Auditory fatigue: retrocochlear components

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Separate and combined effects of a benzodiazepine (alprazolam) and noise on auditory brainstem responses in man

Auditory brainstem responses (ABRs) were recorded in 60 male or female, anxious or anxiety-free university students, before and after separated or simultaneous intake of alprazolam and exposure to noise. A significant increase of the latencies of the ABRs was found when subjects took alprazolam. This effect is consistent with the presence of gamma-aminobutyric acid (GABA), one of the neurotransmitters at terminals of cochlear efferent fibres A significant increase of the latencies was observed after noise alone. In subjects taking alprazolam when they are exposed to noise, the effect of noise on the ABR latencies is reduced, but not abolished. The effects of alprazolam on the ABR are consistent with the presence of GABA in the medulla and pons. Significant effects of noise upon III-V and I-V intervals suggest that auditory 'fatigue' may involve a retrocochlear component. Differences due to sex appear to be abolished by anxiety.

Auditory effects of aircraft noise on people living near an airport

Two groups of randomly chosen individuals who lived in two communities located different distances from the airport were studied. We monitored audiometry and brainstem auditory-evoked potentials to evaluate cochlear and retrocochlear functions in the individuals studied. The results of audiometry measurements indicated that hearing ability was reduced significantly in individuals who lived near the airport and who were exposed frequently to aircraft noise. Values of pure-tone average, high pure-tone average, and threshold at 4 kHz were all higher in individuals who lived near the airport, compared with those who lived farther away. With respect to brainstem auditory-evoked potentials, latencies between the two groups were not consistently different; however, the abnormality rate of such potentials was significantly higher in volunteers who lived near the airport, compared with less-exposed counterparts. In addition, a positive correlation was found between brainstem auditory-evoked potential latency and behavioral hearing threshold of high-frequency tone in exposed volunteers. We not only confirmed that damage to the peripheral cochlear organs occurred in individuals exposed frequently to aircraft noise, but we demonstrated involvement of the central auditory pathway.

Enhancement of the auditory cortex evoked responses in awake guinea pigs after noise exposure

In a previous paper [Popelár et al., Hear. Res. 26, 239-247 (1987)] we have shown that amplitudes of the auditory cortex evoked responses (AC-ER) in awake guinea pigs were enhanced for several hours after 1 h of noise exposure whereas amplitudes of the compound potential of the auditory nerve (CAP) and of the inferior colliculus evoked responses (IC-ER) declined. The present study demonstrates that the duration of the AC-ER amplitude increase is related to the intensity of the noise exposure (white noise, for 30 min or 1 h, intensity range 105-125 dB). The AC-ER amplitude as well as the threshold shift increased linearly with increasing intensity of the noise. The maximum AC-ER increase occurred when clicks served as stimuli; amplitude enhancement was smaller for 1 kHz tone pips and was absent when 20 kHz tone pips were used. The amplitude enhancement was specific for the auditory cortex since the amplitude of visually evoked responses, recorded in the occipital cortex, was unchanged after noise exposure. It is suggested that the postexposure amplitude enhancement of the AC-ER is produced by temporary exhaustion of inhibitory processes in the auditory cortex.

Effects of aircraft noise on hearing and auditory pathway function of school-age children

This study was conducted to investigate the influence of high-frequency aircraft noise on the function of the auditory system of school-age children. A total of 228 students attending a school near an airport (school A) and 151 students attending a school far from an airport (school B) were analyzed. Audiometry and brainstem auditory evoked potential (BAEP) detection were performed in all subjects to evaluate cochlear and retrocochlear function. The results of audiometry indicated that hearing ability was significantly worse in the children of school A, which was located under the flight paths. The values of pure tone average, high pure tone average, and threshold at 4 kHz were all higher in children who were frequently exposed to aircraft noise. There was no consistent difference in BAEP latencies between the two schools. These results indicate that central transmission is not affected in children who have been exposed to aircraft noise for several years. The results of the present study showed a significant association between aircraft noise exposure and prevalence of noise-induced hearing loss. Although damage to peripheral cochlear organs was confirmed in school-age children, involvement of the central auditory pathway could not be verified.

Acoustically induced hearing loss: intracellular studies in the guinea pig cochlea

Intracellular recordings were made from both inner and outer hair cells (IHC, OHC) in the basal coil of the guinea pig cochlea before, during and after the animal was exposed to loud, pure tones. Following multiple loud tones both types of sensory cells demonstrate a culmulative decrease in their voltage responses to a test tone. A loss in sensitivity of the compound action potential (CAP) of the eighth nerve co-incides with a decrease in both the amplitude of the IHC receptor potential and the positive summating potential (+SP) recorded at the round window. The largest decreases in sensitivity of IHCs are found at the characteristic or best frequency (CF) of each cell and this frequency selective loss of sensitivity results in a decrease in the tuning of the hair cell. During and following a loud tone the nonlinear properties of IHCs are also reduced.

Effect of noise on auditory evoked responses in awake guinea pigs

Changes in the auditory nerve action potential (AP), evoked responses from the inferior colliculus (IC-ER) and auditory cortex (AC-ER) were assessed after exposure to white noise of 120 dB SPL for 1 h in awake guinea pigs. Auditory thresholds were estimated with the aid of averaged AP, IC-ER and AC-ER, besides the threshold shifts also the changes in amplitude-intensity functions were evaluated. Auditory thresholds for tone pips and clicks increased by 20-30 dB 1 h after exposure and were similar in all the three investigated structures. The maximum threshold shifts for tone pips were observed at 8 kHz and were 33.2 +/- 12.9 dB for AP, 30.4 +/- 12.7 dB for IC-ER and 30.8 +/- 13.0 dB for AC-ER (n = 20). The thresholds recovered to preexposure levels within one week. Reduction in AP and IC-ER amplitudes 1 h after exposure was similar, the amplitude-intensity functions were shifted by 20-40 dB. In contrast, the amplitude-intensity functions in the auditory cortex 1 h after exposure were steeper than before exposure and this amplitude enhancement was present for 24 h after exposure. The enhancement of the AC-ER which resembles recruitment and which may be a sign of hypersensitivity of the animal to auditory stimuli was present only when the animals exposed to noise were awake. The noise exposure in animals anaesthetized with urethane reduced the amplitude-intensity functions of all three recorded potentials.

Auditory brainstem evoked potentials in blast injury

Blast injury typically consists of a mixed conductive and sensorineural hearing loss. The sensorineural component includes temporary as well as permanent threshold elevations. Auditory brainstem evoked potentials (ABEP) are sensitive to functional changes in various levels along the auditory pathway. ABEP were recorded from 37 survivors of blasts and latency measures were correlated with clinical findings. Prolongation of peak latencies was correlated with the conductive component of blast-induced hearing loss, as well as with the TTS component of the sensorineural impairment. No central effects of blast on the auditory system were detected. In addition to their objectivity, ABEP hold the promise of differentiating between the permanent and temporary effects of blast on hearing.

Auditory evoked potentials and audiological follow-up of subjects developing noise-induced permanent threshold shift

Three examinations, including cochlear microphonics (CM) to 99/s 1-kHz tones and auditory brainstem evoked potentials (ABEP) to 10/s and 55/s clicks, as well as psychoacoustical tests, were performed on 31 normally hearing subjects, exposed to occupational noise for over a year. The results showed prolongation of the absolute latency of peaks I, III and V, without significant changes in interpeak latency differences (IPLD) or CM latency. The site affected by increasing stimulus rate, giving rise to increased IPLD, appears to be central rather than cochlear. The efficacy of increased stimulus rate in detecting noise-induced auditory changes was higher than achieved applying the 10/s click rate.

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.

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.

Brainstem-evoked responses of guinea pigs exposed to high noise levels in utero

Pregnant guinea pigs were exposed to loom room noise at 115 dB A for 7.5 hr/day for various periods during the last one-third of pregnancy. When the hearing of their offspring was tested by auditory brain stem-evoked response techniques at 6-dB intervals, peak IV latencies of exposed pups were found to be significantly longer than those of otherwise similar control pups. The latency differences corresponded to a 5-dB increase in stimulus at medium stimulus levels and 10-12 dB near threshold. The results indicate that it is possible for noise-induced loss to occur in utero in mammals whose auditory maturation process is complete, or nearly so, before birth.

Effect of high-intensity sound on cochlear microphonics and activity of inferior colliculus neurons in the guinea pig

The input-output functions of cochlear microphonics (recorded from the round window) in guinea pigs exposed briefly to high-intensity sound (2 kHz, 130 dB, 30 min) were shifted toward higher intensities by about 15 dB in comparison with normal animals. 2-4 h after the exposure this shift decreases to 10 dB. The response characteristics of the inferior colliculus neurons were compared under similar conditions. Thresholds at the characteristc frequencies (CF) measured 2-4 h after exposure, were raised by 30-50 dB, a higher threshold increase was observed at frequencies from 5-7 kHz. 48 h after exposure the thresholds approached those obtained in normal animals. There was no pronounced hair cell loss after exposure. It is assumed that in addition to the impairment of sound transduction in hair cells there may be a specific effect of high-intensity sound exposure on neuronal transmission in the auditory pathway.