The sharpening of cochlear frequency selectivity in the normal and abnormal cochlea

Usefulness of phase gradients of otoacoustic emissions in auditory health screening: An exploration with swept tones

Otoacoustic emissions (OAEs) are low-level sounds generated by the cochlea and widely used as a noninvasive tool to inspect cochlear impairments. However, only the amplitude information of OAE signals is used in current clinical tests, while the OAE phase containing important information about cochlear functions is commonly discarded, due to the insufficient frequency-resolution of existing OAE tests. In this study, swept tones with time-varying frequencies were used to measure stimulus frequency OAEs (SFOAEs) in human subjects, so that high-resolution phase spectra that are not available in existing OAE tests could be obtained and analyzed. The results showed that the phase of swept-tone SFOAEs demonstrated steep gradients as the frequency increased in human subjects with normal hearing. The steep phase gradients were sensitive to auditory functional abnormality caused by cochlear damage and stimulus artifacts introduced by system distortions. At low stimulus levels, the group delays derived from the phase gradients decreased from around 8.5 to 3 ms as the frequency increased from 1 to 10 kHz for subjects with normal hearing, and the pattern of group-delay versus frequency function showed significant difference for subjects with hearing loss. By using the swept-tone technology, the study suggests that the OAE phase gradients could provide highly sensitive information about the cochlear functions and therefore should be integrated into the conventional methods to improve the reliability of auditory health screening.

Acoustic Reflexes in Individuals Having Hyperacusis of the Auditory Origin

Functional role of the acoustic reflex in preventing over stimulation of the inner auditory system by decreasing sound intensity along with the previous reports of acoustic reflex abnormalities in individuals having hyperacusis point towards the involvement of acoustic reflex deficit in the origin of hyperacusis especially when any medical condition leading to hyperacusis is not associated. However this issue remains contradictory owing to limited comprehensive investigation. This study was undertaken to ascertain the relationship between hyperacusis and the acoustic reflex. Threshold, amplitude and latency of the acoustic reflex were measured in two different groups of individuals having hyperacusis; Group 1: 14 individuals having hyperacusis with hearing loss (HwHL) and Group 2: 17 individuals having hyperacusis without hearing loss (HwoHL). Control group (Group 3) consisted of 15 normal hearing individuals who never experienced hyperacusis. Result showed a significant group effect on all the measured characteristics of the acoustic reflex. ARTs were found to be significantly higher in HwHL and HwoHL when compared to NHwoH. ARTs were statistically similar for HwoHL and NHwoH. HwoHL's ARAs and ARLs were significantly smaller and prolonged, respectively, when compared to HwHL and NHwoH. HwHL and NHwoH had statistically similar ARAs and ARLs. This study confirms acoustic reflex abnormalities in some individuals having hyperacusis with or without hearing loss. It further highlight the importance of involving acoustic reflex testing in the assessment of hyperacusis especially when hyperacusis is not associated with hearing loss or any other medical condition that may lead to hyperacusis.

How Do Age and Hearing Loss Impact Spectral Envelope Perception?

Purpose

The goal was to evaluate the potential effects of increasing hearing loss and advancing age on spectral envelope perception.

Method

Spectral modulation detection was measured as a function of spectral modulation frequency from 0.5 to 8.0 cycles/octave. The spectral modulation task involved discrimination of a noise carrier (3 octaves wide from 400 to 3200 Hz) with a flat spectral envelope from a noise having a sinusoidal spectral envelope across a logarithmic audio frequency scale. Spectral modulation transfer functions (SMTFs; modulation threshold vs. modulation frequency) were computed and compared 4 listener groups: young normal hearing, older normal hearing, older with mild hearing loss, and older with moderate hearing loss. Estimates of the internal spectral contrast were obtained by computing excitation patterns.

Results

SMTFs for young listeners with normal hearing were bandpass with a minimum modulation detection threshold at 2 cycles/octave, and older listeners with normal hearing were remarkably similar to those of the young listeners. SMTFs for older listeners with mild and moderate hearing loss had a low-pass rather than a bandpass shape. Excitation patterns revealed that limited spectral resolution dictated modulation detection thresholds at high but not low spectral modulation frequencies. Even when factoring out (presumed) differences in frequency resolution among groups, the spectral envelope perception was worse for the group with moderate hearing loss than the other 3 groups.

Conclusions

The spectral envelope perception as measured by spectral modulation detection thresholds is compromised by hearing loss at higher spectral modulation frequencies, consistent with predictions of reduced spectral resolution known to accompany sensorineural hearing loss. Spectral envelope perception is not negatively impacted by advancing age at any spectral modulation frequency between 0.5 and 8.0 cycles/octave.

Tinnitus and patterns of hearing loss

Tinnitus is strongly linked with the presence of damaged hearing. However, it is not known why tinnitus afflicts only some, and not all, hearing-impaired listeners. One possibility is that tinnitus patients have specific inner ear damage that triggers tinnitus. In this study, differences in cochlear function inferred from psychophysical measures were measured between hearing-impaired listeners with tinnitus and hearing-impaired listeners without tinnitus. Despite having similar average hearing loss, tinnitus patients were observed to have better frequency selectivity and compression than those without tinnitus. The results suggest that the presence of subjective tinnitus may not be strongly associated to outer hair cell impairment, at least where hearing impairment is evident. The results also show a different average pattern of hearing impairment amongst the tinnitus patients, consistent with the suggestion that inner hair cell dysfunction with subsequent reduced auditory innervation is a possible trigger of tinnitus.

The prevention of noise induced hearing loss in children

Increasingly, our acoustic environment is filled with amplified sound sources (e.g., MP3 players, video game stations, and sports/entertainment venues). There is serious concern and also some controversy about the risks of acoustic trauma in children. This overview provides some basic information on the physiological mechanisms that lead to noise induced hearing loss, a survey of various studies that, on balance, indicates that there is cause for concern, and finally a discussion on measures that can help to prevent noise induced hearing loss in children. This paper is designed for public health and other healthcare professions (ENT, audiologists, family doctors, and pediatricians) who should understand the risks of noise induced hearing loss and its prevention.

Tuning of the tectorial membrane in the basilar papilla of the northern leopard frog

The basilar papilla (BP) in the frog inner ear is a relatively simple auditory receptor. Its hair cells are embedded in a stiff support structure, with the stereovilli connecting to a flexible tectorial membrane (TM). Acoustic energy passing the papilla presumably causes displacement of the TM, which in turn deflects the stereovilli and stimulates the hair cells. Auditory neurons that contact the BP’s hair cells are known to have nearly identical characteristic frequencies and frequency selectivity. In this paper, we present optical measurements of the mechanical response of the TM. Results were obtained from five specimens. The TM displacement was essentially in phase across the membrane, with the largest amplitudes occurring near the hair cells. The response was tuned to a frequency near 2 kHz. The phase accumulated over at least 270° across the measured frequencies. The tuning quality Q_10dB values were calculated; the average Q_10dB was 2.0 ± 0.8 (standard deviation). Our results are comparable to those of neural-tuning curves in the same and a similar species. Also, they are in agreement with the response of an associated structure—the contact membrane—in a closely related species. Our data provides evidence for a mechanical basis for the frequency selectivity of the frog’s BP.

Neural correlates of age-related declines in frequency selectivity in the auditory midbrain

Reduced frequency selectivity is associated with an age-related decline in speech recognition in background noise and reverberant environments. To elucidate neural correlates of age-related alteration in frequency selectivity, the present study examined frequency response areas (FRAs) of multi-unit clusters in the inferior colliculus of young, middle-aged, and old CBA/CaJ mice. The FRAs in middle-aged and old mice were found to be broader and more asymmetric in shape. In addition to a decrease of closed/complex FRAs in both middle age and old groups, there was a transient decrease in V-shaped FRAs and a concomitant increase in multipeak FRAs in middle age. Intensity coding was also affected by age, as observed in an increase of monotonic responses in middle-aged and old mice. While a decline in low-level activity began in middle age, reduced driven rates at suprathreshold levels occurred later in old age. Collectively, these results support the view that aging alters frequency selectivity by widening excitatory FRAs and that these changes begin to appear in middle age.

Loudness perception in the domestic cat: reaction time estimates of equal loudness contours and recruitment effects

The domestic cat is the primary physiological model of loudness coding and recruitment. At present, there are no published descriptions of loudness perception in this species. This study used a reaction time task to characterize loudness perception in six behaviorally trained cats. The psychophysical approach was based on the assumption that sounds of equal loudness elicit responses of equal latency. The resulting equal latency contours reproduced well-known features of human equal loudness contours. At the completion of normal baseline measures, the cats were exposed to intense sound to investigate the behavioral correlates of loudness recruitment, the abnormally rapid growth of loudness that is commonly associated with hearing loss. Observed recruitment effects were similar in magnitude to those that have been reported in hearing-impaired humans. Linear hearing aid amplification is known to improve speech intelligibility but also exacerbate recruitment in impaired listeners. The effects of speech spectra and amplification on recruitment were explored by measuring the growth of loudness for natural and amplified vowels before and after sound exposure. Vowels produced more recruitment than tones, and the effect was exacerbated by the selective amplification of formant structure. These findings support the adequacy of the domestic cat as a model system for future investigations of the auditory processes that underlie loudness perception, recruitment, and hearing aid design.

Encoding intensity in ventral cochlear nucleus following acoustic trauma: implications for loudness recruitment

Loudness recruitment, an abnormally rapid growth of perceived loudness with sound level, is a common symptom of sensorineural hearing loss. Following acoustic trauma, auditory-nerve rate responses are reduced, and rate grows more slowly with sound level, which seems inconsistent with recruitment (Heinz et al., J. Assoc. Res. Otolaryngol. 6:91-105, 2005). However, rate-level functions (RLFs) in the central nervous system may increase in either slope or saturation value following trauma (e.g., Salvi et al., Hear. Res. 147:261-274, 2000), suggesting that recruitment may arise from central changes. In this paper, we studied RLFs of neurons in ventral cochlear nucleus (VCN) of the cat after acoustic trauma. Trauma did not change the general properties of VCN neurons, and the usual VCN functional classifications remained valid (chopper, primary-like, onset, etc.). After trauma, non-primary-like neurons, most noticeably choppers, exhibited elevated maximum discharge rates and steeper RLFs for frequencies at and near best frequency (BF). Primary-like neurons showed the opposite changes. To relate the neurons' responses to recruitment, rate-balance functions were computed; these show the sound level required to give equal rates in a normal and a traumatized ear and are analogous to loudness balance functions that show the sound levels giving equal perceptual loudness in the two ears of a monaurally hearing-impaired person. The rate-balance functions showed recruitment-like steepening of their slopes in non-primary-like neurons in all conditions. However, primary-like neurons showed recruitment-like behavior only when rates were summated across neurons of all BFs. These results suggest that the non-primary-like, especially chopper, neurons may be the most peripheral site of the physiological changes in the brain that underlie recruitment.

Spectral modulation masking patterns reveal tuning to spectral envelope frequency

Auditory processing appears to include a series of domain-specific filtering operations that include tuning in the audio-frequency domain, followed by tuning in the temporal modulation domain, and perhaps tuning in the spectral modulation domain. To explore the possibility of tuning in the spectral modulation domain, a masking experiment was designed to measure masking patterns in the spectral modulation domain. Spectral modulation transfer functions (SMTFs) were measured for modulation frequencies from 0.25 to 14 cycles/octave superimposed on noise carriers either one octave (800-1600 Hz, 6400-12,800 Hz) or six octaves wide (200-12,800 Hz). The resulting SMTFs showed maximum sensitivity to modulation between 1 and 3 cycles/octave with reduced sensitivity above and below this region. Masked spectral modulation detection thresholds were measured for masker modulation frequencies of 1, 3, and 5 cycles/octave with a fixed modulation depth of 15 dB. The masking patterns obtained for each masker frequency and carrier band revealed tuning (maximum masking) near the masker frequency, which is consistent with the theory that spectral envelope perception is governed by a series of spectral modulation channels tuned to different spectral modulation frequencies.

Identifying dead regions in the cochlea through the TEN Test

An audiogram is not sufficient to indicate cochlear dead regions.

Aim

To investigate cochlear dead regions in sensorineural hearing loss subjects using the TEN test. Site: CEDALVI/ HRAC-USP-Bauru/Sao Paulo/Brazil, August 2003 to February 2004.

Study Design

A contemporary cross-sectional cohort study. Material and Methods: The TEN test was applied in three groups: G1(5 women with pure-tone thresholds within normal limits); G2(4 women and 5 men with moderate sensorineural flat hearing loss); G3(19 women and 24 men with mild to severe sloping sensorineural hearing loss).

Results

In the G1 group the TEN value required to eliminate the test tone was, on average, close to the absolute threshold for all frequencies. No dead regions were found in the ears tested in group G2. 76 ears were tested in group G3, and six showed no evidence of dead regions in the cochlea.

Conclusions

The TEN test was an effective test to indicate a dead region in the cochlea of subjects with sloping sensorineural hearing loss. There is evidence that pure-tone detection is different for subjects with high frequency sensorineural hearing loss and flat hearing loss; we observed a significant difference between the masked threshold and the absolute threshold only in sloping hearing loss and not for flat hearing loss.

Auditory-nerve rate responses are inconsistent with common hypotheses for the neural correlates of loudness recruitment

A number of perceptual phenomena related to normal and impaired level coding can be accounted for by the degree of compression in the basilar-membrane (BM) magnitude response. However, the narrow dynamic ranges of auditory-nerve (AN) fibers complicate these arguments. Because the AN serves as an information bottleneck, an improved understanding of the neural coding of level may clarify some of the limitations of current hearing aids. Here three hypotheses for the neural correlate of loudness recruitment were evaluated based on AN responses from normal-hearing cats and from cats with a noise-induced hearing loss (NIHL). Auditory-nerve fiber rate-level functions for tones were analyzed to test the following hypotheses: Loudness recruitment results from steeper AN rate functions after impairment. This hypothesis was not supported; AN rate functions were not steeper than normal following NIHL, despite steeper estimated BM responses based on the AN data. Loudness is based on the total AN discharge count, and recruitment results from an abnormally rapid spread of excitation after impairment. Whereas abnormal spread of excitation can be observed, steeper growth of total AN rate is not seen over the range of sound levels where recruitment is observed in human listeners. Loudness of a narrowband stimulus is based on AN responses in a narrow BF region, and recruitment results from compression of the AN-fiber threshold distribution after impairment. This hypothesis was not supported because there was no evidence that impaired AN threshold distributions were compressed and the growth of AN activity summed across BFs near the stimulus frequency was shallower than normal.Overall, these results suggest that loudness recruitment cannot be accounted for based on summed AN rate responses and may depend on neural mechanisms involved in the central representation of intensity.

The Allen-Fahey experiment extended

An ingenious experiment has been performed by Allen and Fahey [J. Acoust. Soc. Am. 92, 178-188 (1992)], in which they attempted to estimate the gain of the cochlear amplifier by comparing responses to the 2 f1-f2 distortion product (DP) in the outer ear canal (otoacoustic emissions) and from an auditory-nerve fiber. Results were essentially negative: no evidence of cochlear amplification was found in that experiment. A variation of that experiment is reported here, where DP responses in the outer ear canal are compared with mechanical responses of the basilar membrane. This variation does not suffer from the major limitation in the original experiment in the choice of possible frequency ratios. Results confirm and extend those of Allen and Fahey entirely. Apparently, the gain of the cochlear amplifier cannot be measured in this way. It is argued that the retrograde wave going to the stapes is most likely reduced in magnitude by wave interference when the two primary frequencies approach each other. Such a reduction does not take place in the forward-going wave to the location tuned to the DP frequency. This explanation is illustrated on the basis of results of earlier experiments on the movements of the basilar membrane.

An ENU-induced mutation in AP-2α leads to middle earand ocular defects in Doarad mice

One of the advantages of N-ethyl- N-nitrosourea (ENU)-induced mutagenesis is that, after randomly causing point mutations, a variety of alleles can be generated in genes leading to diverse phenotypes. For example, transcription factor AP-2alpha ( Tcfap2a) null homozygote mice show a large spectrum of developmental defects, among them missing middle ear bones and tympanic ring. This is the usual occurrence, where mutations causing middle ear anomalies usually coincide with other abnormalities. Using ENU-induced mutagenesis, we discovered a new dominant Tcfap2a mutant named Doarad ( Dor) that has a missense mutation in the PY motif of its transactivation domain, leading to a misshapen malleus, incus, and stapes without any other observable phenotype. Dor homozygous mice die perinatally, showing prominent abnormal facial structures and ocular defects. In vitro assays suggest that this mutation causes a "gain of function" in the transcriptional activation of AP-2alpha. These mice enable us to address more specifically the developmental role of Tcfap2a in the eye and middle ear and are the first report of a mutation in a gene specifically causing middle ear abnormalities, leading to conductive hearing loss.

Response growth with sound level in auditory-nerve fibers after noise-induced hearing loss

People with sensorineural hearing loss are often constrained by a reduced acoustic dynamic range associated with loudness recruitment; however, the neural correlates of loudness and recruitment are still not well understood. The growth of auditory-nerve (AN) activity with sound level was compared in normal-hearing cats and in cats with a noise-induced hearing loss to test the hypothesis that AN-fiber rate-level functions are steeper in impaired ears. Stimuli included best-frequency and fixed-frequency tones, broadband noise, and a brief speech token. Three types of impaired responses were observed. 1) Fibers with rate-level functions that were similar across all stimuli typically had broad tuning, consistent with outer-hair-cell (OHC) damage. 2) Fibers with a wide dynamic range and shallow slope above threshold often retained sharp tuning, consistent with primarily inner-hair-cell (IHC) damage. 3) Fibers with very steep rate-level functions for all stimuli had thresholds above approximately 80 dB SPL and very broad tuning, consistent with severe IHC and OHC damage. Impaired rate-level slopes were on average shallower than normal for tones, and were steeper in only limited conditions. There was less variation in rate-level slopes across stimuli in impaired fibers, presumably attributable to the lack of suppression-induced reductions in slopes for complex stimuli relative to BF-tone slopes. Sloping saturation was observed less often in impaired fibers. These results illustrate that AN fibers do not provide a simple representation of the basilar-membrane I/O function and suggest that both OHC and IHC damage can affect AN response growth.

The sound-level-dependent growth in the extent of fMRI activation in Heschl's gyrus is different for low- and high-frequency tones

fMRI (functional magnetic resonance imaging) was used to investigate whether the growth in activation of the human auditory cortex, with increasing sound level, is discernibly different for high- and low-frequency tones. Ten volunteers were scanned whilst listening to sequences of low-frequency (0.30-kHz) tones at sound levels between 42 and 96 dB sound pressure level (SPL), and 10 whilst listening to high-frequency (4.75-kHz) tones at the same sound levels. Activation was measured in Heschl's gyrus (including primary auditory cortex) which has been shown to be most sensitive to changes in sound level. For the 0.30-kHz tone, the extent of activation was flat up to 66 dB and then showed a rapid growth which continued up to the highest level studied (96 dB SPL). In contrast, increasing the level of 4.75-kHz tones produced a steady growth in the extent of activation across the range of levels studied. These results are consistent with physiological evidence suggesting that recruitment of primary auditory cortical neurones may be different at high and low frequencies.

Heschl's gyrus is more sensitive to tone level than non-primary auditory cortex

Previous neuroimaging studies generally demonstrate a growth in the cortical response with an increase in sound level. However, the details of the shape and topographic location of such growth remain largely unknown. One limiting methodological factor has been the relatively sparse sampling of sound intensities. Additionally, most studies have either analysed the entire auditory cortex without differentiating primary and non-primary regions or have limited their analyses to Heschl's gyrus (HG). Here, we characterise the pattern of responses to a 300-Hz tone presented in 6-dB steps from 42 to 96 dB sound pressure level as a function of its sound level, within three anatomically defined auditory areas; the primary area, on HG, and two non-primary areas, consisting of a small area lateral to the axis of HG (the anterior lateral area, ALA) and the posterior part of auditory cortex (the planum temporale, PT). Extent and magnitude of auditory activation increased non-linearly with sound level. In HG, the extent and magnitude were more sensitive to increasing level than in ALA and PT. Thus, HG appears to have a larger involvement in sound-level processing than does ALA or PT.

Electrophysiological characteristics of inferior colliculus neurons in mutant mice with hereditary absence of cochlear outer hair cells

One strain of homozygous Kit(W-v) mice (formerly known as W(v)/W(v)) lack 98% of the cochlear outer hair cells (OHCs) from birth. Inner hair cells (IHCs) and supporting cells develop normally. Thus, this strain is an attractive model to study the effect of complete OHC absence on central frequency representation. Frequency threshold curves were recorded along the tonotopic axis of inferior colliculus (IC) in mutant and control mice of the genetic background strain (C57BL/6J) and a different outbred strain (NMRI/wild mouse hybrids) known to be free of any cochlear pathology. The average threshold level of neurons in the mutants was 100 dB sound pressure level, 60 dB higher than in C57BL/6J and NMRI mice. Their tuning curves lacked the sharply tuned tip. In the C57BL/6J mice, although younger than four months, abnormal tuning curves were found for about 30% of the neurons, especially in the high frequency range. No abnormal tuning curves were found in the NMRI mice. The bandwidth of the tuning curves, measured at 10 dB above threshold, was on average 1.27 octaves in mutants, 0.62 octaves in C57BL/6J mice, and 0.34 octaves in NMRI mice. The range for the high cut-off frequency of the tuning curves at 10 dB above threshold was 6.4-61.1 kHz in the NMRI and 7-59.5 kHz in C57BL/6J. In the mutants, the range was limited to 11.1-41.7 kHz. The tonotopic gradient based on the cut-off frequency was less steep in the IC of the mutants than in both control groups.

A test for the diagnosis of dead regions in the cochlea

Hearing impairment may sometimes be associated with complete loss of inner hair cells (IHCs) over a certain region of the basilar membrane. We call this a 'dead region'. Amplification (using a hearing aid) over a frequency range corresponding to a dead region may not be beneficial and may even impair speech intelligibility. However, diagnosis of dead regions is not easily done from the audiogram. This paper reports the design and evaluation of a method for detecting and delimiting dead regions. A noise, called 'threshold equalizing noise' (TEN), was spectrally shaped so that, for normally hearing subjects, it would give equal masked thresholds for pure tone signals at all frequencies within the range 250-10,000 Hz. Its level is specified as the level in a one-ERB (132 Hz) wide band centred at 1000 Hz. Measurements obtained from 22 normal-hearing subjects and TEN levels of 30, 50 and 70 dB/ERB confirmed that the signal level at masked threshold was approximately equal to the noise level/ERB and was almost independent of signal frequency. Masked thresholds were measured for 20 ears of 14 subjects with sensorineural hearing loss, using TEN levels of 30, 50 and 70 dB/ERB. Psychophysical tuning curves (PTCs) were measured for the same subjects. When there are surviving IHCs corresponding to a frequency region with elevated absolute thresholds, a signal in that frequency region is detected via IHCs with characteristic frequencies (CFs) close to that region. In such a case, threshold in the TEN is close to that for normal-hearing listeners, provided that the noise intensity is sufficient to produce significant masking. Also, the tip of the PTC lies close to the signal frequency. When a dead region is present, the signal is detected via IHCs with CFs different from that of the signal frequency. In such a case, threshold in the TEN is markedly higher than normal, and the tip of the PTC is shifted away from the signal frequency. Generally, there was a very good correspondence between the results obtained using the TEN and the PTCs. We conclude that the measurement of masked thresholds in TEN provides a quick and simple method for the diagnosis of dead regions.

The effects of maturation and stimulus parameters on the optimal f(2)/f(1) ratio of the 2f(1)-f(2) distortion product otoacoustic emission in neonates(1)

Distortion product otoacoustic emission (DPOAE) measurements are becoming popular in the clinical realm because they have been shown to reflect cochlear function. The primary tones used to evoke the DPOAE are important in determining the amplitude of the emission recorded in the ear canal. This study examined the ratio of the primaries necessary to determine the maximum amplitude emission as a function of development, stimulus level and frequency. Optimum f(2)/f(1) ratios were measured utilizing the f(1)-sweep technique from 105 neonates between 30-42 weeks conceptional age (CA) and 40 adults. No significant difference for optimum ratio was shown between the neonatal and the adult groups. Primary tone frequency had a significant effect on optimum ratio for both neonates and adults. Low f(2) frequencies (<4 kHz) were associated with higher optimum ratios than high f(2) frequencies (>4 kHz). The adult group was used to investigate the effect of stimulus level on the optimum f(2)/f(1) ratio for f(2) frequencies from 1.7 to 10 kHz. Regression analysis showed significant differences across levels of the primaries at all frequencies except for f(2)=3.4 and 7.0 kHz. These differences in f(2)/f(1) ratio across stimulus frequency and level may be attributed to the change in the shape of the excitation profiles along the basilar membrane.

Modulation of responses and frequency tuning of thalamic and collicular neurons by cortical activation in mustached bats

In the Jamaican mustached bat, Pteronotus parnellii parnellii, one of the subdivisions of the primary auditory cortex is disproportionately large and over-represents sound at approximately 61 kHz. This area, called the Doppler-shifted constant frequency (DSCF) processing area, consists of neurons extremely sharply tuned to a sound at approximately 61 kHz. We found that a focal activation of the DSCF area evokes highly specific corticofugal modulation in the inferior colliculus and medial geniculate body. Namely a focal activation of cortical DSCF neurons tuned to, say, 61. 2 kHz with 0.2-ms-long, 100-nA electric pulses drastically increases the excitatory responses of thalamic and collicular neurons tuned to 61.2 kHz without shifting their best frequencies (BFs). However, it decreases the excitatory responses of subcortical neurons tuned to frequencies slightly higher or lower than 61.2 kHz and shifts their BFs away from 61.2 kHz. The BF shifts are symmetrical and centrifugal around 61.2 kHz. These corticofugal effects are larger on thalamic neurons than on collicular neurons. The cortical electrical stimulation sharpens the frequency-tuning curves of subcortical neurons. These corticofugal effects named "egocentric selection" last </=2.5 h after the cessation of a 7-min-long cortical electrical stimulation. In the mustached bat, corticofugal modulation serves to increase the contrast in neural representation of sound at approximately 61 kHz, which is an important component of an echo bearing velocity information. It is also most likely that the corticofugal system plays an important role in plasticity of the central auditory system. Another subdivision of the auditory cortex of the mustached bat is called the FM-FM area. This area consists of delay-tuned combination-sensitive neurons, called FM-FM neurons, and has the echo-delay axis for the systematic representation of target distances. A focal electric stimulation of the FM-FM area evokes changes in the responses of collicular and thalamic FM-FM neurons. These changes are basically the same as those described in the present paper. Therefore corticofugal modulation takes place for frequency domain analysis in exactly the same way as it does in time domain analysis.

Reorganization of the frequency map of the auditory cortex evoked by cortical electrical stimulation in the big brown bat

In a search phase of echolocation, big brown bats, Eptesicus fuscus, emit biosonar pulses at a rate of 10/s and listen to echoes. When a short acoustic stimulus was repetitively delivered at this rate, the reorganization of the frequency map of the primary auditory cortex took place at and around the neurons tuned to the frequency of the acoustic stimulus. Such reorganization became larger when the acoustic stimulus was paired with electrical stimulation of the cortical neurons tuned to the frequency of the acoustic stimulus. This reorganization was mainly due to the decrease in the best frequencies of the neurons that had best frequencies slightly higher than those of the electrically stimulated cortical neurons or the frequency of the acoustic stimulus. Neurons with best frequencies slightly lower than those of the acoustically and/or electrically stimulated neurons slightly increased their best frequencies. These changes resulted in the over-representation of repetitively delivered acoustic stimulus. Because the over-representation resulted in under-representation of other frequencies, the changes increased the contrast of the neural representation of the acoustic stimulus. Best frequency shifts for over-representation were associated with sharpening of frequency-tuning curves of 25% of the neurons studied. Because of the increases in both the contrast of neural representation and the sharpness of tuning, the over-representation of the acoustic stimulus is accompanied with an improvement of analysis of the acoustic stimulus.

Inter-relationship between different psychoacoustic measures assumed to be related to the cochlear active mechanism

The active mechanism in the cochlea is thought to depend on the integrity of the outer hair cells (OHCs). Cochlear hearing loss is usually associated with damage to both inner hair cells (IHCs) and OHCs, with the latter resulting in a reduction in or complete loss of the function of the active mechanism. It is believed that the active mechanism contributes to the sharpness of tuning on the basilar membrane (BM) and is also responsible for compressive input-output functions on the BM. Hence, one would expect a close relationship between measures of sharpness of tuning and measures of compression. This idea was tested by comparing three different measures of the status of the active mechanism, at center frequencies of 2, 4, and 6 kHz, using subjects with normal hearing, with unilateral or highly asymmetric cochlear hearing loss, and with bilateral loss. The first measure, HLOHC, was an indirect measure of the amount of the hearing loss attributable to OHC damage; this was based on loudness matches between the two ears of subjects with unilateral hearing loss and was derived using a loudness model. The second measure was the equivalent rectangular bandwidth (ERB) of the auditory filter, which was estimated using the notched-noise method. The third measure was based on the slopes of growth-of-masking functions obtained in forward masking. The ratio of slopes for a masker centered well below the signal frequency and a masker centered at the signal frequency gives a measure of BM compression at the place corresponding to the signal frequency; a ratio close to 1 indicates little or no compression, while ratios less than 1 indicate that compression is occurring at the signal place. Generally, the results showed the expected pattern. The ERB tended to increase with increasing HLOHC. The ratio of the forward-masking slopes increased from about 0.3 to about 1 as HLOHC increased from 0 to 55 dB. The ratio of the slopes was highly correlated with the ERB (r = 0.92), indicating that the sharpness of the auditory filter decreases as the compression on the BM decreases.

Cochlear function: hearing in the fast lane

The cochlea amplifies sound over a wide range of frequencies. Outer hair cells have been thought to play a mechanical part in this amplification, but it has been unclear whether they act rapidly enough. Recent work suggests that outer hair cells can indeed work at frequencies that cover the auditory range.

Corticofugal modulation of the midbrain frequency map in the bat auditory system

The auditory system, like the visual and somatosensory systems, contains topographic maps in its central neural pathways. These maps can be modified by sensory deprivation, injury and experience in both young and adult animals. Such plasticity has been explained by changes in the divergent and convergent projections of the ascending sensory system. Another possibility, however, is that plasticity may be mediated by descending corticofugal connections. We have investigated the role of descending connections from the cortex to the inferior colliculus of the big brown bat. Electrical stimulation of the auditory cortex causes a downward shift in the preferred frequencies of collicular neurons toward that of the stimulated cortical neurons. This results in a change in the frequency map within the colliculus. Moreover, similar changes can be induced by repeated bursts of sound at moderate intensities. Thus, one role of the mammalian corticofugal system may be to modify subcortical sensory maps in response to sensory experience.

Distortion-product otoacoustic emissions in middle-aged subjects with normal versus potentially presbyacusic high-frequency hearing loss

Distortion-product otoacoustic emissions (DPOAEs) are still undergoing evaluation for clinical use. Although the effects of ageing on otoacoustic emissions have been studied quite extensively in the past, DPOAE response-growth or input-output (I-O) measures, which are well suited as an objective method for monitoring cochlear function at specific frequencies, have been less thoroughly examined. The aim of the present study was to assess the 2f1-f2 DPOAEs in a clinical setting in order to examine the response of 20 normally hearing middle-aged adults and to compare the results with those of 20 people of the same age with ears of sensorineural high-frequency hearing loss (HL). The experiment consisted of two stages. First, the DPOAE-gram was recorded in 1-4-octave steps at a stimulus level of 70 dB SPL over a frequency range of the f2 primary tone which extended from 1.001 to 6.299 kHz. Secondly, in order to elicit DPOAE I-O functions, the two primary stimuli were presented at equilevel intensities ranging from 20 to 71 dB SPL. The stimulus-level step size was 3 dB. The I-O functions were recorded at five separate DPOAE frequencies, with the f2 frequency most closely related to the clinical audiogram (f2 = 1.0, 1.5, 2.0, 4.0 and 6.0 kHz). Two clearly separated portions in the form of the I-O function for normally hearing ears were found. The first portion, in response to primary levels of 60 dB SPL and below, showed a plateau (saturating) behaviour. If primary levels exceeded 60 dB SPL, I-O functions became more linear. The attenuation of the saturation portion of the I-O function in ears with high-frequency HL across the frequency-test range is difficult to explain because elevated behavioural thresholds were observed only for frequencies > 1.5 kHz. Thus, the more linear I-Os associated with the hearing-loss frequencies may indicate deficiencies in the active properties of outer hair cells (OHCs), whereas those for I-Os < 1.5 kHz, where hearing was normal, may indicate a beginning of damage to active OHC micromechanical processes prior to their clinical manifestation. DPOAE recordings from people with high-frequency HL, possibly age-related, supplement recordings of TEOAEs and give complementary information on degenerative changes in the outer hair-cells. DPOAE I-O functions may reveal discrete pathological alterations both in the active cochlear signal processing and in the passive mechanisms of the cochlea prior to their detection by clinical audiometric tests.

Psychoacoustic consequences of compression in the peripheral auditory system

Input-output functions on the basilar membrane of the cochlea show a strong compressive nonlinearity at midrange levels for frequencies close to the characteristic frequency of a given place. This article shows how many different phenomena can be explained as consequences of this nonlinearity, including the "excess" masking produced when 2 nonsimultaneous maskers are combined, the nonlinear growth of forward masking with masker level, the influence of component phase on the effectiveness of complex forward maskers, changes in the ability to detect increments and decrements with level, temporal integration, and the influence of component phase and level on the perception of vowellike sounds. Cochlear hearing loss causes basilar-membrane responses to become more linear. This can account for loudness recruitment, linear additivity of nonsimultaneous masking, linear growth of forward masking, reduced temporal resolution for sounds with fluctuating envelopes, and reduced temporal integration.

Auditory processing of complex sounds: an overview

The past 30 years has seen a remarkable development in our understanding of how the auditory system - particularly the peripheral system - processes complex sounds. Perhaps the most significant has been our understanding of the mechanisms underlying auditory frequency selectivity and their importance for normal and impaired auditory processing. Physiologically vulnerable cochlear filtering can account for many aspects of our normal and impaired psychophysical frequency selectivity with important consequences for the perception of complex sounds. For normal hearing, remarkable mechanisms in the organ of Corti, involving enhancement of mechanical tuning (in mammals probably by feedback of electro-mechanically generated energy from the hair cells), produce exquisite tuning, reflected in the tuning properties of cochlear nerve fibres. Recent comparisons of physiological (cochlear nerve) and psychophysical frequency selectivity in the same species indicate that the ear’s overall frequency selectivity can be accounted for by this cochlear filtering, at least in band width terms. Because this cochlear filtering is physiologically vulnerable, it deteriorates in deleterious conditions of the cochlea - hypoxia, disease, drugs, noise overexposure, mechanical disturbance - and is reflected in impaired psychophysical frequency selectivity. This is a fundamental feature of sensorineural hearing loss of cochlear origin, and is of diagnostic value. This cochlear filtering, particularly as reflected in the temporal patterns of cochlear fibres to complex sounds, is remarkably robust over a wide range of stimulus levels. Furthermore, cochlear filtering properties are a prime determinant of the ‘place’ and ‘time’ coding of frequency at the cochlear nerve level, both of which appear to be involved in pitch perception. The problem of how the place and time coding of complex sounds is effected over the ear’s remarkably wide dynamic range is briefly addressed. In the auditory brainstem, particularly the dorsal cochlear nucleus, are inhibitory mechanisms responsible for enhancing the spectral and temporal contrasts in complex sounds. These mechanisms are now being dissected neuropharmacologically. At the cortical level, mechanisms are evident that are capable of abstracting biologically relevant features of complex sounds. Fundamental studies of how the auditory system encodes and processes complex sounds are vital to promising recent applications in the diagnosis and rehabilitation of the hearing impaired.

Recording from the inferior colliculus following cochlear inner hair cell damage

The anti-cancer drug carboplatin has been used to generate inner hair cell (IHC) lesions in the cochleae of chinchilla. This model has provided a valuable physiological tool for the study of the auditory system, particularly concerning the relative roles of IHCs and outer hair cells (OHCs). We recorded responses to contralateral sound stimuli of single units (SU) in the central nucleus (CN) of the inferior colliculus (IC) from normal and carboplatin treated animals. Normal single unit thresholds and frequency tuning curves (FTCs) were found, despite gross IHC damage within the cochleae of carboplatin treated animals. No evoked afferent responses could be detected in CN regions which represented cochlear loci where total IHC loss had occurred. Normal frequency selectivity in the auditory system is possible with small numbers of surviving IHCs provided OHCs remain normal.

Perceptual consequences of cochlear hearing loss and their implications for the design of hearing aids

This paper provides an overview of changes in the perception of sound that result from cochlear damage. It starts with a brief introduction to the physiology of the cochlea, emphasizing the role of the "active mechanism" and describing how cochlear function is altered by cochlear damage. Then the effects of cochlear damage on various aspects of perception are described, including absolute sensitivity, frequency selectivity, loudness perception and intensity discrimination, temporal resolution, temporal integration, pitch perception and frequency discrimination, and sound localization and other aspects of binaural and spatial hearing. The possible role of each of these aspects of auditory perception in the ability to understand speech in quiet and in noise is discussed and evaluated. It is concluded that, for losses up to about 45 dB, audibility is the single most important factor. However, for greater losses, poor discrimination of suprathreshold (audible) stimuli is also of major importance. The final section of the paper describes applications of the findings to hearing aid design. It is concluded that linear amplification can be of only limited benefit in compensating for the effects of cochlear damage. Hearing aids incorporating compression can help to compensate for the effects of reduced dynamic range. Digital signal processing to enhance spectral contrast may be of some help in compensating for the effects of reduced frequency selectivity.

Relations among compression, suppression, and combination tones in mechanical responses of the basilar membrane: data and MBPNL model

Recent observations of combination tones and low-frequency suppression in the mechanical response of the basilar membrane have eliminated uncertainty of the existence of these non-linear phenomena at this stage of the cochlea (Robles et al., 1991; Ruggero et al., 1992b; Rhode and Cooper, 1993), and provide the first opportunity to test and extend theoretical understanding of these non-linear responses. This paper reports a systematic study of the new data for the chinchilla with the MBPNL (multiple band-pass non-linearity) non-linear model for rapid compression in cochlear filtering (Goldstein, 1990). The two key findings are that: (1) the large differences in suppression growth rates for low- and high-frequency suppressors are closely predicted by the model, although a statistically significant small bias in the prediction was found; and (2) combination tones of frequency 2f2-f1 were well predicted by the model, while reported levels of 2f1-f2 were underestimated by the model. The most likely explanation of the result for 2f1-f2 is that the model correctly predicts the distributed generation of combination tones, but does not include a propagating medium that allows combination tones to accumulate at their tuned site. Alternative explanations requiring experimental clarification are suggested. The general finding of this study is that the complex level-dependent properties of suppression and combination tones are closely related to the three regions characterizing the tuned response of the basilar membrane, in which linear-like responses at low and high sound levels are joined by a compressive range. The MBPNL model describes this general relationship quantitatively and is a reliable basis for further modeling research.

Effect of hypoxemia and ethacrynic acid on ABR and distortion product emission thresholds

Various studies have shown that induction of hypoxemia in animals such that arterial blood oxygen tensions reach 20-30 mm Hg is accompanied by reversible threshold elevations of the auditory nerve-brain-stem evoked response (ABR). In this state, the endocochlear potential (EP) is depressed, causing a smaller potential difference across the hair cells and/or reduced activity of the cochlear amplifier of the outer hair cells. In order to test these possibilities, ABR threshold (an expression of the overall sensitivity of the cochlea) and changes in threshold of the cubic (2f1-f2) distortion product emissions (DPE) (an expression of activity of the cochlear amplifier) were measured in the same cats while the EP was depressed by hypoxemia or by ethacrynic acid. During the episodes of hypoxemia, DPE thresholds were elevated by 10 dB while ABR thresholds were elevated by 22.8 dB. Therefore, it seems that a normal EP is necessary both for normal cochlear transduction (inner hair cells) and for normal cochlear amplification (outer hair cells). The human fetus in utero is relatively hypoxic and there is evidence that its auditory threshold is also similarly elevated. Therefore the threshold elevation in the fetus in utero, estimated to be about 20 dB, is a consequence of both reduced transduction current through the inner hair cells (about 10 dB) and an additional 10 dB reduction in the activity of the cochlear amplifier of the outer hair cells.

OHC response recruitment and its correlation with loudness recruitment

After proper noise exposure, Hensen's cells, which have been shown to follow closely the response characteristics of the outer hair cells, suffered a loss of sensitivity at low and moderate SPLs. The lower the stimulus level, the greater was the loss. When the low-SPL loss did not exceed about 40 dB, input-output functions showed an increased rate of amplitude growth, so that the post-exposure response caught up with its pre-exposure counterpart between 60 and 90 dB SPL, depending on the severity of the loss. These results, together with preceding clinical observations, led us to the conclusion that loudness recruitment occurs at least in part at the hair cell level and is basically a local event as opposed to a pathological spread of excitation. The response recruitment we have discovered appears to result from a decreased effect of the active feedback when the passive cochlear mechanisms are intact. Evidence for these relationships is presented and an explanation is offered for previous experimental successes and failures in observing a steepening of rate-intensity functions in auditory nerve fibers after noise exposures or administration of ototoxic drugs.

Effects of chronic cochlear damage on threshold and frequency tuning of neurons in AI auditory cortex

We describe the effects of long-term cochlear lesions on the frequency response properties of AI cortical neurons in the cat. Young animals were treated with amikacin to produce bilateral, basal to mid-turn cochlear lesions. After 12-24 months the response properties of single neurons or small unit clusters in primary auditory cortex were recorded in anesthetized animals. Responses to stimulus frequency and intensity were mapped in detail and frequency threshold curves (FTCs) and Q10dB values were derived. Subsequent to recording experiments, scanning electron microscopy of the sensory epithelium was used to characterize the degree and extent of the cochlear damage. In normal control animals, Q10dB values were, on average, lower than those derived by others from cochlear nerve fibre recordings in the same species. In amikacin-treated animals, deterioration was evident in the threshold and tuning properties of cortical neurons, particularly in those cells whose input originated in damaged cochlear regions. Often, neurons associated with 'normal' cochlear areas (as assessed by scanning microscopy) also had poor frequency tuning compared with controls. As an animal model of sensorineural hearing loss, we consider the cat with long-term cochlear lesions to be more appropriate than animals with acute or short-term pathology. We also suggest that in making physiological-psychophysical correlations, neural responses from the central auditory system (e.g. cortex) should perhaps be given more consideration than data derived at the cochlear level.

Functional development of auditory sensitivity in the fetus and neonate

The human fetus responds to sound stimuli while still in utero. The rat and cat begin to hear only after birth. Therefore neonatal rat and cat are used as models of the development of auditory sensitivity in the human fetus. The inner ear of rat responds to stimuli delivered directly to it (bone conduction) before the middle ear can conduct sounds to the inner ear. During this period, middle ear development involves mesenchyme resorption, ossicular hardening and opening of the external canal. The latter stages of inner ear development involve increased magnitude of the endocochlear potential which augments cochlear transduction and the active cochlear amplifier. These developmental stages are probably controlled by thyroid hormone which activates several genes leading to the synthesis of proteins and enzymes required for the structural and functional maturation of the ear. This likely includes the Na+,K(+)-ATPase of the stria vascularis which generates the endocochlear potential. The magnitude of the endocochlear potential is dependent on oxygen supply so that the human fetus in utero whose blood carries less oxygen than the newborn has a hypoxia-induced sensorineural hearing loss. Upon birth and transition from placental to pulmonary oxygenation, the oxygen content of blood is increased, the magnitude of the endocochlear potential is elevated and auditory sensitivity is enhanced.

Comparison of measures of frequency resolution and recruitment in patients undergoing neuro-otological investigation

Loudness recruitment and reduced frequency resolution both occur in cochlear types of hearing loss. One theory of loudness recruitment suggests that, as intensity is coded partly by spread of excitation across the nerve fibre array, recruitment is a direct consequence of the broad spread of excitation associated with poor frequency resolution. The present study investigated the relationship between these two quantities. The study involved a simple measure of frequency resolution (three-point psychoacoustical tuning curve, PTC) and conventional measures of recruitment obtained from patients undergoing neuro-otological investigation. Results from 376 ears of 226 patients without any material conductive impairment are presented. Measures of recruitment included the alternate binaural loudness balance test (ABLB) and estimates of dynamic range given by the sensation levels of the uncomfortable loudness level and the acoustic reflex threshold. Once covariation with hearing threshold level had been accounted for, no clear relationship emerged between frequency resolution and any of the measures of recruitment. This finding does not support the notion that frequency resolution and recruitment are specifically related. Rather, PTCs provide information complementary to measures of recruitment.

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.

Neonatal cochlear hearing loss results in developmental abnormalities of the central auditory pathways

We have used animal models of long term neonatal cochlear hearing loss to study developmental plasticity of the central auditory pathways. Newborn chinchilla pups and feline kittens were treated with the ototoxic drug amikacin, so as to induce basal lesions in the cochlea. At maturity these animals were used in single unit electrophysiological mapping studies, in which the cochleotopic organization of primary auditory cortex (of the cat) and the inferior colliculus of the midbrain (in the chinchilla) were mapped. We have observed, both in the midbrain and auditory cortex, massive reorganization of frequency representation. Most striking were the presence of large monotonic regions (i.e. large areas in which all neurons have similar tuning properties). Cochlear lesions which involve inner hair cells clearly modify the normal development of cochleotopic representation in the midbrain and cortical regions. We suggest that similar abnormal patterns of frequency representation will exist in human subjects with long term neonatal hearing loss.

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.

Effects of envelope fluctuations on gap detection

The inherent fluctuations present in narrowbands of noise may limit the ability to detect gaps in the noise; 'dips' in the noise may be confused with the gap to be detected. For subjects with cochlear hearing loss, loudness recruitment may effectively magnify the fluctuations and this could partly account for the reduced ability to detect gaps in noise bands that is usually found in subjects with cochlear hearing loss. In the present experiments we tested these ideas by processing noise bands to alter the amount of envelope fluctuation. The envelopes of the noise bands were raised to a power, N. Powers greater than 1 result in expansion of the envelope (magnified fluctuations, simulating loudness recruitment), while powers less than 1 result in compression of the envelope (decreased fluctuations). Thresholds for detecting gaps in processed noise bands centered at 1 kHz were measured as a function of noise bandwidth and of N. To prevent the detection of spectral changes introduced by the gap or by the processing, stimuli were either presented in background noise, or at a low sensation level (20 dB). Three normally hearing subjects, two subjects with unilateral cochlear hearing loss and two subjects with bilateral cochlear hearing loss were tested. Gap thresholds generally increased with increasing N. This effect was large for small noise bandwidths (50 Hz or less) and smaller for larger noise bandwidths (200 Hz or more). For both the normal and impaired ears, gap thresholds at narrow bandwidths were improved relative to those for unprocessed noise bands (N = 1) by compressing the envelope fluctuations (N < 1). The results support the idea that fluctuations in narrowband noises affect gap detection, and that loudness recruitment may adversely affect the ability to detect gaps in noise bands. They also show that compression of the fluctuations in the noise can improve gap detection.

Frequency selectivity in workers with noise-induced hearing loss

This study was undertaken in order to document, in a group of subjects affected by a noise-induced hearing loss, the relation between the loss of auditory sensitivity and the loss of frequency selectivity at mid-frequencies, namely 1000 and 3000 Hz. Auditory filter shapes were estimated using the notched noise method. Twelve notch widths were tested, six symmetrical and six asymmetrical with respect to the signal frequency; the spectral level of the noise was set at 50 dB SPL. Data were collected with 22 noise-exposed workers having different degrees of hearing loss. The findings indicate that above a certain degree of hearing loss, which seems to be around 30 dB HL, frequency selectivity tends to decrease linearly with increase in loss of sensitivity. Even when the degree of hearing loss is similar in origin and in magnitude, there is a wide variation among subjects in auditory filter bandwidth. Based on the data collected in this study, it is not possible to adequately predict the auditory filter bandwidth of an individual from hearing threshold levels.

Behavioural evidence for recovery of auditory function in guinea pigs following kanamycin administration

Deterioration followed by recovery of behavioural absolute threshold and frequency selectivity has been observed in guinea pigs following kanamycin administration of 200 mg/kg body weight daily for 16 days. Deterioration in function consistently follows a high-to-low frequency pattern and recovery generally occurs at the lowest of the high (8-32 kHz) frequencies affected. The degree of recovery is related to the magnitude of the threshold elevation; where large (40-45 dB) elevations occur initially, the process appears to be partial since threshold recovers only to within 5-12 dB of pre-administration levels. In instances where smaller threshold elevations (5-20 dB) take place initially, recovery can sometimes be complete. However, when threshold elevations of over 50 dB occur, no recovery is apparent. Recovery is relatively slow, taking place over periods of up to 100 days post-kanamycin administration. Hair cell counts have established that the threshold elevation which remains in instances of partial recovery is not related to a reduction in hair cell numbers at the light microscope level.

Postnatal development of central auditory frequency maps

In the early postnatal period of many mammals and in the perihatching period of chicks the auditory ranges are restricted to the species-specific low- and mid-frequency ranges. During subsequent development, the high frequency hearing expands (depending on the species) by 1-4 octaves. Adult-like audition is established between the 4th and the 7th week. It is still discussed controversially, how the extension of the auditory ranges relates to the maturation of orderly frequency representation in the cochleae of the respective species. The present review summarizes investigations of the development of tonotopy in nuclei of the central auditory system, and discusses how the centrally acquired data might contribute to the understanding of the maturation of cochlear stimulus transduction and to the development of frequency maps.

Psychoacoustical tuning curves. Normative data for clinical use

This normative study presents a clinically applicable method with which to obtain Psychoacoustical Tuning Curves: data are obtained with clinical audiometers, and measurements are performed on normal-hearing non-trained volunteers. A simultaneous masking method with test frequency of 4,000 Hz was used, withmasking being incremented in 5 dB or 1 dB steps. One group of subjects was tested three times with 5 dB incrementing steps to evaluate the method's reproducibility. A second group of subjects was measured twice with 1 dB incrementing steps, to compare the reproducibility of the 1-dB step measurements, and the difference between the 1-dB step and 5-dB step measurements. All the data were evaluated as normative values. The method presented proves to have a good reproducibility. Although testing with 1-dB incrementing steps provides more accurate measurements and curves with steeper slopes, the 5-dB incrementing step method is more practicable. Interest variability evaluation indicates that differences between two measurements performed on the same person exceeding 120 Hz (2 SD) bandwidth can be interpreted as significant.