Comparison of psychophysical and evoked-potential tuning curves in the chinchilla

The rat animal model for noise-induced hearing loss

Rats make excellent models for the study of medical, biological, genetic, and behavioral phenomena given their adaptability, robustness, survivability, and intelligence. The rat's general anatomy and physiology of the auditory system is similar to that observed in humans, and this has led to their use for investigating the effect of noise overexposure on the mammalian auditory system. The current paper provides a review of the rat model for studying noise-induced hearing loss and highlights advancements that have been made using the rat, particularly as these pertain to noise dose and the hazardous effects of different experimental noise types. In addition to the traditional loss of auditory function following acoustic trauma, recent findings have indicated the rat as a useful model in observing alterations in neuronal processing within the central nervous system following noise injury. Furthermore, the rat provides a second animal model when investigating noise-induced cochlear synaptopathy, as studies examining this in the rat model resemble the general patterns observed in mice. Together, these findings demonstrate the relevance of this animal model for furthering the authors' understanding of the effects of noise on structural, anatomical, physiological, and perceptual aspects of hearing.

Narrow-band chirp and tone burst auditory brainstem response as an early indicator of synaptopathy in industrial workers exposed to occupational noise

This study aims at characterizing and comparing the findings of auditory brainstem response (ABR) using narrow-band chirp (NB-chirp) and tone burst (TB) for both latency and amplitude parameters among those exposed to occupational noise and to determine which among the two serves as a better indicator of noise-induced cochlear neuropathy. Forty adult males in the age range of 20-35 years were considered, wherein 20 of them were exposed to noise > 80 dB (A) for 8 hours per day constituting Noise-exposed group; and Control group consisted of 20 individuals without occupational noise exposure. ABR was recorded using NB-chirp and TB for four frequencies at 80 dB nHL through Etymotic Research - 3A (ER-3A) Insert phones using Interacoustics Eclipse EP-25 in individuals with and without noise exposure. MANOVA was performed to compare between TB ABR and NB-chirp ABR between the two groups. Statistical analysis revealed a notable difference for NB-chirp comparisons between the two groups at three frequencies: 500 Hz, F(1, 38) = 10.6; 1000 Hz, F(1, 38) = 7.91; and 2000 Hz, F(1, 38) = 6.64. Whereas, the difference was evident at only 500 Hz: F(1, 38) = 4.98 in case of TB ABR. However, there was no significant difference seen at any of the frequencies for amplitude parameters in both TB and NB-chirp ABR. Latency of wave V using NB-chirp was considered to be a better indicator compared to TB, acting as a better clinical tool in early identification, diagnosis, and monitoring of noise induced hearing loss (NIHL).

The value of a kurtosis metric in estimating the hazard to hearing of complex industrial noise exposures

A series of Gaussian and non-Gaussian equal energy noise exposures were designed with the objective of establishing the extent to which the kurtosis statistic could be used to grade the severity of noise trauma produced by the exposures. Here, 225 chinchillas distributed in 29 groups, with 6 to 8 animals per group, were exposed at 97 dB SPL. The equal energy exposures were presented either continuously for 5 d or on an interrupted schedule for 19 d. The non-Gaussian noises all differed in the level of the kurtosis statistic or in the temporal structure of the noise, where the latter was defined by different peak, interval, and duration histograms of the impact noise transients embedded in the noise signal. Noise-induced trauma was estimated from auditory evoked potential hearing thresholds and surface preparation histology that quantified sensory cell loss. Results indicated that the equal energy hypothesis is a valid unifying principle for estimating the consequences of an exposure if and only if the equivalent energy exposures had the same kurtosis. Furthermore, for the same level of kurtosis the detailed temporal structure of an exposure does not have a strong effect on trauma.

Understanding tinnitus: the dorsal cochlear nucleus, organization and plasticity

Tinnitus, the perception of a phantom sound, is a common consequence of damage to the auditory periphery. A major goal of tinnitus research is to find the loci of the neural changes that underlie the disorder. Crucial to this endeavor has been the development of an animal behavioral model of tinnitus, so that neural changes can be correlated with behavioral evidence of tinnitus. Three major lines of evidence implicate the dorsal cochlear nucleus (DCN) in tinnitus. First, elevated spontaneous activity in the DCN is correlated with peripheral damage and tinnitus. Second, there are somatosensory inputs to the DCN that can modulate spontaneous activity and might mediate the somatic-auditory interactions seen in tinnitus patients. Third, we have found a subpopulation of DCN neurons in the adult rat that express doublecortin, a plasticity-related protein. The expression of this protein may reflect a role of these neurons in the neural reorganization causing tinnitus. However, there is a problem in extending the findings in the rodent DCN to humans. Classic studies state that the structure of the primate DCN is quite different from that of rodents, with primates lacking granule cells, the recipients of somatosensory input. To address the possibility of major species differences in DCN organization, we compared Nissl-stained sections of the DCN in five different species. In contrast to earlier reports, our data suggest that the organization of the primate DCN is not dramatically different from that of the rodents, and validate the use of animal data in the study of tinnitus. This article is part of a Special Issue entitled: Tinnitus Neuroscience.

Forward masking estimated by signal detection theory analysis of neuronal responses in primary auditory cortex

Psychophysical forward masking is an increase in threshold of detection of a sound (probe) when it is preceded by another sound (masker). This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation. Studies in the auditory nerve and cochlear nucleus using signal detection theory techniques to derive neuronal thresholds showed that in centrally projecting neurons, increases in masked thresholds were significantly smaller than the changes measured psychophysically. Larger threshold shifts have been reported in the inferior colliculus of awake marmoset. The present study investigated the magnitude of forward masking in primary auditory cortical neurons of anaesthetised guinea-pigs. Responses of cortical neurons to unmasked and forward masked tones were measured and probe detection thresholds estimated using signal detection theory methods. Threshold shifts were larger than in the auditory nerve, cochlear nucleus and inferior colliculus. The larger threshold shifts suggest that central, and probably cortical, processes contribute to forward masking. However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically. Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.

Role of the kurtosis statistic in evaluating complex noise exposures for the protection of hearing

OBJECTIVE

To highlight a selection of data that illustrate the need for better descriptors of complex industrial noise environments for use in the protection of hearing.

DESIGN

The data were derived using a chinchilla model. All noise exposures had the same total energy and the same spectrum; that is, they were equal energy exposures presented at an overall 100 dB(A) SPL that differed only in the scheduling of the exposure and the value of the kurtosis, beta(t), a statistical metric. Hearing thresholds were determined before and after noise exposure using the auditory-evoked potential measured from the inferior colliculus in the brain stem. Cochlear damage was estimated from sensory-cell counts (cochleograms).

RESULTS

(1) For equivalent energy and spectra, exposure to a high-kurtosis, non-Gaussian noise produced substantially greater hearing and sensory-cell loss in the chinchilla model than a low-kurtosis, Gaussian noise. (2) beta(t) computed on the amplitude distribution of the noise could clearly differentiate between the effects of Gaussian and non-Gaussian noise environments. (3) beta(t) can order the extent of the trauma as determined by hearing thresholds and sensory-cell loss.

CONCLUSIONS

The noise level in combination with the statistical properties of the noise quantified by beta(t) clearly differentiate the effects between both continuous and interrupted and intermittent Gaussian and non-Gaussian noise environments. For the same energy and spectrum, the non-Gaussian environments are clearly the more hazardous. The use of both an energy and kurtosis metric can better predict the hazard of a high-level complex noise than the use of an energy metric alone (as is the current practice). These results point out the need for a new approach to the analysis and quantification of industrial noise for the purpose of hearing conservation practice.

Psychophysical evidence of damaged active processing mechanisms in Belgian Waterslager Canaries

Belgian Waterslager canaries (BWC), bred for a distinct low-pitched song, have an inherited high-frequency hearing loss associated with hair cell abnormalities. Hair cells near the abneural edge of the papilla, which receive primarily efferent innervation in normal birds, are among the most severely affected. These cells are thought to support nonlinear active processing in the avian ear, though the mechanisms are poorly understood. Here we present psychophysical evidence that suggests degraded active processing in BWC compared to normal-hearing non-BWC. Critical ratios, psychophysical masking patterns and phase effects on masking by harmonic complexes were measured in BWC and non-BWC using operant conditioning procedures. Critical ratios were much larger in BWC than in non-BWC at high frequencies. Psychophysical tuning curves derived from the masking patterns for BWC were broadened at high frequencies. BWC also showed severely reduced phase effects on masking by harmonic complexes compared to non-BWC. As has been hypothesized previously for hearing-impaired humans, these results are consistent with a loss of active processing mechanisms in BWC.

The effectiveness of N-acetyl-L-cysteine (L-NAC) in the prevention of severe noise-induced hearing loss

Three groups of chinchillas were exposed to a nonGaussian continuous broadband noise at an Leq=10 5dB SPL, 8h/d for 5d. One group (N=6) received only the noise. A second group (N=6) received the noise and was additionally treated with L-NAC (325 mg/kg, i.p.). Treatment was administered twice daily for 2d prior to exposure and for 2d following the exposure. During exposure the animals received the L-NAC just prior to and immediately after each daily exposure. The third group (N=4) was exposed to the noise and received saline injections on the same schedule as the L-NAC treated animals. Auditory evoked potential recordings from the inferior colliculus were used to estimate pure tone thresholds and surface preparations of the organ of Corti quantified the sensory cell population. In all three groups PTS exceeded 50 dB at 2.0k Hz and above with severe sensory cell loss in the basal half of the cochlea. There was no statistically significant difference among the three groups in all measures of noise-induced trauma. Treatment with L-NAC did not reduce the trauma produced by a high-level, long duration, broadband noise exposure.

Hearing loss from interrupted, intermittent, and time varying non-Gaussian noise exposure: The applicability of the equal energy hypothesis

Sixteen groups of chinchillas (N=140) were exposed to various equivalent energy noise paradigms at 100 dB(A) or 103 dB(A) SPL. Eleven groups received an interrupted, intermittent, and time varying (IITV) non-Gaussian exposure quantified by the kurtosis statistic. The IITV exposures, which lasted for 8 hday, 5 daysweek for 3 weeks, were designed to model some of the essential features of an industrial workweek. Five equivalent energy reference groups were exposed to either a Gaussian or non-Gaussian 5 days, 24 hday continuous noise. Evoked potentials were used to estimate hearing thresholds and surface preparations of the organ of Corti quantified the sensory cell population. For IITV exposures at an equivalent energy and kurtosis, the temporal variations in level did not alter trauma and in some cases the IITV exposures produced results similar to those found for the 5 day continuous exposures. Any increase in kurtosis at a fixed energy was accompanied by an increase in noise-induced trauma. These results suggest that the equal energy hypothesis is an acceptable approach to evaluating noise exposures for hearing conservation purposes provided that the kurtosis of the amplitude distribution is taken into consideration. Temporal variations in noise levels seem to have little effect on trauma.

Hearing loss from interrupted, intermittent, and time varying Gaussian noise exposures: the applicability of the equal energy hypothesis

Eight groups of chinchillas (N=74) were exposed to various equivalent energy [100 or 106 dB(A) sound pressure level (SPL)] noise exposure paradigms. Six groups received an interrupted, intermittent, time varying (IITV) Gaussian noise exposure that lasted 8 h/d, 5 d/week for 3 weeks. The exposures modeled an idealized workweek. At each level, three different temporal patterns of Gaussian IITV noise were used. The 100 dB(A) IITV exposure had a dB range of 90-108 dB SPL while the range of the 106 dB(A) IITV exposure was 80-115 dB SPL. Two reference groups were exposed to a uniform 100 or 106 dB(A) SPL noise, 24 h/d for 5 days. Each reference group and the three corresponding IITV groups comprised a set of equivalent energy exposures. Evoked potentials were used to estimate hearing thresholds and surface preparation histology quantified sensory cell populations. All six groups exposed to the IITV noise showed threshold toughening effects of up to 40 dB. All IITV exposures produced hearing and sensory cell loss that was similar to their respective equivalent energy reference group. These results indicate that for Gaussian noise the equal energy hypothesis for noise-induced hearing loss is an acceptable unifying principle.

Nonconstant quality of auditory filters in the porpoises, Phocoena phocoena and Neophocaena phocaenoides (Cetacea, Phocoenidae)

Simultaneous tone-tone masking in conjunction with the envelope-following response (EFR) recording was used to obtain tuning curves in porpoises Phocoena phocoena and Neophocaena phocaenoides asiaeorientalis. The EFR was evoked by amplitude-modulated probes with a modulation rate of 1000 Hz and carrier frequencies from 22.5 to 140 kHz. Equivalent rectangular quality QERB of the obtained tuning curves varied from 8.3-8.6 at lower (22.5-32 kHz) probe frequencies to 44.8-47.4 at high (128-140 kHz) frequencies. The QERB dependence on probe frequency could be approximated by regression lines with a slope of 0.83 to 0.86 in log-log scale, which corresponded to almost frequency-proportional quality and almost constant bandwidth of 3-4 kHz. Thus, the frequency representation in the porpoise auditory system is much closer to a constant-bandwidth rather that to a constant-quality manner.

Increased resistance to free radical damage induced by low-level sound conditioning

Conditioning is the phenomenon where exposure to moderate-level acoustic stimuli can increase the ear's resistance to subsequent more intense sound exposures. In recent years, research has shown that conditioning increases the availability of antioxidant enzymes which presumably protects the ear from oxidative stress induced by a traumatic noise exposure [Jacono, A.A., Hu, B., Kopke, R.D., Henderson, D., Van De Water, T.R., Steinman, H.M., 1998. Changes in cochlear antioxidant enzyme activity after sound conditioning and noise exposure in the chinchilla. Hear Res 117, 31-8]. The current study was designed to assess whether the increase in endogenous antioxidants seen following conditioning could provide protection from oxidative stress induced by Paraquat, a potent generator of superoxide. Chinchillas were exposed to a conditioning noise, 500 Hz OBN at 95 dB for 6 h/day for 10 days, followed 5 days later with Paraquat application to the round window. Controls underwent the Paraquat application surgery, without prior conditioning. Evoked potential thresholds were determined prior to conditioning, at day 1, 5 and 10 during conditioning, at day 15 (5 days after conditioning), and at day 17, 19, 23, and 35 (1, 3, 7, and 20 days post-Paraquat). The conditioned animals showed reductions in permanent threshold shift and reduced inner hair cell loss relative to controls. These results reinforce the hypothesis that antioxidants are primary mediators of the conditioning effect.

Unexceptional sharpness of frequency tuning in the human cochlea

The responses to sound of auditory-nerve fibers are well known in many animals but are topics of conjecture for humans. Some investigators have claimed that the auditory-nerve fibers of humans are more sharply tuned than are those of various experimental animals. Here we invalidate such claims. First, we show that forward-masking psychophysical tuning curves, which were used as the principal support for those claims, greatly overestimate the sharpness of cochlear tuning in experimental animals and, hence, also probably in humans. Second, we calibrate compound action potential tuning curves against the tuning of auditory-nerve fibers in experimental animals and use compound action potential tuning curves recorded in humans to show that the sharpness of tuning in human cochleae is not exceptional and that it is actually similar to tuning in all mammals and birds for which comparisons are possible. Third, we note that the similarity of frequency of tuning across species with widely diverse cochlear lengths and auditory bandwidths implies that for any given stimulus frequency the "cochlear amplifier" is confined to a highly localized region of the cochlea.

Prevention of noise-induced hearing loss with Src-PTK inhibitors

Studies from our lab show that noise exposure initiates cell death by multiple pathways [Nicotera, T.M., Hu, B.H., Henderson, D., 2003. The caspase pathway in noise-induced apoptosis of the chinchilla cochlea. J. Assoc. Res. Otolaryngol. 4, 466-477] therefore, protection against noise may be most effective with a multifaceted approach. The Src protein tyrosine kinase (PTK) signaling cascade may be involved in both metabolic and mechanically induced initiation of apoptosis in sensory cells of the cochlea. The current study compares three Src-PTK inhibitors, KX1-004, KX1-005 and KX1-174 as potential protective drugs for NIHL. Chinchillas were used as subjects. A 30 microl drop of one of the Src inhibitors was placed on the round window membrane of the anesthetized chinchilla; the vehicle (DMSO and buffered saline) alone was placed on the other ear. After the drug application, the middle ear was sutured and the subjects were exposed to noise. Hearing was measured before and several times after the noise exposure and treatment using evoked responses. At 20 days post-exposure, the animals were anesthetized their cochleae extracted and cochleograms were constructed. All three Src inhibitors provided protection from a 4 h, 4 kHz octave band noise at 106 dB. The most effective drug, KX1-004 was further evaluated by repeating the exposure with different doses, as well as, substituting an impulse noise exposure. For all conditions, the results suggest a role for Src-PTK activation in noise-induced hearing loss (NIHL), and that therapeutic intervention with a Src-PTK inhibitor may offer a novel approach in the treatment of NIHL.

The use of distortion product otoacoustic emissions in the estimation of hearing and sensory cell loss in noise-damaged cochleas

Distortion product otoacoustic emissions (DPOAE), permanent threshold shifts (PTS) and outer hair cell (OHC) losses were analyzed in a population of 187 noise-exposed chinchillas to determine the predictive accuracy (sensitivity and specificity) of the DPOAE for PTS and OHC loss. Auditory evoked potentials (AEP) recorded from the inferior colliculus of the brainstem were used to estimate hearing thresholds and surface preparation histology was used to determine sensory cell loss. The overlapping cumulative distributions and high variability in emission responses for both PTS and OHC loss made it difficult to predict AEP threshold and OHC loss from DPOAE level measurements alone. Using a strict criterion (i.e. emissions better than the 5th percentile of the preexposure DPOAE level, and PTS< or = 5 dB or OHC loss< or = 5%), it was found that the postexposure DPOAE level could be used with reasonable confidence to determine if the status of peripheral auditory system was either normal (i.e. PTS< or = 5 dB) or abnormal (PTS>30 dB or OHC loss>40%). However, the high variability of individual DPOAE responses resulted in a broad region of 'uncertainty' (i.e. 5<PTS< or = 30 dB and 5%<OHC loss< or = 40%) making it difficult in the chinchilla model to use the postexposure DPOAE level with confidence to predict in individual subjects the amount of PTS or OHC loss. Our results also indicate that significant reductions in the amplitude of the DPOAE are related primarily to a systematic loss of OHCs, and that a postexposure DPOAE level< or = 10 dB SPL, obtained with a low frequency primary level of 65 dB SPL, represents a criterion value which can serve as an indication of significant OHC loss (> or = 50%) or PTS (> or = 35 dB) in noise-exposed chinchillas. Based on an exponential regression analysis of individual subjects, correlations were higher for PTS/DPOAE than for OHC loss/DPOAE.

Noise-induced hearing loss in chinchillas pre-treated with glutathione monoethylester and R-PIA

The protective effects of glutathione monoethylester (GEE) and GEE in combination with R-N6-phenylisopropyladenosine (R-PIA) were evaluated in the chinchilla when exposed to impulse (145 dB pSPL) or continuous (105 dB SPL, 4 kHz OB) noise. Six groups of 10 chinchillas were used as subjects. Before exposure to noise, the subjects were anesthetized, a 30 microl drop of drug was placed on the round window (GEE [50, 100, 150 mM], GEE 50 mM and R-PIA). Forty minutes later the subject was exposed to either impulse or continuous noise. The 50 mM treatment provided significant protection from impulse noise, but not from continuous noise exposure. The combination provided significant protection from both the continuous and impulse noise. In a separate set of experiments, glutathione (GSH) levels were measured in the perilymph. All the drug treatments elevated GSH levels. The results are discussed in terms of antioxidant treatments as a prophylactic measure against noise-induced hearing loss.

Functional reorganization in chinchilla inferior colliculus associated with chronic and acute cochlear damage

This paper describes some of the unexpected functional changes that occur in the inferior colliculus (IC) following noise- and drug-induced cochlear pathology. A striking example of this is the compensation that is seen in IC responsiveness after drug-induced selective inner hair cell (IHC) loss. Despite a massive reduction in the compound action potential (CAP) caused by partial IHC loss, the evoked potential amplitude from the IC shows little or no reduction. Acoustic trauma, which impairs cochlear sensitivity and tuning, also reduces the CAP amplitude. Despite this reduced neural input, IC amplitude sometimes increases at a faster than normal rate and the response amplitude is enhanced at frequencies below the hearing loss. Single unit recordings suggest the IC enhancement phenomenon may be due to the loss of lateral inhibition. After an acute traumatizing exposure to a tone located above the characteristic frequency (CF), approximately 50% of IC neurons show a significant increase in their spike rate, a significant expansion of the low frequency tail of the tuning curve and a significant improvement in sensitivity in the tail of the tuning curve. These changes suggest that IC neurons receive inhibition from a high frequency side band and that this inhibition is diminished by acoustic trauma above CF. To determine if side band inhibition was locally mediated, specific antagonist(s) to inhibitory neurotransmitters were applied and found to produce effects similar to acoustic trauma. The results suggest that lesioned-induced central auditory plasticity could contribute to several symptoms associated with sensorineural hearing loss such as loudness recruitment, tinnitus and poor speech discrimination in noise.

Correlations among evoked potential thresholds, distortion product otoacoustic emissions and hair cell loss following various noise exposures in the chinchilla

Changes in cubic distortion product otoacoustic emissions (DeltaDPOAEs), evoked potential threshold shifts (TSs) and outer hair cell (OHC) losses were measured in a population of 95 noise-exposed chinchillas. Each animal was exposed to one of 23 different noises in an asymptotic threshold shift (ATS) producing paradigm or an interrupted noise paradigm which typically produced a toughening effect. Noises were narrow band (400 Hz) impacts with center frequencies of 0.5, 1.0, 2.0, 4.0 or 8.0 kHz presented 1 impact/s at peak SPLs of 109, 115, 121 or 127 dB. The duration of the exposures was 24 h/day for 5 days (ATS paradigm) or 6 h/day for 20 days (toughening paradigm). Based on a linear regression analysis of individual subject and group mean data, correlations among the following dependent variables were made: DeltaDPOAEs, ATS, toughening or TS recovery (TS(r)), permanent threshold shift (PTS) and OHC loss. Correlations among these metrics were generally highest for DPOAE primary frequency levels, L(1)=L(2)=70 dB. Correlation between DeltaDPOAE and TS(r) was typically low, while a considerably higher correlation was found between DeltaDPOAE and ATS. Correlations among the permanent measures of noise-induced effects, i.e. for DeltaDPOAE/PTS and DeltaDPOAE/OHC loss were typically poor when there was only a small or a moderate noise-induced effect (PTS<25 dB and DeltaDPOAE<20 dB). However, for PTS<25 dB the correlation between PTS and OHC loss was considerably better than the correlation between DeltaDPOAE and OHC loss. For more severe noise-induced changes there was generally a good correspondence between OHC loss, PTS and DeltaDPOAE metrics.

Auditory plasticity and hyperactivity following cochlear damage

This paper will review some of the functional changes that occur in the central auditory pathway after the cochlea is damaged by acoustic overstimulation or by carboplatin, an ototoxic drug that selectively destroys inner hair cells (IHCs) in the chinchilla. Acoustic trauma typically impairs the sensitivity and tuning of auditory nerve fibers and reduces the neural output of the cochlea. Surprisingly, our results show that restricted cochlear damage enhances neural activity in the central auditory pathway. Despite a reduction in the auditory-nerve compound action potential (CAP), the local field potential from the inferior colliculus (IC) increases at a faster than normal rate and its maximum amplitude is enhanced at frequencies below the region of hearing loss. To determine if this enhancement was due to loss of sideband inhibition, we recorded from single neurons in the IC and dorsal cochlear nucleus before and after presenting a traumatizing above the unit's characteristic frequency (CF). Following the exposure, some neurons showed substantial broadening of tuning below CF, less inhibition, and a significant increase in discharge rate, consistent with a model involving loss of sideband inhibition. The central auditory system of the chinchilla can be deprived of some of its cochlear inputs by selectively destroying IHCs with carboplatin. Selective IHC loss reduces the amplitude of the CAP without affecting the threshold and tuning of the remaining auditory nerve fibers. Although the output of the cochlea is reduced in proportion to the amount of IHC loss, the IC response shows only a modest amplitude reduction, and remarkably, the response of the auditory cortex is enhanced. These results suggest that the gain of the central auditory pathway can be up- or down regulated to compensate for the amount of neural activity from the cochlea.

Effects of inner hair cell loss on inferior colliculus evoked potential thresholds, amplitudes and forward masking functions in chinchillas

The effects of outer hair cell (OHC) loss on evoked potential (EVP) thresholds, amplitudes and forward masking (FWM) functions have been fairly well characterized. In contrast, the effects of inner hair cell (IHC) losses are largely unknown, primarily due to the difficulty of producing selective IHC lesions. Recent studies have shown that IHCs of the chinchilla are preferentially damaged by the anticancer drug, carboplatin. In this study, we administered a single 100 mg/kg dose of carboplatin to four chinchillas, to examine the effects of IHC lesions on EVPs measured from the inferior colliculus (IC-EVPs). Thresholds and amplitude functions were measured for 0.25-16 kHz tone bursts, and FWM functions were measured at 1, 2 and 4 kHz, using masker probe intervals of 2, 5, 10, 20, 40 and 80 ms, before and 1-2 months after carboplatin treatment. Histology revealed IHC lesions ranging from approximately 15 to 90%, with virtually no loss of OHCs. Surprisingly, even massive IHC lesions were not associated with elevations of IC-EVP thresholds. IC-EVP amplitudes at suprathreshold levels were sometimes depressed, sometimes enhanced, and in some cases unchanged. IHC lesions increased susceptibility to FWM, particularly at intermediate (10-20 ms) masker-probe intervals, without significantly changing the overall time course of FWM. The results provide new perspectives on the contribution of IHCs to FWM, and on the ability of the central auditory system to adapt to a significant reduction of neural input from the cochlea.

Noise-induced threshold shift dynamics measured with distortion-product otoacoustic emissions and auditory evoked potentials in chinchillas with inner hair cell deficient cochleas

Chinchillas (n = 6) were treated with carboplatin and, following a 30-day recovery period, were exposed to a 115 dB peak SPL impact noise presented at a rate of l/s for 6 h/day for 10 days. A second group (n = 6) received only the noise treatment. Cubic distortion product otoacoustic emissions (2f1-f2) and auditory evoked potential (AEP) detection thresholds in response to tone bursts were measured before and 30 days after drug treatment and following the first and 10th day of the noise exposure. Thirty days after the final exposure day, permanent changes in AEP detection thresholds and emissions were measured and cochleograms constructed. The drug treatment eliminated over 80% of the inner hair cells (IHC) in the cochlea, leaving the outer hair cell (OHC) population essentially intact prior to the interrupted noise exposure. The drug treatment alone had very little or no effect on AEP detection thresholds and emission metrics. Following the noise exposure, the IHC-deficient animals showed clear 'toughening' effects in the AEP and emission measures which were the same as measured in the group receiving only the noise. After a 30-day post-exposure recovery period. AEP thresholds were elevated about 10 dB at the low frequencies in the drug-noise group whereas emissions returned to near normal despite the massive IHC losses. These results are consistent with the idea that an intact OHC population is required for toughening. However, sound-evoked efferent pathways activated by the few remaining IHCs (approximately 20%) which, in this preparation, are distributed throughout the cochlea, may still contribute significantly to the toughening phenomena.

Evoked-potential thresholds and cubic distortion product otoacoustic emissions in the chinchilla following carboplatin treatment and noise exposure

Twenty-two chinchillas were given either a single intraperitoneal (i.p.) or intravenous (i.v.) injection (50 or 75 mg/kg) of Paraplatin, an asymptotic threshold shift-producing noise or a combination of the drug and noise in series. Auditory evoked potential (pure-tone) audiograms and cubic distortion product otoacoustic emissions were obtained on each animal before and after treatment, and the sensory epithelium of the cochlea was evaluated using the surface preparation method. Anatomical analysis indicated that the carboplatin alone caused relatively severe but scattered losses of inner hair cells throughout most of the cochlea which were dependent on dose and administration route. The outer sensory cell population remained essentially intact. In animals that had up to 40% scattered losses of only inner hair cells, evoked potential thresholds were near normal and the emission functions either were normal or showed an enhanced output. The severe losses of inner hair cells produced by the drug had no effect on the threshold shift dynamics produced by a five-day uninterrupted noise exposure. In general, there was not a consistent relation between the emission data and both the permanent threshold shift and outer hair cell losses.

ABR frequency tuning curves in dolphins

Tone-tone masking was used to determine auditory brain-stem response tuning curves in dolphins (Tursiops truncatus) in a simultaneous-masking paradigm. The Q10 of the curves was as large as 16-19 in the frequency range 64-128 kHz. In the range 45-16 kHz, Q10 decreased proportionally to the frequency with the bandwidth of the curves being constant, about 3.5-4 kHz at the 10-dB level. Tuning curves below 45 kHz are supposed to reflect broad spectral bandwidth of the probe's effective part which is no longer than 0.5 ms, irrespective of actual probe duration. Tuning curves above 64 kHz are supposed to reflect the real frequency tuning of the dolphin's auditory system.

The role of tuning curve variables and threshold measures in the estimation of sensory cell loss

Auditory-evoked potential tuning curves were collected at six frequencies before and 30 days after various noise exposures in 363 chinchillas using a simultaneous masking paradigm. Traditional bivariate and multiple linear regression/correlation analyses were performed in an effort to determine the extent to which sensory cell damage could be estimated from a knowledge of audiometric and tuning curve variables. The results showed strong correlations between percent outer hair cell (%OHC) loss and permanent threshold shift (PTS) and between %OHC loss and the tuning curve variables Q10 dB and high- and low-frequency slopes (SHF, SLF). The correlations were strongest between PTS and %OHC loss. However, the proportion of variability (r2) in %OHC loss attributable to variability in the predictor variable(s) (i.e., PTS) could be increased significantly by adding the Q10 dB of the tuning curve whose probe frequency was centered in the octave band length of the cochlea corresponding to the frequency at which the PTS occurred. The r2 values could be further increased by including audiometric and tuning curve variables from frequencies adjacent to the octave band being evaluated.

Frequency selectivity in noise-damaged cochleas

Measures of auditory threshold and masked threshold were obtained at six audiometric test frequencies along with cochleograms on a total population of 363 noise-exposed chinchillas. Seventy animals were chosen from this sample and were separated into five relatively homogeneous groups based upon the amount of permanent threshold shift and sensory cell losses the animals incurred. Tuning curve (TC) metrics were compared to the mean preexposure TC metrics for each group and to the reference preexposure TC metrics obtained from the sample of 363 animals. These data show that in animals with relatively little hearing loss changes in TC metrics can provide evidence for noise-induced sensory cell losses and that the low frequency slope of the TC is a sensitive index of trauma.

Auditory evoked potentials in cats with neonatal high frequency hearing loss. Evidence of abnormal frequency representation in the midbrain

We have recorded auditory evoked potentials, of both neurogenic and myogenic origin, in cats having neonatal high frequency cochlear hearing loss. Using frequency specific stimuli (tone pips) and by measuring responses near to threshold, we have probed tonotopic (or cochleotopic) representation within the brainstem-midbrain auditory pathway. At stimulus frequencies corresponding to the high frequency cut-off of the cats' audiograms we have observed enhanced amplitudes of both auditory brainstem evoked responses (ABR) and postauricular myogenic (PAM) potentials. We interpret our findings as evidence of a larger than normal population of neurons tuned to this frequency region. We suggest that such abnormal frequency representation results from a long-term sensory deficit caused by lesions to the basal, high frequency region of the cochleas.

Physiological and histological changes associated with the reduction in threshold shift during interrupted noise exposure

The compound action potential (AP) was recorded from one group of chinchillas exposed to interrupted noise (95 dB SPL, octave band centered at 500 Hz, 3 h on, 9 h off) for 15 days. A second group of chinchillas was exposed to the same interrupted noise for 1, 2 or 15 days and their cochleas were analyzed by scanning electron microscopy (SEM). During the first few days of the exposure, the AP threshold was elevated approximately 40 dB at the low-to-mid frequencies; however, the threshold shifts decreased with increasing exposure duration so that the threshold shift was only about 10 dB after the 15th day of exposure. The amplitude of the AP also recovered with exposure time. In contrast to the improvement in AP threshold, the number of missing hair cells increased and the condition of the stereocilia on inner and outer hair cells deteriorated between the first and 15th day of the exposure.

Effects of noise and salicylate on auditory evoked-response thresholds in the chinchilla

The combined effects of noise and sodium salicylate on auditory sensitivity were examined in the chinchilla. Sensitivity was monitored by recording the evoked response recorded with an electrode implanted in the inferior colliculus. Sodium salicylate (300 mg/kg/day), an octave band of noise centered at 500 Hz (80 or 105 dB SPL), or both of these agents were delivered for 15 days. Threshold testing was performed at 7 frequencies before, during, and after exposure to the ototraumatic agent(s). The salicylate alone caused an average temporary threshold shift of less than 10 dB and essentially no permanent shift. Animals exposed to noise alone had temporary and permanent threshold shifts which were not significantly different from those observed in animals exposed to noise plus salicylate. The data suggest that a single daily injection of sodium salicylate, resulting in peak serum salicylate concentrations of 28 to 34 mg% 2 to 4 hours after delivery, does not exacerbate the temporary or permanent threshold shifts induced by 15-day, 24-hour-per day exposure to either a moderate- or high-level, low-frequency noise. A second series of experiments utilizing a higher dose of salicylate (450 mg/kg/day) was not completed due to a high mortality rate among subjects that received salicylate and were exposed to noise. This result was consistent with other recent examinations of the interaction of these agents.

The effect of quinine on psychoacoustic tuning curves, stapedius reflexes and evoked otoacoustic emissions in healthy volunteers

Quinine causes reversible hearing loss, closely related to the quinine plasma concentration. The effects of quinine on psychoacoustic tuning curves, stapedius reflex thresholds and evoked otoacoustic emissions were studied in healthy volunteers. The tuning curves became shallower, whereas reflex thresholds were unaffected. The shift in the emission thresholds paralleled that of the pure-tone thresholds. There were also qualitative changes in the emissions: 1) the exponent of the stimulus-response function changed from 0.34 to 0.56; 2) decay time shortened; 3) the power spectrum shifted towards lower frequencies. The results are discussed in relation to various aspects of cochlear performance and are suggested to depend on an outer hair cell dysfunction.

Enhanced evoked response amplitudes in the inferior colliculus of the chinchilla following acoustic trauma

Evoked response amplitude-level functions were measured from electrodes in the inferior colliculus of the chinchilla before and after exposure to a 2 kHz pure tone of 105 dB SPL. The exposure produced approximately 20-30 dB of permanent threshold shift from 2 to 8 kHz, but little or no hearing loss at higher or lower frequencies. Generally less than 60% of the outer hair cells were missing in the region of hearing loss. The amplitude-level functions measured at 4 and 8 kHz generally showed a loss in sensitivity at low sound levels, a reduction in the maximum amplitude and sometimes steeper than normal slopes. The amplitude-level functions measured at 2 kHz also showed a loss in sensitivity; however, the maximum amplitude was often greater than normal. Even though there was no loss in sensitivity at 0.5 kHz, the amplitude-level function was steeper than normal and the maximum amplitude of the evoked response was almost always substantially larger than normal. The enhancement of the evoked response amplitude from the inferior colliculus does not appear to originate in the cochlea, but may reflect a reorganization of neural activity in the central auditory pathway.

Interaction of cisplatin and noise on the peripheral auditory system

The potentiation of cisplatin ototoxicity by noise was explored in the chinchilla. The effects of exposure to cisplatin alone, noise alone or concurrent exposure to both agents were compared in terms of the threshold shift of the auditory evoked potential and the amount of hair cell loss. The combination of cisplatin plus noise produced significantly more hair cell loss and hearing loss at the high frequencies than did either the noise or cisplatin alone when the noise level was 85 dB SPL or higher; no interaction was seen when the noise level was 70 dB SPL. The amount of the interaction, when present, was constant regardless of the noise level. These results indicate that moderate to high levels of noise can exacerbate cisplatin ototoxicity.

Evoked response narrow-band noise masking patterns in the chinchilla

Narrow-band noise masking patterns were measured at 0.5, 1, 2 and 4 kHz in the chinchilla using the auditory evoked response from the inferior colliculus. At low masker levels, the masking profiles were symmetrical and centered on the masker. However, as masker level increased, the masking profiles spread predominantly toward the high frequencies. The masking profiles obtained at 0.5 and 1 kHz, exhibited a broad plateau extending 1-2 octaves above the masker at the highest masker level (70 dB SPL) whereas those obtained at 2 and 4 kHz showed a peak. In contrast to tone-on-tone masking profiles, none of the narrow-band noise masking profiles contained a low-threshold notch on the high frequency side of the masker. The evoked response masking profiles obtained in the chinchilla were slightly wider, but qualitatively similar to those measured psychophysically in humans. Thus, the evoked response narrow-band noise masking profiles may provide a convenient way of evaluating the spread of masking in difficult-to-test subjects.

Cochlear threshold assessment using tone-derived action potentials

An evoked-potential technique has been evaluated which detects whether the cochlea responds to a continuous, low level tone. The technique involves recording the cochlear action potential (AP) response to a suprathreshold probe tone, first in the absence and then in the presence of a continuous masking tone at the same frequency. Subtraction of the masked AP waveform from the unmasked AP yields a 'derived' potential, provided the continuous tone is above the threshold of cochlear sensitivity. Derived AP responses may be recorded with continuous masking tones over 10 dB below the threshold to the probe stimulus. In normally hearing guinea pigs, the mean best derived threshold using a 10-microV response criterion was 7.1 dB SPL, compared to 18.9 dB for conventional AP thresholds. The tone-derived response appears to provide a more sensitive and frequency-specific method for determining cochlear thresholds.

Frequency selectivity of the middle latency response

A forward masking paradigm was used to assess the frequency selectivity of the middle latency response (MLR). Tuning curves of the MLR were obtained in unanesthetized gerbils. Changes in the amplitudes of MLR waves A, B, and C with latency values of 10 to 13 ms, 14 to 17 ms, and 20 to 25 ms, respectively, were analyzed as a function of masker frequency and intensity. Tuning curves of the MLR were also compared to tuning curves of the auditory brainstem response (ABR), which was recorded simultaneously with the MLR. The MLR and ABR differed in their response to forward masking. The MLR was reduced in amplitude or eliminated by masker stimuli that had minimal or no effect on the ABR. Forward masking often caused variable and non-monotonic changes in the amplitude of the MLR. Tuning curves of the MLR indicate that the MLR is less frequency selective than the ABR. The MLR is an electrophysiological measure of auditory function central to the auditory brainstem. Therefore, it may provide information concerning central components of normal and pathological auditory function. However, because of the variability of MLR amplitudes with forward masking, tuning curves of the MLR are difficult to obtain and are not efficient for routine measurements of frequency selectivity.

Evoked-response tone-on-tone masking in the chinchilla: effect of masker frequency

Tone-on-tone masking patterns were measured at 0.5, 1, and 4 kHz using the auditory-evoked response from the inferior colliculus of the chinchilla. Masking profiles obtained with a masking level of 30 dB SPL were relatively symmetrical; however, as masker level increased, masking spread toward the high frequencies, particularly with the 0.5-kHz masker. For masker frequencies of 1 and 4 kHz, a low-threshold notch was observed in the masking profile 2/3 of an octave above the frequency of the masker. The low-threshold notch may represent a response to the distortion tone 2F1-F2. The notch was absent in the 0.5-kHz masking profile. The masking profiles obtained with the evoked response are somewhat wider, but otherwise qualitatively similar to those measured psychophysically in humans. Thus, the evoked-response procedure may provide a convenient way of evaluating the spread of masking and the presence of distortion tones in difficult-to-test subjects.

Effects of sodium salicylate on evoked-response measures of hearing

The effects of sodium salicylate on auditory sensitivity, frequency selectivity, and temporal resolution were monitored in chinchillas using evoked responses recorded from the inferior colliculus. Administration of sodium salicylate (450 mg/kg/day for 5-15 days) resulted in serum salicylate levels of 25-50 mg/100 ml. The salicylate administration caused elevation of evoked response thresholds (up to 30 dB), mainly at the high frequencies. Frequency selectivity, as estimated by the Q10 dB value of evoked-response tuning curves, was poorer at 4 kHz during drug administration than during the no drug (control) condition. Temporal resolution, as estimated by recovery from forward masking was not significantly poorer during salicylate administration than before administration. However, there was a trend toward longer recovery in animals with large threshold shifts. These results are consistent with models suggesting that salicylate affects the auditory system at the level of the organ of Corti. They also suggest that while the changes in hearing that occur after salicylate administration are consistent with those seen in sensorineural hearing loss, the changes in hearing in the chinchilla are small enough to preclude the use of the drug as a practical model for sensorineural loss.

The relation among hearing loss, sensory cell loss and tuning characteristics in the chinchilla

Evoked-potential tuning curves were obtained on over 150 chinchillas before and after acoustic overstimulation in order to relate the effects of changes in frequency selectivity to sensory cell loss over a wide range of hearing loss. Pre- and post-exposure measures of auditory thresholds and masked thresholds (simultaneous tone-on-tone paradigm) were obtained in each animal at 0.5, 1.0, 2.0, 4.0, 8.0 and 11.2 kHz, using the auditory evoked potential recorded from the inferior colliculus. Three tuning curve variables (Q10dB, low-frequency slope and high-frequency slope) were compared to the amount of noise-induced permanent threshold shift and to the percent sensory cell loss produced by a variety of noise exposures. Based upon large sample averages, frequencies showing permanent threshold shifts in excess of 10 dB also showed statistically significant differences between pre- and post-exposure measures of all three tuning curve variables. Shifts of less than 10 dB were not accompanied by statistically significant changes in the tuning curve variables. The percentage of outer hair cell loss, and percentage change in tuning curve characteristics showed systematic and parallel increases as threshold shifts increased at all probe tone frequencies except 8.0 and 11.2 kHz. In general, the results were consistent in showing that there is a systematic change in the variables which define the quality of tuning as hearing loss progressively increases and that these changes are clearly related to outer hair cell losses.

Evoked-response forward-masking functions in chinchillas with noise-induced permanent hearing loss

Evoked-response forward-masking functions were measured by chronic electrodes in the inferior colliculus of the chinchilla before and after exposure to an intense tone that produced a permanent hearing loss. Before exposure, the forward-masking time constants ranged from 50 to 90 ms. After exposure, the forward-masking time constants increased significantly in the region of hearing loss, but not in regions where hearing was normal. The effect of the hearing loss on the time course of forward masking was most pronounced once the hearing loss exceeded 20-25 dB. These physiological changes in the evoked-response forward-masking functions appear to parallel those observed psychophysically in human listeners.

Comparative psychoacoustics

Psychophysical data on unspecialized mammals commonly used in auditory research were compiled from the literature, and an attempt was made to compare the hearing capacities of these species with man. Binaural hearing and sound localization were not considered. The most complete psychoacoustic data exist for chinchilla, cat, various primates, and the mouse. The existing data include audiograms, frequency and intensity discrimination thresholds, critical masking ratios, critical bandwidths, temporal summation functions at threshold, psychophysical tuning curves, gap detection thresholds, temporal modulation transfer functions, temporal discriminations, and auditory filter shapes. In general, the qualitative forms of most all psychoacoustic functions for these mammals are similar to those for man, and there is little reason to believe that the mechanisms underlying these capacities are different across mammals. Although the discriminative capacities of humans are generally more acute than those of non-humans, the database on the capacities of non-humans is not yet sufficient for systematic comparisons across species to be made with confidence.

Evoked response 'forward masking' patterns in chinchillas with temporary hearing loss

Evoked response "forward masking" data were measured from the inferior colliculus of the chinchilla before and during a temporary threshold shift. The hearing loss was induced by a 2 kHz pure tone of 85 dB SPL presented from 5-8 days. The exposure elevated thresholds by approximately 35 dB at the mid frequencies, but had no effect on low frequency hearing. The exposure also altered the time course of the evoked response forward masking data. Time constants fitting the forward masking data increased by up to a factor of three at the frequency with the greatest loss, but remained within normal limits at the low frequencies where hearing was normal. The increase in the forward masking time constants became most noticeable once the hearing loss exceeded 25 dB. These physiological results are consistent with psychophysical forward masking data from hearing impaired listeners.

Evoked response 'forward masking' functions in chinchillas

Evoked potentials were obtained from the inferior colliculus of awake chinchillas using a 'forward masking' stimulus paradigm. The response was elicited using a brief probe tone presented 10 dB above threshold. Forward masking functions were then measured by determining the intensity of the masker needed to produce a 50% reduction in evoked response amplitude over a range of time intervals between masker offset and probe onset. The time course of forward masking was evaluated by fitting an exponential function to the results. The average time constant values of forward masking functions ranged from 50 to 90 ms. Although the constants at 0.5 and 8.0 kHz were somewhat longer than those at the midfrequencies, the differences were not statistically significant. The evoked response forward masking functions were quite similar to those obtained psychophysically; this suggests that it may be an efficient method of estimating the time course of forward masking in difficult to test subjects.

Threshold sensitivity and frequency selectivity measured with round window whole nerve action potentials in the awake, restrained chinchilla

Chronic electrodes, placed on the round windows of four adult chinchillas, were used to measure whole-nerve action potential (AP) thresholds and tuning curves. The AP quiet thresholds were within 17 dB of thresholds measured by behavioral methods and within 3 dB of those measured by other evoked response procedures. The AP tuning curves, obtained by a simultaneous masking procedure, were also similar to previously measured tuning curves in the chinchilla. These results indicate that long-term indwelling electrodes can be successfully placed on the chinchilla round window and used to measure threshold sensitivity and frequency selectivity. Recordings from these electrodes could be used in a variety of situations and may be particularly useful in studying the effects of noise on the cochlea.

A comparison of brainstem, whole-nerve AP and single-fiber tuning curves in the gerbil: normative data

Tuning curves were obtained from brainstem responses (BSRs) and whole-nerve action potential (AP) responses of gerbils using a forward masking procedure. These are compared with single-fiber tuning curves of the gerbil. The broadness of the tips, tip-to-tail ratios and the slopes of the high frequency sides of the BSR tuning curves are similar to those of the AP curves for probe frequencies ranging from 1 to 8 kHz. Also, the BSR and AP curves share a number of characteristics with single-fiber tuning curves. These results suggest that, like the AP, components of the BSR can be used to measure frequency selectivity in the periphery of an intact auditory system.

Hypersensitivity to electrical stimulation of auditory nuclei follows hearing loss in cats

The purpose of the study was to determine if permanent, sound-induced hearing loss altered behaviorally measured thresholds for the detection of electrical stimulation applied to auditory nuclei. Electrodes were placed in cochlear nucleus and inferior colliculus in four cats. Behaviorally measured thresholds for the detection of brief trains of electrical pulses were determined before and after a 48 h exposure to a 1 kHz tone of approximately 110 dB SPL. The mean decrease in electrical stimulation threshold as a result of the sound exposure was 10.4 dB. The ongoing electrical activity (in microV, rms) recorded from the electrodes showed a mean 2.2 dB decrease after the sound exposure. In some electrodes, there was partial recovery towards pre-exposure levels for stimulation threshold and for ongoing activity, but typically, the changes persisted until the animals were terminated 30 days later. The magnitudes of the decreases in stimulation threshold and background activity proved not to be highly correlated. The permanent auditory threshold shift across all cats and all frequencies was 19 dB. This mild hearing loss produced a marked alteration in certain characteristics of the central auditory mechanisms.