Single auditory nerve fiber and action potential latencies in normal and noise-treated chinchillas

Psychophysical changes in temporal processing in chinchillas with noise-induced hearing loss: A literature review

It is well-established that excessive noise exposure can systematically shift audiometric thresholds (i.e., noise-induced hearing loss, NIHL) making sounds at the lower end of the dynamic range difficult to detect. An often overlooked symptom of NIHL is the degraded ability to resolve temporal fluctuations in supra-threshold signals. Given that the temporal properties of speech are highly dynamic, it is not surprising that NIHL greatly reduces one's ability to clearly decipher spoken language. However, systematic characterization of noise-induced impairments on supra-threshold signals in humans is difficult given the variability in noise exposure among individuals. Fortunately, the chinchilla is audiometrically similar to humans, making it an ideal animal model to investigate noise-induced supra-threshold deficits. Through a series of studies using the chinchilla, the authors have elucidated several noise-induced deficits in temporal processing that occur at supra-threshold levels. These experiments highlight the importance of the chinchilla model in developing an understanding of noise-induced deficits in temporal processing.

Noise trauma induced plastic changes in brain regions outside the classical auditory pathway

The effects of intense noise exposure on the classical auditory pathway have been extensively investigated; however, little is known about the effects of noise-induced hearing loss on non-classical auditory areas in the brain such as the lateral amygdala (LA) and striatum (Str). To address this issue, we compared the noise-induced changes in spontaneous and tone-evoked responses from multiunit clusters (MUC) in the LA and Str with those seen in auditory cortex (AC) in rats. High-frequency octave band noise (10-20 kHz) and narrow band noise (16-20 kHz) induced permanent threshold shifts at high-frequencies within and above the noise band but not at low frequencies. While the noise trauma significantly elevated spontaneous discharge rate (SR) in the AC, SRs in the LA and Str were only slightly increased across all frequencies. The high-frequency noise trauma affected tone-evoked firing rates in frequency and time-dependent manner and the changes appeared to be related to the severity of noise trauma. In the LA, tone-evoked firing rates were reduced at the high-frequencies (trauma area) whereas firing rates were enhanced at the low-frequencies or at the edge-frequency dependent on severity of hearing loss at the high frequencies. The firing rate temporal profile changed from a broad plateau to one sharp, delayed peak. In the AC, tone-evoked firing rates were depressed at high frequencies and enhanced at the low frequencies while the firing rate temporal profiles became substantially broader. In contrast, firing rates in the Str were generally decreased and firing rate temporal profiles become more phasic and less prolonged. The altered firing rate and pattern at low frequencies induced by high-frequency hearing loss could have perceptual consequences. The tone-evoked hyperactivity in low-frequency MUC could manifest as hyperacusis whereas the discharge pattern changes could affect temporal resolution and integration.

Perception of across-frequency asynchrony by listeners with cochlear hearing loss

Cochlear hearing loss is often associated with broader tuning of the cochlear filters. Cochlear response latencies are dependent on the filter bandwidths, so hearing loss may affect the relationship between latencies across different characteristic frequencies. This prediction was tested by investigating the perception of synchrony between two tones exciting different regions of the cochlea in listeners with hearing loss. Subjective judgments of synchrony were compared with thresholds for asynchrony discrimination in a three-alternative forced-choice task. In contrast to earlier data from normal-hearing (NH) listeners, the synchronous-response functions obtained from the hearing-impaired (HI) listeners differed in patterns of symmetry and often had a very low peak (i.e., maximum proportion of "synchronous" responses). Also in contrast to data from NH listeners, the quantitative and qualitative correspondence between the data from the subjective and the forced-choice tasks was often poor. The results do not provide strong evidence for the influence of changes in cochlear mechanics on the perception of synchrony in HI listeners, and it remains possible that age, independent of hearing loss, plays an important role in temporal synchrony and asynchrony perception.

Auditory brainstem responses predict auditory nerve fiber thresholds and frequency selectivity in hearing impaired chinchillas

Noninvasive auditory brainstem responses (ABRs) are commonly used to assess cochlear pathology in both clinical and research environments. In the current study, we evaluated the relationship between ABR characteristics and more direct measures of cochlear function. We recorded ABRs and auditory nerve (AN) single-unit responses in seven chinchillas with noise-induced hearing loss. ABRs were recorded for 1-8 kHz tone burst stimuli both before and several weeks after 4 h of exposure to a 115 dB SPL, 50 Hz band of noise with a center frequency of 2 kHz. Shifts in ABR characteristics (threshold, wave I amplitude, and wave I latency) following hearing loss were compared to AN-fiber tuning curve properties (threshold and frequency selectivity) in the same animals. As expected, noise exposure generally resulted in an increase in ABR threshold and decrease in wave I amplitude at equal SPL. Wave I amplitude at equal sensation level (SL), however, was similar before and after noise exposure. In addition, noise exposure resulted in decreases in ABR wave I latency at equal SL and, to a lesser extent, at equal SPL. The shifts in ABR characteristics were significantly related to AN-fiber tuning curve properties in the same animal at the same frequency. Larger shifts in ABR thresholds and ABR wave I amplitude at equal SPL were associated with greater AN threshold elevation. Larger reductions in ABR wave I latency at equal SL, on the other hand, were associated with greater loss of AN frequency selectivity. This result is consistent with linear systems theory, which predicts shorter time delays for broader peripheral frequency tuning. Taken together with other studies, our results affirm that ABR thresholds and wave I amplitude provide useful estimates of cochlear sensitivity. Furthermore, comparisons of ABR wave I latency to normative data at the same SL may prove useful for detecting and characterizing loss of cochlear frequency selectivity.

Noise-induced hearing loss alters the temporal dynamics of auditory-nerve responses

Auditory-nerve fibers demonstrate dynamic response properties in that they adapt to rapid changes in sound level, both at the onset and offset of a sound. These dynamic response properties affect temporal coding of stimulus modulations that are perceptually relevant for many sounds such as speech and music. Temporal dynamics have been well characterized in auditory-nerve fibers from normal-hearing animals, but little is known about the effects of sensorineural hearing loss on these dynamics. This study examined the effects of noise-induced hearing loss on the temporal dynamics in auditory-nerve fiber responses from anesthetized chinchillas. Post-stimulus-time histograms were computed from responses to 50-ms tones presented at characteristic frequency and 30 dB above fiber threshold. Several response metrics related to temporal dynamics were computed from post-stimulus-time histograms and were compared between normal-hearing and noise-exposed animals. Results indicate that noise-exposed auditory-nerve fibers show significantly reduced response latency, increased onset response and percent adaptation, faster adaptation after onset, and slower recovery after offset. The decrease in response latency only occurred in noise-exposed fibers with significantly reduced frequency selectivity. These changes in temporal dynamics have important implications for temporal envelope coding in hearing-impaired ears, as well as for the design of dynamic compression algorithms for hearing aids.

Follow-up of latency and threshold shifts of auditory brainstem responses after single and interrupted acoustic trauma in guinea pig

Thresholds of auditory brainstem responses (ABRs) are widely used to estimate the level of noise-induced hearing loss or the level of acquired resistance to acoustic trauma after repeated exposures, i.e., the "toughening" effect. Less is known about ABR latencies and their relation to threshold changes. Guinea pigs were exposed to a traumatic pure tone at 5 kHz, 120 dB SPL, as either single (2 h, 4 h) or repeated (1 h every 48 h, four times) sessions. Thresholds and latencies of ABRs were monitored up to 45 days following the acoustic trauma. We show that latencies are prolonged in the case of large temporary threshold shifts observed in the days following trauma. The latency shift decreases after several repeated exposures, then stabilizes, similar to thresholds, suggesting that the "toughening" effect also applies to latencies. Permanent latency shift is usually very small compared to the permanent threshold shift. This effect could produce a recovery in the ability to process auditory information through the precise timing of neuronal events. Our study indicates that when estimated at suprathreshold stimulation level (70 dB SPL), latency provides complementary information to the sole threshold.

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.

Mechanics of the mammalian cochlea

In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the "base" of the cochlea (near the stapes) and low-frequency waves approaching the "apex" of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the "cochlear amplifier." This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.

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.

Basilar-membrane responses to clicks at the base of the chinchilla cochlea

Basilar-membrane responses to clicks were measured, using laser velocimetry, at a site of the chinchilla cochlea located about 3.5 mm from the oval window (characteristic frequency or CF: typically 8-10 kHz). They consisted of relatively undamped oscillations with instantaneous frequency that increased rapidly (time constant: 200 microseconds) from a few kHz to CF. Such frequency modulation was evident regardless of stimulus level and was also present post-mortem. Responses grew linearly at low stimulus levels, but exhibited a compressive nonlinearity at higher levels. Velocity-intensity functions were almost linear near response onset but became nonlinear within 100 microseconds. Slopes could be as low as 0.1-0.2 dB/dB at later times. Hence, the response envelopes became increasingly skewed at higher stimulus levels, with their center of gravity shifting to earlier times. The phases of near-CF response components changed by nearly 180 degrees as a function of time. At high stimulus levels, this generated cancellation notches and phase jumps in the frequency spectra. With increases in click level, sharpness of tuning deteriorated and the spectral maximum shifted to lower frequencies. Response phases also changed as a function of increasing stimulus intensity, exhibiting relative lags and leads at frequencies somewhat lower and higher than CF, respectively. In most respects, the magnitude and phase frequency spectra of responses to clicks closely resembled those of responses to tones. Post-mortem responses were similar to in vivo responses to very intense clicks.

Auditory-nerve fiber responses to clicks in guinea pigs with a damaged cochlea

This paper describes auditory-nerve single-fiber responses to clicks in noise-damaged cochleas. Poststimulus time histograms (PSTHs) were recorded for various click intensities and for the two click polarities. The PSTHs found in fibers with elevated thresholds are discussed in relation to the frequency threshold curves (FTCs) measured in these fibers. Five types of abnormal FTCs are distinguished. Type I is elevated as a whole, type II has an elevated (and often broadened) tip and a tail at normal level, type III has low thresholds in the tail (often hypersensitive), type IV represents a flat tuning, and type V has no tip but shows a clear appearance of the tail (often hypersensitive). The click PSTHs of abnormal fibers were compared to normal PSTHs at equal sound-pressure levels, and various abnormal trends were found corresponding to the type of FTC. PSTHs for type I have longer dominant-peak latencies and smaller amplitudes; PSTHs for type II were normal well above the fiber's threshold; PSTHs for type III revealed remarkable patterns with multiple peaks, part of them with a latency strongly varying with polarity; PSTHs for type IV showed narrow peaks and steep amplitude/intensity curves; PSTHs for type V showed a multiple peaked pattern and large amplitudes and steep amplitude/intensity curves to rarefaction polarity. The various features in the click responses were in most cases consistent with the type of FTC. The results can be used to explain deviations in whole-nerve recordings in abnormal cochleas.

Effect of unilateral partial cochlear lesions in adult cats on the representation of lesioned and unlesioned cochleas in primary auditory cortex

We examined the effect of unilateral restricted cochlear lesions in adult cats on the topographic representations ("maps") of the lesioned and unlesioned cochleas in the primary auditory cortex (AI) contralateral to the lesioned cochlea. Frequency (tonotopic) maps were derived by conventional multineuron mapping procedures in anesthetized animals. In confirmation of a study in adult guinea pigs (Robertson and Irvine [1989] J. Comp. Neurol. 282:456-471), we found that 2-11 months after the unilateral cochlear lesion the map of the lesioned cochlea in the contralateral AI was altered so that the AI region in which frequencies with lesion-induced elevations in cochlear neural sensitivity would have been represented was occupied by an enlarged representation of lesion-edge frequencies (i.e., frequencies adjacent to those with elevated cochlear neural sensitivity). Along the tonotopic axis of AI the total representation of lesion-edge frequencies could extend up to approximately 2.6 mm rostal to the area of normal representation of these frequencies. There was no topographic order within this enlarged representation. Examination of threshold sensitivity at the characteristic frequency (CF, frequency to which the neurons were most sensitive) in the reorganized regions of the map of the lesioned cochlea established that the changes in the map reflected a plastic reorganization rather than simply reflecting the residue of prelesion input. In contrast to the change in the map of the lesioned contralateral cochlea, the map of the unlesioned ipsilateral cochlea did not differ from those in normal animals. Thus, in contrast to the normal very good congruency between ipsilateral and contralateral AI maps, in the lesioned animals ipsilateral and contralateral maps differed in the region of AI in which there had been a reorganization of the map of the lesioned cochlea. Outside the region of contralateral map reorganization, ipsilateral and contralateral AI maps remained congruent within normal limits. The difference between the two maps in the region of contralateral map reorganization suggested, in light of the physiology of binaural interactions in the auditory pathway, that the cortical reorganization reflected subcortical changes. Finally, response properties of neuronal clusters within the reorganized map of the lesioned cochlea were compared to normative data with respect to threshold sensitivity at CF, the size of frequency "response areas," and response latencies. In the majority of cases, CF thresholds were similar to normative data. The frequency "response areas" were slightly less sharply tuned than normal, but not significantly. Response latencies were significantly shorter than normal in three animals and significantly longer in one animal.

Age-related changes in auditory evoked potentials of gerbils. II. Response latencies

Auditory brainstem responses (ABR) were recorded in young (6-10 month) and aged (36 month) Mongolian gerbils. Data from the young animals served as the baselines for comparison to aged animals which were categorized on the basis of ABR thresholds. Aged gerbils with normal thresholds (re young controls) had wave i and ii latencies of the ABR which were relatively normal at 1-4 kHz and slightly reduced at 8 and 16 kHz. Wave iv latencies in the aged gerbils with normal thresholds were reduced at all frequencies. Aged gerbils with 10-30 dB of hearing loss had wave i, ii, and iv latencies which were prolonged at low sound pressure levels and normal at high stimulus levels. Aged gerbils with 30 dB or greater losses had prolonged wave i, ii, and iv latencies at most levels. Slopes of latency-intensity (L/I) functions were steeper at 1-4 kHz than controls in aged subjects with hearing losses of 10 dB or greater. Slopes of L/I functions for wave iv were normal in aged subjects. The wave i-iv interval was shorter than normal in aged subjects with no hearing loss, normal in aged subjects with 10-30 dB of loss, and prolonged in subjects with greater than 30 dB of loss.

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.

Latency enhancement of the cochlear nerve compound action potential (CAP)

Forward maskers within two frequency-intensity domains are capable of decreasing (enhancing) CAP latency: one region flanks the low frequency tail, the other flanks the tip/high frequency slope regions of the latency tuning curve (TC). By contrast, amplitude enhancement typically does not flank the high frequency slope region.

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.

Single-fibre and whole-nerve responses to clicks as a function of sound intensity in the guinea pig

This paper describes a study of the intensity dependence of click-evoked responses of auditory-nerve fibres in relation to the simultaneously recorded compound action potential (CAP). Condensation and rarefaction clicks were presented to normal hearing guinea pigs over an intensity range of 60 dB. The recorded poststimulus time histograms (PSTHs) were characterized by the latency (tp), amplitude (Ap) and synchronization (Sp) of their dominant peak, parameters that are particularly important for the understanding of the CAP. For all fibres tp decreased monotonically with increasing intensity, in a continuous way for fibres with high characteristic frequency (CF greater than 3 kHz), and in discrete steps of one CF-cycle for low-CF (CF less than or equal to 3 kHz) fibres. An additional analysis of PSTH envelopes revealed that average latency shifts with intensity are similar for all CFs above 2 kHz. For all fibres Ap increased monotonically with intensity; the increase was stronger and maximum values were larger for low-CF than for high-CF fibres. A schematic model PSTH was then formulated on the basis of the experimental data. A sum of these model PSTHs from a hypothesized fibre population was convolved with an elemental unit response (Versnel et al., 1992) in order to simulate the compound action potential. Synthesized CAPs agreed with experimental CAPs in their main aspects.

Neural correlates of gap detection in auditory nerve fibers of the chinchilla

The neural correlates of gap detection were examined in a population of single auditory nerve fibers in the chinchilla. Acoustic stimuli consisted of 120 ms noise bursts (30-80 dB SPL) which contained silent intervals (gaps: 1, 2, 3, 4,5, 6 and 10 ms) at the midpoint. The neural response to the gap was quantified by the modulation index, (MAX-MIN)/AVE, which accounts for the steady state discharge rate before the gap (AVE), the minimum firing rate during the gap (MIN), and the maximum firing rate after the gap (MAX). In general, the modulation index increased as a function of gap width and stimulus level. Furthermore, there was a positive correlation between the modulation index and the characteristic frequency of the fiber. To estimate how detection could be based on the neuronal response, a criterion-free measure, analogous to d'. was calculated using z-scores obtained from the distributions of modulation index values collected before and during the gap and used to predict percent correct values for chinchilla psychophysical studies. The values increased with gap duration in a sigmoidal manner much like the psychometric functions in the chinchilla. In general, the neural gap thresholds obtained approximated those obtained psychophysically, although they were less affected by stimulus level.

Noise and hypoxia induced temporary threshold shifts in rats studied by ABR

Rats were exposed for 2 h either to 115 dB SPL noise, to 5% oxygen in nitrogen (hypoxia) or to both hypoxia and noise. Auditory nerve-brainstem evoked responses (ABR) to 80 dB HL clicks and threshold were recorded prior to exposure, immediately (5-10 min) after the exposure, 2 h after and 2 weeks after the exposure. The findings in each experimental animal were compared to those in the control (non-exposed) group and to those in the other groups. Thresholds were elevated in each of the experimental groups, but these were temporary threshold shifts, since 2 weeks following the exposure, threshold had returned to normal. Latencies (wave I and the IV-I interpeak latency difference- (IPLD] were prolonged in the groups exposed to hypoxia (hypoxia alone and hypoxia + noise). These results are discussed in view of possible mechanisms of these noise and hypoxia induced temporary threshold shifts.

Electrophysiology of the auditory system

This review has attempted to summarise the properties of electro physiological responses in the auditory system. The treatment was broad and consequently somewhat sketchy. For a more detailed recent treatment of the physiology of the auditory system the reader is referred to Pickles (1982), Møller (1983), or Altschuller et al (1986). The data on acoustic injury have been reviewed recently by Schmiedt (1984). Discussions of a number of topics such as development, hair cell function and speech encoding are found in Berlin (1984).

N1 latency prolongation in the guinea pig cochlea treated with nitrogen mustard-N-oxide studied by narrow band analysis

The effect of nitrogen mustard-N-oxide (NMO) upon the click and tone burst-evoked N1 latency was examined in 14 albino guinea pigs. In all animals except one, the pseudothresholds of action potentials were elevated, especially in the high tone area. In addition to the amplitude reduction, the N1 latency was prolonged in 12 animals. The narrow band analysis of N1 revealed that the latency was equally prolonged in all frequency areas, although the amount of the amplitude reduction was much larger in the high frequency area. It was concluded that the prolongation of the N1 latency in NMO-treated animals was due to dysfunction of outer hair cells along the entire cochlear partition.

Considerations in the differentiation between brainstem lesions and sensori-neural hearing loss using auditory nerve-brainstem evoked responses

When auditory nerve-brainstem evoked responses are used in neurological diagnosis, an important response parameter to be evaluated is brainstem transmission time - the time interval between the auditory nerve response (wave 1) and the response from the rostal brainstem (wave P4). Occasionally the only response wave present is wave P4 and its latency is prolonged. This can be compatible with a conductive or a sensorineural hearing loss (SNHL), or a neurological (brain stem) lesion such as a retrocochlear hearing loss (RCHL). A conductive hearing loss can be excluded by otoscopic, audiometric and tympanometric examination. In order to differentiate between SNHL and RCHL, several techniques have been suggested in order to enhance the visibility of wave 1, but these are not always applicable or successful. In this study, P4 latency was plotted as a function of the audiometric hearing loss at 4 kHz for 26 ears with SNHL only. From this, the correlation coefficient (r = 0.71) was calculated along with the linear regression equation and the standard error of the prediction. This gave an estimation of the range of P4 latency to be expected for a given SNHL for comparison with the actual latency obtained in patients with neurological complaints. An actual latency greater than the expected latency range would be considered corroboratory of a RCHL. Several clinical examples are presented. These considerations also point out the need to bear in mind the otological context of these evoked responses even when used in neurological diagnosis.

Responses from AVCN units in the cat before and after inducement of an acute noise trauma

Acoustically evoked responses of single units in the anteroventral part of the cochlear nucleus (AVCN) in the cat were studied together with compound eighth-nerve action potentials (AP). Halfway through the experiments the cats were exposed for half an hour to pink noise at 105 dB SPL, producing an average threshold shift of 30 dB (maximum 50 dB) in the 2-6 kHz region. The effect of noise exposure was studied in two ways. On the one hand we compared the results for one population of units measured before the noise exposure with those found for another population measured afterwards. On the other hand we compared the results before and after the noise exposure for one unit that could be kept under observation during the noise exposure. After the noise exposure spontaneous activity and phase-locking of the responses of the units to the stimulus waveform were not significantly different from the pre-exposure findings. Response latency tended to increase. Therefore, the decrease of latency found for APs must be due to a shift to higher frequencies of the population of units contributing to the AP. The sharply tuned tip segments of tuning curves shift to higher levels whereas the low-frequency tails remain at about the same level. Q10 decreases by at most 50%, which was also found for AP tuning curves. Response spectra for clicks and noise (reverse correlation function) did not show a significant decrease of frequency selectivity. Units with CF less than 3 kHz may show a shift of CF to a lower frequency by 10-20%. After inducement of the noise trauma the sharply tuned tip segment of a tuning curve may not be found and CF may be assigned to a local minimum in the low-frequency tail of the tuning curve.

N1 latency following acute pure-tone trauma

The latency of the N1 component of tone burst evoked compound action potentials was examined in chinchilla following acute pure-tone trauma. At and below the trauma frequency (4 kHz) the N1 latency at threshold generally increased, while above the trauma frequency it decreased; tonotopically paralleling pitch shifts observed in humans following pure-tone trauma. When N1 latency at threshold is considered across animals as a linear function of dB SPL at threshold, after trauma a high degree of linear correlation was found at 6 and 8 kHz, while a low degree of linear correlation was found at 4 kHz. An interpretation and the significance of the data are discussed.

Response patterns of auditory nerve fibers during temporary threshold shift

Temporary threshold shifts were studied in chinchillas exposed to noise (octave-band noise centered at 500 Hz, 95 dB SPL, 5 days duration) and the response properties of their auditory nerve fibers were measured. The threshold shifts of the fibers were approximately 35 to 65 dB; these values were equal to or slightly greater than those measured behaviorally. Most units had broad V-shaped tuning curves due to a greater loss in sensitivity near the characteristic frequency (CF) than in the low-frequency tail. In 17% of the units, the thresholds were actually lower in the tail than at CF, so that the tuning curves were W-shaped. The latencies of the fibers were within normal limits in terms of absolute intensity, but shorter than normal in terms of intensity relative to threshold. Other measures such as the spontaneous discharge rate, the discharge rate-intensity functions, and the firing patterns to tone bursts at CF appeared normal. These results indicate that neural response patterns during noise-induced temporary threshold shift are similar to those measured during permanent threshold shift.

Hyperthermia increases aminoglycoside ototoxicity

Similarities were noted between the nature of inner ear damage produced by loud sounds and by aminoglycoside antibiotics. Since body temperature affects cochlear function and influences the effects of noise on the ear, a similar effect was predicted for the aminoglycoside, kanamycin. By environmentally elevating the body temperatures of preweanling mice to approximately 1 degree C above that of the normal adult, kanamycin ototoxicity was increased (an average 20.4 dB threshold elevation, vs. 9.3 dB for kanamycin injected mice reared at room temperature). Hyperthermia per se had no influence on auditory thresholds. This may be of relevance to humans with fever who are treated with aminoglycoside antibiotics.

Noise impairment in the guinea pig. I. Changes in electrical evoked activity along the auditory pathway

Changes in the cochlear microphonics (CM), auditory nerve action potential (AP), and evoked responses from the inferior colliculus (IC-ER) and auditory cortex (AC-ER) of the guinea pig were assessed after exposure to white noise of 115 dB for 30 min. Both continuous and intermittent (200 ms noise and 200 ms pause) exposures were used. In comparison with the pre-exposure level, CM isopotential curves were shifted by 1.1 +/- 0.5 dB (means +/- S.E.) on the average in the range of 0.5-8 kHz (recorded at the round window). The amplitude-intensity function of the click-evoked auditory nerve action potential decreased by 8.4 +/- 1.2 dB, that of the inferior colliculus evoked response by 20.9 +/- 3.7 dB, and the amplitude-intensity function of the auditory cortex evoked potential decreased by 6.2 +/- 4.7 dB. A similar reduction in the amplitude was found after both continuous and intermittent noise exposure. In contrast to the decrease in amplitudes of evoked potentials, the latency-intensity functions of the individual waves of potentials evoked along the auditory pathway did not change when compared at the same click intensity before and after the exposure. The results suggest that individual auditory nuclei are impaired by the noise to different extents and that the impairment does not increase linearly up to the auditory cortex.

Age-related changes in sensitivity of the postpubertal ear to acoustic trauma

Although the age of the postpubertal mammal is typically ignored in evaluating the damaging effects of noise on the ear, it was shown to account for over 65% of the variability in the mouse. 44 inbred CBA/J mice, ranging in age from 60 (early postpuberty) to 360 days (late middle age), were tested for cochlear AP thresholds at frequencies from 2 to 64 kHz. They were then subjected to 5 min of a 124 dB octave band (12-24 kHz) noise. Although all the mice had similar pre-exposure thresholds, the extent of noise-induced AP threshold elevation and the frequencies most severely affected depended upon the ages of the mice. The youngest subjects had the greatest threshold elevations, being most pronounced at, and 1 octave above, the center frequency of the noise exposure. With increasing age there was a progressively less severe effect. The oldest subjects had cochlear AP threshold elevations which were restricted to a frequency 2 octaves above the exposure frequency.

Noise impairment in the guinea pig. II. Changes of single unit responses in the inferior colliculus

Spontaneous and evoked activity of neurons in the inferior colliculus of guinea pigs was recorded before and after exposure to noise (continuous or intermittent white noise, 115 dB SPL for 30 min). A single unit was investigated in each animal, and its activity was monitored for several hours. Exposure to noise elevated the threshold of the tip of the tuning curve, resulting in a broadening of the tuning curve. Threshold elevation at the characteristic frequency was greater after exposure to intermittent noise (200 ms noise and 200 ms pause), reaching values of 22.8 +/- 3.7 dB (means +/- S.E.) than it was after exposure to continuous noise (threshold elevation of 13.1 +/- 1.7 dB). The average threshold shift was 17.1 +/- 2 dB. Neither the shape of the poststimulus histograms nor the slope of the spike-intensity curves changed with the noise exposure. The total number of spikes during the response was, however, reduced, and the reduction was in proportion to the threshold elevation. Monaural noise exposure had no effect on the neuronal activity evoked by stimulation of the opposite, nonexposed ear. The latencies of responses recorded after exposure to noise were also longer than the latencies at the same absolute intensity recorded before the exposure. Thus the latencies during the original pre-exposure and acquired postexposure thresholds were practically identical.

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.

Eighth-nerve action potentials evoked by tone bursts in cats before and after inducement of an acute noise trauma

Properties of eighth-nerve action potentials (AP) evoked by single-frequency tone-bursts (test tone) were studied in cats. Curves representing AP threshold as a function of test-tone frequency have a shape similar to behavioral and single-fiber threshold curves. The absolute level of AP thresholds is higher than that of behavioral and single-fiber thresholds. Cats were exposed to broad-band noise (equal intensities per octave) halfway into the experiments. This exposure resulted in a long-term temporary threshold shift (TTS) which remained fairly steady during the measurements. APs were measured before and during an acute noise trauma in the same animal. After inducement of the trauma the greatest threshold shift is found between 2 and 6 kHz. Curves representing AP amplitude as a function of stimulus SPL are displaced to higher stimulus SPLs. Sometimes the slope of the curve is steeper after the noise exposure than before. AP latency at threshold did not change due to the excessive noise exposure. AP-latency values compared at equal sound pressure levels before and after inducement of the trauma showed higher values during the trauma than before.

The latency of auditory nerve-brainstem responses in sensorineural hearing loss

The use of auditory nerve-brainstem responses in differential diagnosis of hearing loss is based on several properties of these responses including response latency. The auditory nerve response latency has been shown to be prolonged in conductive hearing loss. The latency of the brainstem responses is also often prolonged in retrocochlear hearing loss. However, the effect of sensorineural hearing losses on auditory nerve response latency is not clear. Several authors report that response latency is prolonged in sensorineural loss, whereas others claim that it is unchanged. To study this, auditory nerve-brainstem responses to 75 dB HL clicks were recorded in normal-hearing subjects and in those with various degrees of high-frequency sensorineural hearing loss. In the more extreme hearing losses, the auditory nerve response could not be seen in the response trace, so the latency of the earlobe positive wave from the region of the inferior colliculus was considered as mirroring auditory nerve response latency, since the time interval between these two waves has been shown to be constant. The average latency of the more severe hearing loss group (more than 40 dB hearing loss at 4kHz) was found to be only 0.35 ms longer than that of the normal-hearing group. This value is smaller than that seen in most conductive and retrocochlear hearing losses. This result warrants continued use of prolonged auditory nerve response latency (greater than 0.35 ms) as an indicator of conductive hearing loss. Possible explanations for smaller latency prolongation than expected of the auditory nerve response in sensorineural hearing loss are discussed based on the properties of single auditory nerve fibers.

Hearing threshold shifts from prolonged exposure to noise in guinea pigs

Auditory thresholds were assessed in three guinea pigs with a conditioning procedure based on the positive reinforcement paradigm. Thereafter, the guinea pigs were exposed for 5 days to third octave band noise centred at 2 kHz at 100 dB SPL. Thresholds at 0.5, 2 and 4 kHz were controlled during the exposure and up to 120 days after exposure. After 6 h of exposure, the threshold shift at 4 kHz reached 40 dB and increased slowly to 45 dB by the fifth day. The quasi-asymptotic shift was less expressed at 2 kHz, where the initial threshold shift amounted to 20 dB and rose to 40 dB by the fifth day. Thresholds recovered in the time course of 5 days after exposure and a significant permanent threshold shift was present 120 days after exposure. It amounted to 35 dB at 4 kHz and to 20 dB at 2 kHz. Auditory thresholds were dependent upon the duration of the stimulus: the decrease in duration of a tone from 200 ms to 2 ms caused a rise in threshold of about 10 dB before as well as after the exposure. The effects of prolonged noise exposure upon guinea pigs are similar to those found in chinchillas with the exception of the permanent threshold shift, which is more marked in guinea pigs.

VIII nerve response to click stimuli in normal and pathological cochleas

A flat 30--50 dB hearing loss was established in chinchillas following a 5 day exposure to an octave band of noise (354--708 Hz, 95 dB SPL). After exposure, single auditory nerve fiber recordings were obtained using click and tone burst stimuli. The thresholds of units from the noise-treated animals were elevated 30--70 dB and the tuning curves were abnormally broad. At the threshold for click stimulation, the fiber latencies were shorter in the noise-treated animals than those in normal animals. However, the latencies for the two groups were similar when stimulated at the same intensities. As indicated by the number of peaks in the PST histograms obtained with clicks, the units from the noise-treated animals showed considerably more damping in the neural response than those from normal units. The temporal spacing between the peaks in the histograms for units of similar CF was the same in the normal and noise-treated groups, although this cannot be taken to infer that an individual unit PST histogram would remain the same after noise exposure as before. These limited neural data therefore show changes in the same direction as those in the transient mechanical response of the basilar membrane reported by Robles et al.