicians who had experienced repeated exposures to intense sound levels during at least 5 yr of musical activity. DESIGN: An evaluation of both ears in 42 nonprofessional pop/rock musicians included pure-tone audiometry in the conventional and extended high-frequency range, the measurement of uncomfortable loudness levels, and an assessment of tinnitus and hypersensitivity to sound. Exclusion criteria were (a) the occurrence of acoustic trauma, (b) excessive noise exposure during occupational activities, (c) a history of recurrent otitis media, (d) previous ear surgery, (e) a fracture of the cranium, (f) ingestion of potentially ototoxic drugs, and (g) reported hearing difficulties within the immediate family. These audiometric results were then compared with a control group of 20 otologically normal young adults with no history of long-term noise exposure. RESULTS: After adjusting for age and gender, relative to ISO 7029, the mean hearing threshold in the frequency range of 3 to 8 kHz was 6 dB in the musicians and 1.5 dB in the control group. This difference was statistically significant (Mann-Whitney rank sum test, p < 0.001). A significant difference was also observed between musicians using regular hearing protection during their activities (average 3 to 8 kHz thresholds = 2.4 dB) and musicians who never used such hearing protection (average 3 to 8 kHz thresholds = 8.2 dB), after adjusting for age and gender (Mann-Whitney rank sum test, p = 0.006). Eleven of the musicians (26%) were found to be hypersensitive to sound, and seven (17%) presented with tinnitus. Tinnitus assessment, however, did not reveal any clinically significant psychological distress in these individuals. CONCLUSIONS: Tinnitus and hypersensitivity to sound were observed in a significant minority within a group of nonprofessional pop/rock musicians who had experienced repeated exposure to intense sound levels over at least 5 yr but with minimal impact on their lives. Moreover, hearing loss was minimal in the subjects who always used ear protection, being only 0.9 dB higher than the control group. In contrast, hearing loss was significantly more pronounced, at 6.7 dB higher than the control group, in those musicians who never used ear protection. Continued education about the risk to hearing and the benefits of the persistent use of ear protection is warranted for musicians who are exposed frequently to intense sound levels.     

噪声性听觉损害的声反射增长函数

目的　探讨噪声暴露对声反射增长函数 (acousticreflexgrowthfunction ,ARGF )的影响。方法　对有长期噪声暴露史的工人 5 7例 (10 6耳 )及正常听力青年人 2 5例 (5 0耳 )进行ARGF测试 ,观察受试耳对 10 0 0Hz纯音刺激级由声反射阈 (acousticreflexthreshold ,ART)水平增加至 12 5dBHL时声抗量值变化的函数关系。根据纯音平均听阈 (pure-tonethresholdaverage ,PTA)将噪声暴露耳分为三组。结果　噪声暴露组的声反射 (acousticreflex ,AR)动态范围变窄 ,声抗增长量值减小 ,ARGF曲线坡度变缓 ,反射振幅及声抗增长量值与正常青年组相比有显著下降 (P <0 .0 1) ,而听力损失程度不同的三个病变组间的差异不显著 (P >0 .0 5 )。分别用声抗量值及声抗量增幅比随AR刺激级的增加而变化两种方法分析所得数据 ,发现噪声暴露组的ARGF与正常耳有明显差别 (P <0 .0 5 ,P <0 .0 1)。结论　噪声暴露对ARGF有显著影响。ARGF可用作监测早期噪声性听觉损害 (noiseinducedhearingloss,NIHL)的敏感指标     

Impact noise and hearing. A study from audiometry screening in noisy occupational environments]

The hearing loss of 46,776 subjects exposed to hazardous impact or continuous noise was measured: 11% were exposed to impact noise (10.3% in men, 12.7% in women). Impact noise causes less hearing loss than continuous noise up to a noise emission level (NIL) of 115 dB(A). This result is still compatible with the concept of equivalent continuous A-weighted sound pressure level (LA,eq) and seems to be caused by the protective effect of the stapedius muscle. However, at emission levels higher than 115 dB(A), not covered by ISO 1999 prognosis, impact noise becomes more detrimental. This could be the effect of mechanical damage to the organ of Corti, as observed in animal experiments at sound pressure levels exceeding 114 dB(A). A diminishing protective function of the stapedius muscle could explain the more serious hearing impairment after long exposure to impact noise compared with shorter exposures to an equal dose. Since the hearing loss caused by impact or continuous noise is clearly different in the highest NIL ranges, safety criteria for impact noise should be revised. The NIL defined by Burns and Robinson underestimates the effect of the exposure time for impact noise.     

可接受噪声级测试在单、双耳佩戴助听器效果评估中的应用

目的 通过比较双侧中重度听力损失患者在单、双耳助听下的可接受噪声级(acceptable noise level,ANL),探讨ANL对助听器验配及预测助听效果的作用.方法 选取15例双侧中重度听力损失患者,分别测得双耳未助听、左耳助听、右耳助听和双耳助听状态下的最舒适响度级(most comfortable levels,MCL)、最大背景噪声级(background noise level,BNL),并计算得到ANL值(ANL=MCL-BNL),对结果进行统计学分析.结果 15例受试者双耳未助听、左耳助听、右耳助听及双耳助听四种状态下测得的ANL值分别为18.87±5.26、12.60±2.47、12.00±2.90、5.13±1.25 dB S/N;MCL值分别为80.40±9.28、63.73±5.15、62.27±5.36、61.80±6.05 dB HL;BNL值分别为61.67±6.14、51.13±3.94、50.27±4.50、56.67±5.16 dB HL;左耳助听与右耳助听下的ANL值差异无统计学意义(P>0.05);单、双耳助听下与未助听的ANL值差异均具有统计学意义(P<0.05);双耳助听状态下ANL值显著低于单耳助听(P<0.05).结论 ANL值较低耳更利于助听器验配,佩戴助听器能有效提高听障患者对噪声的接受能力,并且双耳佩戴助听器的效果明显优于单耳.     

Hearing loss and cochlear pathology in the monkey (Macaca) following exposure to high levels of noise.

Eight Old World monkeys were exposed 8 h daily for 20 days to octave-band noise having center frequencies from 0.5--8 kHz at levels of 117--120 dB SPL. Two additional animals received exposures to wide-band, 120-dB SPL noise on the same schedule, and one animal was exposed to the 2-kHz octave band for 40 h continuously. Behavioral audiograms were measured throughout exposure and during a 1-month recovery period. Following recovery, the animals were sacrificed and their ears examined histologically. Monaural audiograms are presented showing initial and final TTS and PTS measured at the end of the recovery period. These are compared with complete cytocochleograms for each ear. Hair cell loss was generally restricted to the outer rows, and was reasonably well correlated with pattern of hearing loss. Some cell loss, including inner hair cells, was found in the extreme basal turn, usually without associated high-frequency hearing loss. The relationships between exposure frequency, hearing loss, and locus of cochlear pathology are discussed, as are changes in TTS during exposure.     

噪声对作业工人听觉影响的动态观察

目的　通过动态观察噪声环境作业工人的听力变化情况,进一步了解噪声性听力损害的发生发展规律。方法　对2012年1月～2014年12月在本院进行健康检查和听力测试的噪声作业工人的听力学资料,进行统计学分析。结果　随访的120例作业工人中,接触噪声时间2年22例,3～5年43例,6～10年55例。噪声作业2～10年工人中,6 kHz处出现听阈提高分别有15耳、49耳和81耳;4 kHz处出现听阈提高分别有19耳、71耳和89耳;3 kHz处出现听阈提高分别有3耳、31耳和75耳。6、4和3 kHz 3个频率处,接触噪声时 间长,听阈提高发生率多,差异有统计学意义。6、4和3 kHz3个频率处,发生噪声性聋例数比较,P 均＜0.05,差异有统计学意义;4 kHz与6 kHz,4 kHz与3 kHz,6 kHz与3 kHz两两比较,P 均＜0.01,差异有统计学意义。结论　噪声环境作业工人,接触噪声时间越长,越容易发生听力损害,听力损害程度越严重;各频率中4 kHz处最易出现噪声性聋,其次为6 kHz处。     

Damage to sensory hairs of inner hair cells after exposure to noise in rabbits without outer hair cells

We investigated if the noise-induced damage to the stereocilia of inner hair cells (IHCs) was dependent on the integrity of the outer hair cells (OHCs) in rabbit. Prior to the noise exposure a total loss of OHCs in the basal 1.5 to 2 turns was induced by administration of kanamycin (400 mg/kg for 10 days). This left the IHCs apparently normal as observed in the scanning electron microscope. These animals exhibited a 20-60 dB hearing loss before noise exposure. In spite of this pronounced hearing loss, the fusion and inclination of the IHC stereocilia were extensive in these noise-exposed ears. The stereocilia damage occurred at the same noise exposure and was as prominent or even more pronounced than has been noted in ears exposed to noise only. Under the assumption that kanamycin causes selective destruction of OHCs, the results can be interpreted as evidence that the OHCs facilitate the IHCs at low sound levels without being involved in the process which damages IHC stereocilia at high levels of noise.     

Les Réflexes Stapédiens Dans la Presbyacousie et la Surdité Professionnelle

Acoustic reflex thresholds were measured on 251 subjects (418 ears) with varying degrees of sensorineural hearing loss. The subjects are divided in three groups according to the pathology involved: presbyacusis, noise-induced hearing loss and combined hearing loss caused by both presbyacusis and noise exposure. The dynamic range of hearing was found to be reduced for all cases and at all frequencies. The decrease in dynamic range was proportional to the severity of the hearing loss. The sensation levels of the acoustic reflex thresholds seem to be dependent on the degree of hearing loss and not on the aetiology. Practical conclusions are drawn concerning the prediction of thresh-holds of hearing and the diagnosis of recruitment based on reflex measurements.     

Evidence of hearing loss in a 'normally-hearing' college-student population

We report pure-tone hearing threshold findings in 56 college students. All subjects reported normal hearing during telephone interviews, yet not all subjects had normal sensitivity as defined by well-accepted criteria. At one or more test frequencies (0.25-8 kHz), 7% of ears had thresholds ≥25 dB HL and 12% had thresholds ≥20 dB HL. The proportion of ears with abnormal findings decreased when three-frequency pure-tone-averages were used. Low-frequency PTA hearing loss was detected in 2.7% of ears and high-frequency PTA hearing loss was detected in 7.1% of ears; however, there was little evidence for 'notched' audiograms. There was a statistically reliable relationship in which personal music player use was correlated with decreased hearing status in male subjects. Routine screening and education regarding hearing loss risk factors are critical as college students do not always self-identify early changes in hearing. Large-scale systematic investigations of college students' hearing status appear to be warranted; the current sample size was not adequate to precisely measure potential contributions of different sound sources to the elevated thresholds measured in some subjects.     

Headphone listening habits,hearing thresholds and listening levels in Swedish adolescents with severe to profound HL and adolescents with normal hearing

Abstract

Objective: Research has minimally focussed on the music listening habits and preferred sound volumes among adolescents with severe to profound congenital HL. Listening to music played at loud sound volumes and for a long duration of time could imply risks of worsening the HL. Therefore, it is important to investigate the listening habits in adolescents with HL. The aim of the present study was to describe the use of personal music devices, subjective estimated sound levels, measured sound levels, listening habits, and hearing symptoms in adolescents with severe to profound hearing loss compared with adolescents with normal hearing.

Design: The study was conducted in two steps. First, a questionnaire was given to students with or without hearing loss. In step two, hearing and sound level measurements were made in a subsample from both groups.

Study sample: The study sample were based on 112 seventeen-year-old students with severe to profound hearing loss and 279 adolescents with normal hearing. Hearing thresholds and listening levels was measured on two subsamples based on 29 adolescents with severe to profound hearing loss and 50 adolescents from the group with normal hearing.

Results: The results showed that adolescents with severe to profound hearing loss listened to significantly louder sound levels for longer periods. For both groups, those listening at louder sound levels had poorer hearing thresholds. This finding is especially alarming for subjects with hearing loss. Among those listening above 85?dB per occasion, the sound level ranged between 85.8?dB up to 109?dB for those with hearing loss, whereas the sound level ranged between 85.5?dB and 100?dB for those with normal hearing.

Conclusions: Adolescents with congenital hearing loss used portable music devices in the same manner as adolescents with normal hearing. However, adolescents with hearing loss listened to louder sound volumes most likely to compensate for their hearing loss, which significantly increases the risk of further damage to their hearing. From a hearing rehabilitation perspective it could be concluded that aspect of music listening habits should be focussed in order to prevent noise induced hearing loss among individuals with congenital hearing loss.     

老年性聋患者普通话快速噪声下言语测试及评估

目的评估老年性聋患者噪声下言语识别能力,探讨年龄、听力损失程度、认知功能对其噪声下言语识别能力的影响。方法选取2018年10月~2020年4月就诊的70例60岁及以上老年性聋患者为研究对象,按年龄分为60~69岁(20例40耳)、70~79岁(28例56耳)、≥80岁(22例44耳)三组,各组分别进行纯音听阈测试、简易智能精神状态量表(mini-mental state examination,MMSE)评估及普通话快速噪声下言语测试(Mandarin quick speech-in-noise test,M-Quick SIN),分别获得双耳0.5、1、2、4 kHz纯音平均听阈(pure-tone audiometry,PTA)、MMSE量表总得分及双耳信噪比损失(signal-to-noise ratio loss,SNR loss);分析年龄、平均听阈、MMSE量表得分对信噪比损失的影响。结果①60~69岁组SNR loss(5.25±5.42)dB明显小于70~79岁组(11.54±6.05)dB和≥80岁组(11.86±6.06)dB(P<0.01);70~79岁组和≥80岁组间SNR loss差异无统计学意义(P>0.05)。②SNR loss随PTA提高而升高,各组间差异均有显著统计学意义(P<0.01)。③MMSE量表得分对SNR loss的主效应不显著(P>0.05)。结论年龄、听力损失程度为老年性聋患者噪声下言语识别的主要影响因素;在一定范围内随着年龄增加,听力损失加重,其噪声下言语识别能力降低。     

Distortion product otoacoustic emissions in normal and hearing-impaired ears

Otoacoustic emissions of distortion products (DPOAE's) were recorded in normal and hearing-impaired human ears using relatively straightforward methods. Two pure-tone stimuli at fixed frequency levels of 73 dB HL for f1 and of 67 dB HL for f2 were used. The frequencies of the two primaries were chosen so that their geometric mean represented standard audiometric frequencies. Measurements of the emission amplitudes at 2f1-f2 and the adjacent noise floor were achieved by spectral averaging. A total of 101 subjects (199 ears) were tested. Seventy-seven ears in 46 subjects had normal hearing (hearing levels less than or equal to 20 dB at standard audiometric frequencies; average hearing levels, less than or equal to 10 dB). Thirty-six ears in 25 subjects had near-normal hearing (no hearing complaints, hearing levels less than or equal to 40 dB; average hearing levels, less than or equal to 20 dB). No significant differences in mean DPOAE values were apparent between these two groups of ears. All but two of these 113 ears (98%) showed emissions at three or more of the six frequencies tested between 1 and 6 kHz. Emissions were detected in more than 75% at each frequency between 1 and 6 kHz and in more than 85% between 1 and 4 kHz. A further 86 ears in 44 subjects exhibited varying degrees of sensorineural hearing loss caused by different pathologies. In general, emission amplitudes approximated the shapes of the audiograms, and a highly significant correlation between hearing thresholds and emission amplitudes was demonstrated in the frequency range of 1 to 4 kHz.(ABSTRACT TRUNCATED AT 250 WORDS)     

Use of low-pass noise in word-recognition testing.

This study attempted to determine whether word-recognition scores obtained in noise were more sensitive to the presence of a hearing loss than recognition scores obtained in quiet. Subjects with normal hearing, high-frequency cochlear hearing loss, and flat cochlear hearing loss were tested in quiet and in the presence of a 500-Hz low-pass noise. Two signal-to-noise conditions were employed, -4 dB and -12 dB. Words were presented at 40dB SL in one experiment and at 96 dB SPL for normal-hearing subjects in a second experiment. The results indicated that, while the word-recognition scores of groups were similar in quiet, the more negative the signal-to-noise ratio, the greater the separation of group scores, with hearing-impaired subjects having poorer recognition scores than normal-hearing subjects. When the speech and noise were presented at high SPLs, however, the normal-hearing subjects had poorer word recognition than those with flat cochlear losses. The results are interpreted as indicating greater spread of masking in normal-hearing than hearing-impaired subjects at high sound pressure levels.     

Hearing loss and cochlear pathology in the monkey (Macaca) following exposure to high levels of noise

Summary Eight Old World monkeys were exposed 8 h daily for 20 days to octave-band noise having center frequencies from 0.5–8 kHz at levels of 117–120 dB SPL. Two additional animals received exposures to wide-band, 120-dB SPL noise on the same schedule, and one animal was exposed to the 2-kHz octave band for 40 h continuously. Behavioral audiograms were measured throughout exposure and during a 1-month recovery period. Following recovery, the animals were sacrificed and their ears examined histologically. Monaural audiograms are presented showing initial and final TTS and PTS measured at the end of the recovery period. These are compared with complete cytocochleograms for each ear. Hair cell loss was generally restricted to the outer rows, and was reasonably well correlated with pattern of hearing loss. Some cell loss, including inner hair cells, was found in the extreme basal turn, usually without associated high-frequency hearing loss. The relationships between exposure frequency, hearing loss, and locus of cochlear pathology are discussed, as are changes in TTS during exposure.This investigation was supported by research grants NS-05077, NS-05065 and NS-12706 and by Program Project grant NS-05785 from the National Institutes of Health     

Asymmetric hearing loss in a random population of patients with mild to moderate sensorineural hearing loss

OBJECTIVES: We evaluated the correlation of asymmetric hearing loss, in a random population of patients with mild to moderate sensorineural hearing loss, to several clinical factors such as age, sex, handedness, and noise exposure. METHODS: We randomly selected, from 8 hearing institutes in Israel, 429 patients with sensorineural hearing loss of at least 30 dB at one frequency and a speech reception threshold not exceeding 30 dB. Patients with middle ear disease or retrocochlear disorders were excluded. The results of audiometric examinations were compared binaurally and in relation to the selected factors. RESULTS: The left ear's hearing threshold level was significantly higher than that of the right ear at all frequencies except 1.0 kHz (p < .05). One hundred fifty patients (35%) had asymmetric hearing loss (more than 10 dB difference between ears). In most of the patients (85%) the binaural difference in hearing threshold level, at any frequency, was less than 20 dB. CONCLUSIONS: Age, handedness, and sex were not found to be correlated to asymmetric hearing loss. Noise exposure was found to be correlated to asymmetric hearing loss.     

Repeated Moderate Noise Exposure in the Rat—an Early Adulthood Noise Exposure Model

In this study, we investigated the effects of varying intensity levels of repeated moderate noise exposures on hearing. The aim was to define an appropriate intensity level that could be repeated several times without giving rise to a permanent hearing loss, and thus establish a model for early adulthood moderate noise exposure in rats. Female Sprague-Dawley rats were exposed to broadband noise for 90 min, with a 50 % duty cycle at levels of 101, 104, 107, or 110 dB sound pressure level (SPL), and compared to a control group of non-exposed animals. Exposure was repeated every 6 weeks for a maximum of six repetitions or until a permanent hearing loss was observed. Hearing was assessed by the auditory brainstem response (ABR). Rats exposed to the higher intensities of 107 and 110 dB SPL showed permanent threshold shifts following the first exposure, while rats exposed to 101 and 104 dB SPL could be exposed at least six times without a sustained change in hearing thresholds. ABR amplitudes decreased over time for all groups, including the non-exposed control group, while the latencies were unaffected. A possible change in noise susceptibility following the repeated moderate noise exposures was tested by subjecting the animals to high-intensity noise exposure of 110 dB for 4 h. Rats previously exposed repeatedly to 104 dB SPL were slightly more resistant to high-intensity noise exposure than non-exposed rats or rats exposed to 101 dB SPL. Repeated moderate exposure to 104 dB SPL broadband noise is a viable model for early adulthood noise exposure in rats and may be useful for the study of noise exposure on age-related hearing loss.     

中枢掩蔽与耳声发射对侧抑制相关性研究

目的：探讨中枢掩蔽与耳发射对侧抑制的相关性。方法：通过对一组耳科正常青年人及4例蜗后聋患者同时进行相同条件下中枢掩蔽及耳声发射对侧抑制的测试，观察中枢掩蔽与耳声发射对侧抑制在同一实验对象的表现。结果：在正常人组中，当对侧掩蔽声小于60 d BHL时，中枢掩蔽效应及耳声发射的对侧抑制均随对侧掩蔽声增加而增大，掩蔽强度大于60 d BHL时，则出现过度掩蔽。耳声发射记录过程中，本底噪声明显升高。当对侧掩蔽声固定为60 d BHL时，中频1.0、2.0kHz中枢掩蔽效应及耳声发射对侧抑制明显高于低频及高频，2.0kHz处最为明显；在蜗后聋组中，健耳或听力损伤较轻耳，中枢掩蔽及耳声发射对侧抑制接近正常人耳，而患者或听力损失较重耳，中枢掩蔽效应为负值或零，对侧抑制表现为反抑制或不抑制。结论：中枢掩蔽与对侧抑制存在相同的发生机理，表现出相同的频率特异性及强度特异性，无论在正常人耳还是病变耳，均表现出高度的相关性。     

Hearing levels of young patients 10 years after mastoidectomy

In several reports, the functional results of mastoid surgery in children have been stated to be disappointing. In this study, a series of 49 child patients who underwent mastoid surgery for chronic suppurative otitis media were examined 10 years postoperatively. At surgery, a cholesteatoma was detected in 49% of the patients. During the observation period, 33% of the patients were subjected to a second operation. At the last follow-up examination, 57% of the operated ears showed hearing levels of 20 dB or better, but in 16% hearing levels were worse than 40 dB. In 16% of the patients, contralateral disease had required surgical treatment and in only 39% of the patients the contralateral ear was found to be normal in otoscopy. Still, at the last follow-up examination, 90% of the better hearing ears had hearing levels of 20 dB or better and only one patient had hearing levels worse than 30 dB in both ears. Compared with an age- and sex-matched control group, hearing levels of the patients were significantly worse but the difference in the mean hearing levels of the better hearing ears was as little as 5 dB. Thus, 10 years after mastoidectomy, hearing levels of child patients were found to be better than expected.     

The medial cochlear efferent system does not appear to contribute to the development of acquired resistance to acoustic trauma

Noise-induced hearing loss (NIHL) was compared between sound conditioned and unconditioned guinea pigs, in which the left ear in both groups had been perfused with strychnine. Animals in the conditioned group were subjected to moderate sound (85 dB SPL broadband, 5 h/day, 10 days) and then exposed to intense sound (110 dB SPL broadband, 5 h). Unconditioned animals were exposed only to the intense sound. Following intense sound exposure, strychnine-treated ears showed greater NIHL than untreated ears in both unconditioned and conditioned animals, demonstrating the role of the medial efferents to reduce NIHL. Conditioned animals, however, showed smaller hearing loss and cochlear damage in both strychnine-treated and untreated ears compared to unconditioned animals; the protective effects given by conditioning were equivalent between the strychnine-treated and untreated ears. These results suggest that, although the medial efferent system acts to attenuate NIHL, it may not be necessary for the acquired resistance to NIHL provided by conditioning.