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.     

CAP amplitude after impulse noise exposure in guinea pigs

In this study, 21 guinea pigs were submitted to a single high energy impulse noise (gun shot with blank projectiles). The auditory function was evaluated over a 7-day recovery period by recording the compound action potential (CAP) from the round window. The threshold shift and input/output function (CAP amplitude and delay function of the stimulus intensity) were studied at different frequencies. CAP amplitude fell after the noise trauma, especially at the lower sound level, resulting in a threshold shift. Latency was significantly increased. During recovery, whereas latency returned to its initial value, CAP amplitude gradually increased and, in half the animals, exceeded the control value for the higher levels of stimulus. This could have been because of progressive disinhibition or recruitment, and may correspond clinically to hyperacusis. These results are discussed referring to those obtained by other authors using other methods.     

Frequency specificity of ABR evaluated by tuning curves

A tone on tone simultaneous masking paradigm was used to determine tuning curves of ABR both from the normal and hearing-impaired subjects. ABR tuning curves were constructed to define masker intensity that resulted in a 50% reduction in probe elicited wave V amplitude. The frequency specificity of each probe stimulus was evaluated by Q10, low cut-off slope, high cut-off slope and the maximum masker frequency calculated for the tuning curves. The results were as follows; 1) Q10, low cut-off slope and high cut-off slope increased gradually with the increase in rise time. However, prolongations of the rise time beyond 3 cycles of probe frequency yielded little improvement in Q10, low cut-off slope and high cut-off slope. 2) Q10, low cut-off slope and high cut-off slope for normal-hearing subjects increased gradually with the increase in stimulus frequency or the decrease in stimulus pressure. Maximum masker frequency of the tuning curves was not always equal to the frequency of probe without the 2-kHz. For the 0.5, 1kHz probe, the maximum masker frequency of the tuning curves showed higher values than the frequency of probe. For the 4kHz probe, the maximum masker frequency of the tuning curves showed lower values than the frequency of probe. The results indicate that the tone pip stimuli will allow to assess certain conditions of auditory function at different frequencies, and they show wider frequencies' spread in the cochlea area near stimulus frequencies. 3) For subject with abrupt high-frequency hearing loss (30dB/oct), a pronounced down-ward shift of maximum masker frequency, down-ward shift of high cut-off slope and up-ward shift of low cut-off slope were found when the probe was placed in the region of elevated threshold. For subject with low-frequency hearing loss (25dB/oct), a pronounced up-ward shift of maximum masker frequency, down-ward shift of low cut-off slope were found. Maximum masker frequency, low and high cut-off slope of hearing-impaired subjects were not always equal to those of normal subjects for same probe. Especially the value of the maximum masker frequency shifted to the direction in which the most sensitive frequency was observed in audiogram. The threshold of ABR reflected the cochlea function of the most sensitive area near stimulus frequencies. Greatest predictive error was observed in steeply sloping audiograms.     

Follow-up of auditory-evoked potentials and temporary threshold shift in subjects developing noise-induced permanent hearing loss

The effects of auditory fatigue, using a temporary threshold shift (TTS) paradigm, on cochlear microphonics (CM) and on auditory brainstem-evoked potentials (ABEP), were studied in normal-hearing subjects during the development of permanent threshold shift (PTS). Behavioral threshold shifts were accompanied by different effects on CM and on ABEP, as PTS was gradually induced by occupational noise. Measures of the effect of increasing stimulus rate (ISR) on ABEP revealed decreased latency shifts during auditory fatigue. ABEP proved useful in early detection of changes in the auditory system, resulting from exposure to noise. In addition, this study further supports the suggestion that TTS acts as a peripheral attenuator of the effect of ISR on the central auditory pathway.     

Temporary threshold shift after exposure to noise and music of equal energy

Ten voluntary subjects were exposed to 10 min of recorded pop music on five occasions. On five other occasions these subjects were exposed to a noise with level-, frequency-, and time-distribution characteristics, measured in octave-band steps, equal to those of the music. Measurements of temporary threshold shift showed almost equal sensitivity to the two stimuli in four subjects, whereas six others demonstrated marked differences in sensitivity. Differences were always due to more temporary threshold shift after exposure to the nonmusical noise stimulus. These findings imply that factors other than physical properties of the fatiguing sound contribute to the degree of temporary threshold shift.     

Effects of intense pure tones on auditory temporal acuity

Gap detection thresholds (GDTs) were obtained from human listeners before and after exposure to a brief 0.4- or 1.7-kHz tone. The temporary threshold shift (TTS) produced 2 min after an exposure was approximately 10 dB. GDT stimuli were octave-band noises centered at one of three frequencies: the exposure frequency, one-half octave above the exposure frequency or one octave above the exposure frequency. GDTs were obtained at 35, 55, and 75 dB SPL at each center frequency. GDT and TTS recovery were monitored at logarithmically-spaced time intervals after the exposures. Following the 1.7-kHz exposure, shifts in post-exposure GDT were only obtained with the low-level stimulus conditions--the magnitude of GDT shift was correlated with the size of the TTS, and the shifts in GDT and absolute threshold required similar amounts of time to recover. Significant post-exposure shifts in GDT were also observed following the 0.4-kHz exposure. However, shifts were found at frequencies where there was no measurable TTS, and they required longer periods of time to recover than did absolute threshold.     

Diplacusis, hearing threshold and otoacoustic emissions in an episode of sudden, unilateral cochlear hearing loss

Limited data are available on the relationship between diplacusis and otoacoustic emissions and sudden hearing threshold changes, and the detail of the mechanism underlying diplacusis is not well understood. Data are presented here from an intensively studied single episode of sudden, non-conductive, mild hearing loss with associated binaural diplacusis, probably due to a viral infection. Treatment with steroids was administered for 1 week. This paper examines the relationships between the hearing loss, diplacusis and otoacoustic emissions during recovery on a day-by-day basis. The hearing thresholds were elevated by up to 20 dB at 4kHz and upwards, and there was an interaural pitch difference up to 12% at 4 and 8 kHz. There was also a frequency-specific change in transient evoked otoacoustic emission (TEOAE) and distortion-product otoacoustic emission (DPOAE) level. DPOAE level was reduced by up to 20 dB. with the greatest change seen when a stimulus with a wide stimulus frequency ratio was used. Frequency shifts in the 2f2-fi DPOAE fine structure corresponded to changes in the diplacusis. Complete recovery to previous levels was observed for TEOAE, DPOAE and hearing threshold. The diplacusis recovered to within normal limits after 4 weeks. The frequency shift seen in the DPOAE fine structure did not quite resolve, suggesting a very slight permanent change. The time-courses of TEOAE. diplacusis and hearing threshold were significantly different: most notably, the hearing threshold was stable over a period when the diplacusis deteriorated. This suggests that the cochlear mechanisms involved in diplacusis, hearing threshold and OAE may not be identical.     

Diplacusis,hearing threshold and otoacoustic emissions in an episode of sudden,unilateral cochlear hearing loss

Limited data are available on the relationship between diplacusis and otoacoustic emissions and sudden hearing threshold changes, and the detail of the mechanism underlying diplacusis is not well understood. Data are presented here from an intensively studied single episode of sudden, non-conductive, mild hearing loss with associated binaural diplacusis, probably due to a viral infection. Treatment with steroids was administered for 1 week. This paper examines the relationships between the hearing loss, diplacusis and oto-acoustic emissions during recovery on a day-by-day basis. The hearing thresholds were elevated by up to 20 dB at 4 kHz and upwards, and there was an interaural pitch difference up to 12% at 4 and 8 kHz. There was also a frequency-specific change in transient evoked otoacoustic emission ( TEOAE) and distortion-product otoacoustic emission (DPOAE) level. DPOAE level was reduced by up to 20 dB, with the greatest change seen when a stimulus with a wide stimulus frequency ratio was used. Frequency shifts in the 2f2 – f1 DPOAE fine structure corresponded to changes in the diplacusis. Complete recovery to previous levels was observed for TEOAE, DPOAE and hearing threshold. The diplacusis recovered to within normal limits after 4 weeks. The frequency shift seen in the DPOAE fine structure did not quite resolve, suggesting a very slight permanent change. The timecourses of TEOAE, diplacusis and hearing threshold were significantly different; most notably, the hearing threshold was stable over a period when the diplacusis deteriorated. This suggests that the cochlear mechanisms involved in diplacusis, hearing threshold and OAE may not be identical.     

Temporary threshold shift,adaptation, and recovery characteristics of frog auditory nerve fibers

While recording from single auditory nerve fibers in a frog, a monaural 3 min pure tone stimulus at CF was used to induce temporary threshold shift (TTS). TTS magnitude was correlated with the exposure tone intensity relative to the pre-exposure best threshold of the neuron, but not with exposure tone absolute intensity. CF and spontaneous spike rate were also uncorrelated with TTS magnitude. Comparison of frequency-threshold curves (FTCs) made before and successively after exposure revealed either a maximum sensitivity loss at the tip of the FTC or an equal shift at all frequencies. Neurons tended to recover from TTS within 3 min post-exposure, regardless of the initial TTS. Thus, recovery from TTS was more rapid for larger shifts. Recovery dynamics followed single or a double negative exponential functions.     

Single-neuron labeling and chronic cochlear pathology. IV. Stereocilia damage and alterations in rate- and phase-level functions

The rate and phase of auditory-nerve response to tone bursts were studied as a function of stimulus level in normal and acoustically traumatized animals. The rate- and phase-level functions of normal auditory-nerve fibers are often separable into a low-intensity component (component I) and high-intensity component (component II), as defined by a dip in the rate function and a simultaneous abrupt shift in the phase function at stimulus levels near 90 dB SPL [10,12,9]. Baseline data are established by defining the relation between stimulus frequency and the characteristic frequency and spontaneous discharge rate of a fiber normally required for the appearance of these two components in the response. Abnormalities of the level functions are shown to occur in acoustically traumatized ears. Noise-induced threshold shift is often characterized by selective attenuation of component I. In some instances, it appears that component I has been eliminated, leaving a response which is identical in threshold, phase and maximum discharge rate to a normal component II. Results of single-unit labeling in such a case suggest that the selective attenuation of component I is associated with selective loss of the tallest row of stereocilia on the inner hair cells (IHCs). It is suggested that component I is normally generated through an interaction between the outer hair cells and the tall row of IHC stereocilia, while component II requires only the shorter row of IHC stereocilia.     

Growth of threshold shift in hair-cell stereocilia following overstimulation

Sensory hair bundle micromechanics were measured from all four hair-cell rows in the isolated guinea pig cochlea before and during overstimulation. The stereocilia bundle was stimulated by an oscillating water jet, and stroboscopic illumination slightly offset from the frequency of the stimulus revealed their motion. The intensity of the water jet could be varied in decibel steps and a 'visual detection level' threshold of stereocilia movement served as the criterion response. Pre-exposure thresholds provided a reference and were compared to thresholds sampled at 1-min intervals during a 10-min exposure period. The exposure stimulus consisted of the water jet adjusted to either 8, 13, or 18 dB above the visual detection level. Stereocilia on hair cells in each of the four rows showed a loss in stiffness which systematically increased during the first 5 to 6 min of exposure. Between 6 and 10 min of overstimulation the threshold shift exhibited a plateau whose magnitude was proportional to the exposure level. There were also differences in the magnitude of threshold shift between the hair-cell rows. The results clearly showed that overstimulation changed the micromechanical behavior of the stereocilia bundle, and that there was a complex interaction between exposure duration, level, and hair-cell row.     

Dynamic visual acuity during linear acceleration along the inter-aural axis

We investigated visual-vestibular interactions during linear acceleration along the inter-aural axis. Eighteen healthy volunteers and two patients with central neurological diseases were subjected to transaural linear acceleration in the direction of gravity force (frequency: 0.5–1.5 Hz; amplitude: 5 cm). During linear acceleration, eye movements were recorded under three test conditions: eyes closed (EC), while staring at an imaginary target (IT) and during the testing of dynamic visual acuity (DVA). As parameters of evaluation we used the amplitude of horizontal eye movements, phase shift and the decrease of DVA threshold (DVAT). Under all test conditions, eye amplitude increased with rising stimulus frequency and exceeded, especially in the higher frequency range, a hypothetically calculated eye amplitude for smooth pursuit. The combination of a visual and vestibular input (DVA and IT) led to a better compensation (lower phase shift) than under vestibular stimulation alone (EC). Eye movements during low-frequency stimulation depended more on the visual system while responses in the higher frequency range were mainly triggered by the otolith organ. At 1.5 Hz the compensatory function of the visual-vestibular system was limited (rising phase shift) and DVAT decreased even in a significant number of healthy subjects. Patients with diseases of the central nervous system showed a higher phase shift and thus a stronger decrease of DVAT (two levels) already at a stimulus frequency of 1.25 Hz. Received: 29 May 1999 / Accepted: 2 September 1999     

Changes in stereocilia micromechanics following overstimulation in metabolically blocked hair cells

Previous studies have demonstrated that stereocilia micromechanics change during exposure to intense stimulation, and then recover after the stimulus has ended. These changes were associated with a loss of stiffness in the sensory hair bundle. The patterns of growth and recovery in these data suggest that active cellular processes control and limit these phenomena. The present investigation further supports this suggestion. The effects of intense stereocilia stimulation were examined before and after the hair cells were metabolically blocked by cooling or by poisoning with NaCN. In normal cells threshold shift increases with exposure duration, reaching a plateau within 5 to 6 min. Moreover, a post-exposure recovery of the threshold was also noted in the control cells within 15 min. In contrast threshold shift after the first minute in the metabolically blocked cells increased monotonically during the exposure without any indication of a plateau. Similarly, no post-exposure recovery of threshold shift was seen in the stereocilia bundles. These data support the hypothesis that metabolic processes in the region of the sensory hairs are important for limiting the loss of stiffness during exposure, as well as for restoring stiffness during recovery.     

Cochlear action potentials threshold and systemic arterial pO2

A progressive decrease in arterial pO2 was induced in Hartley guinea pigs (GP) by having them rebreathe the air entrapped in a closed circuit from which the CO2 was continuously absorbed. Following this slow-developing hypoxemia, a sequence of events concerning the ear could be observed. Firstly, a fluctuation in cochlear action potentials (CAP) was noted. This appeared only at stimulus intensities near the threshold and was not evident at higher intensities. At a mean arterial pO2 level of 30 mm Hg, this phenomenon was observed in 60% of the GP. At a mean pO2 of 24.66 mm Hg, a shift in threshold occurred in all GP. Finally, when the blood pO2 reached an average of 14.92 mm Hg, no CAP response could be elicited, even at click intensity of 120 dB SPL. The 30 mm Hg oxygen concentration in the arterial blood was considered as a "critical level" at which the cochlear function starts to deteriorate in guinea pigs.     

Changes in spontaneous otoacoustic emissions produced by acoustic stimulation of the contralateral ear

Spontaneous otoacoustic emissions (SOAEs) were measured in human ear canals before, during and after presentation of tonal stimuli to the contralateral ear. Stimuli were presented in 1/8 octave steps from 2 octaves below to 1 octave above the SOAE frequency at sound levels below the observed contralateral acoustic reflex threshold. For certain conditions there was an abrupt upward frequency shift at stimulus onset. For a fixed level the effect was frequency selective; the maximum frequency shift was obtained with tones approximately 1/2 octave below the SOAE. SOAE amplitude usually decreased but in some cases increased or remained unchanged. When amplitude changes were observed, the maximum shifts were observed for tones at or near the SOAE frequency. Changes in SOAEs were not observed for stimulus levels below 60 dB SPL. The effect is believed to be mediated by medial efferent neurons of the uncrossed olivocochlear bundle which arise in the medial region of the superior olivary complex and terminate on outer hair cells (OHCs). These results support those models which attribute SOAE generation to OHCs, and are indicative of an efferent influence on cochlear mechanics. A simple model is presented that proposes that efferent activity alters the tuning of the emission generator by causing changes in OHC membrane conductance.     

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates. IV. Effects of stimulus intensity.

High rate intracochlear electrical stimulation using stimulus intensities well above clinical limits can induce a significant reduction in the excitability of the auditory nerve as measured by a reduction in the amplitude of the electrically evoked auditory brainstem response (EABR). The purpose of the present study was to assess the effect of stimulus intensity on these stimulus induced changes by comparing the effects of acute stimulation using stimulus intensities within normal clinical levels (6 dB and 12 dB above EABR threshold) and significantly above normal clinical levels (> 20 dB above EABR threshold; 0.34 microC/phase). Stimulus rates of 200, 400, or 1000 pulses/s (pps) were delivered to bipolar scala tympani electrodes. EABRs were recorded before and periodically following 2 h of continuous stimulation. No reduction in EABR amplitude was observed following stimulation at 6 dB above EABR threshold for the three stimulus rates examined. However, EABRs were reduced when stimulated at 12 dB above EABR threshold at 400 pps, and significantly reduced when stimulated at a rate of 1000 pps. Immediate post-stimulus response amplitudes of wave III were 63% and 35% of the pre-stimulus amplitude at 400 and 1000 pps respectively. More significant reductions in EABR amplitude were observed following stimulation at levels more than 20 dB above EABR threshold for both 400 and 1000 pps stimuli. Our findings indicate that stimulus induced changes in EABR amplitude are related to both stimulus rate and stimulus intensity. Moreover, stimulation using intensities within the normal clinical range show little evidence of prolonged reductions in auditory nerve excitability at stimulus rates of up to 1000 pps.     

Effect of ethanol on dynamic visual acuity during vertical body oscillation in healthy volunteers

Visual orientation is the most important sensory input during locomotion (e.g. walking, driving a car, riding a bicycle). We investigated dynamic visual acuity (DVA) during vertical body-oscillations (amplitude 5 cm; frequency 1.5 Hz) in 12 healthy subjects before and twice after ethanol consumption. During oscillation, vertical eye movements were recorded under two test conditions: with eyes closed (EC) and during DVA testing. A significant increase in vertical eye-amplitude after ethanol ingestion occurred only during EC tests, as a possible sign of vestibular hyperreaction. During vestibular stimulation alone (EC), ethanol did not affect the phase shift between stimulus and eye movements. However, when the subjects were given an additional visual stimulus (DVA), the post-alcohol phase shift rose significantly. Surprisingly, the post-alcohol phase shift values for the two test conditions showed no significant differences. After ethanol ingestion we found no changes in static visual acuity but a significant loss of DVA. Volunteers with a change of DVA threshold (DVAT) showed significantly (P = 0.004) higher post-alcoholic changes in the phase shift. In summary, low doses of ethanol disturbed the visually guided oculomotor response during fixation of an earth-fixed target while the observer was subject to linear vertical acceleration. This effect led to an increasing delay between the beginning of body and eye movements. The consequence was an increasing phase shift and thus a decrease in DVA during whole-body oscillation which was comparable to movements during human locomotion. Received: 8 March 2000 / Accepted: 18 April 2000     

Effects of discotheque music on audiometric results and central acoustic evoked neuromagnetic responses

Audiograms and auditory evoked magnetic fields (AEFs) were observed in young male and female adults at different ages before and after being exposed to discotheque music for 4 hours. Sound pressure levels (SPLs) ranged from 95 dB (SPL) up to 130 dB (SPL). After exposure, subjects had temporary threshold shifts up to 20-25 dB, which almost disappeared after 2 hours. The majority of the subjects suffered from tinnitus that lasted approximately as long as the temporary threshold shift. Correspondingly, a transient delay and prolongation of the main component of the acoustically evoked magnetic field (AEF) negative wave, occurring 100 msec after stimulus (N100 m), was seen after this exposure; other components of the AEF (positive wave, occurring 50, 160, and 200 msec after stimulus [P50 m, P160 m, and P200 m, respectively]) occurred less often as compared to nonexposed controls. Because effects of vigilance on the AEF could be excluded, these changes can be related to the loud music, indicating an influence of noise on central auditory processing. The transient tinnitus could be caused by acoustic microinjuries (hidden acoustic predamage) of outer hair cells, leading to the persistent hearing threshold shifts from which many young adults aged 20-24 years are suffering. Occurrence of tinnitus closely coincides with the changes in hearing threshold and AEF, thus, a limitation of loudness in discotheques is needed to prevent this kind of hearing hazard.     

Comparison between the frequency specificities of auditory brainstem response thresholds to clicks with and without high-pass masking noise.

In this study, the frequency specificity of the auditory brainstem response (ABR) threshold to a click masked with 1590-Hz high-pass masking noise is compared with the frequency specificity of the unmasked click-evoked ABR threshold. The ABR threshold to the high-pass-noise-masked click stimulus is low frequency specific and corresponds with the 1,000-Hz pure-tone threshold. Although the ABR threshold to the unmasked click stimulus corresponds with the '3,000'-Hz pure-tone threshold, the frequency specificity seems much less pronounced than that of the low-frequency-specific stimulus. This study shows, however, that this apparent lack of frequency specificity can be attributed to the selection of pure-tone hearing losses. The ABR threshold evoked by an unmasked click stimulus is, therefore, preeminently useful as a high-frequency point of a two-point audiogram. The possible reasons why the ABR threshold evoked by a broad-band stimulus as the unmasked click corresponds with the higher frequencies of the pure-tone audiogram are discussed.     

40-Hz auditory event-related potential in normal adults

40-Hz event-related potentials (AERP) in response to 0.5-, 1-, 2- and 4-kHz tone pips were studied in 45 subjects (18 males and 27 females) in order to assess their reliability and threshold in normal adults and to study the effects of stimulus frequency and intensity on their latency and amplitude. In all subjects well-formed and reproducible 40-Hz AERP were detected, thus showing a good reliability of 40-Hz AERP to tone pips. The response was always detectable within 15 dB nHL intensity level and showed a sequence of positive (P1, P2 and P3) and negative (N1, N2 and N3) waves. It has also been observed that the latency of the first component following the acoustic stimulus decreased at increasing stimulus frequency and intensity, while the amplitude of the whole response increased upon increasing stimulus intensity. It can be suggested that the 40-Hz AERP to tone pips may represent a useful tool in assessing auditory threshold in the low-frequency range.