Roger P. Hamernik

Anatomical correlates of impulse noise-induced mechanical damage in the cochlea

Changes in the surface morphology of the organ of Corti in the chinchilla were studied following exposure to blast waves at 160 dB peak SPL. The generation and development of a mechanically induced lesion on the organ of Corti was followed over a period of 30 days using scanning electron microscopy. The most prominent feature of the lesion was the complete separation of a 5-7 mm strip of the sensory epithelia consisting of outer hair cells, Deiter cells and Hensen cells from the reticular lamina and the basilar membrane. The inner hair cells in this same area survived for several days in a remarkably normal condition. A spectrum of ciliary changes was observed on the inner and outer hair cells that differ somewhat from those commonly reported following continuous noise exposure. Some of the observed changes in morphology can be related to a variety of inferred mechanical events on the basilar membrane.

Impulse noise: Critical review

A review of the last 10 years of research on impulse noise reveals certain insights and perspectives on the biological and audiological effects of exposures to impulse noise. First, impulse noise may damage the cochlea by direct mechanical processes. Second, after exposure to impulse noise, hearing may recover in an erratic, nonmonotonic pattern. Third, even though the existing damage-risk criteria evaluate impulse noise in terms of level, duration, and number, often parameters such as temporal pattern, waveform, and rise time are also important in the production of a hearing loss. Fourth, the effects of impulse noise are often inconsistent with the principle of the equal energy hypothesis. Fifth, impulse noise can interact with background continuous noise to produce greater hearing loss than would have been predicted by the simple sum of the individual noises.

The quantitative relation between sensory cell loss and hearing thresholds

On the basis of experimental data obtained from 420 noise-exposed animals (chinchilla), the amount of sensory cell loss has been quantitatively related to the amount of permanent threshold shift at eight audiometric test frequencies between 0.125 and 16 kHz. The noise exposures, which varied extensively in spectrum, intensity and duration, produced permanent threshold shifts that ranged from 0 to 70 dB across a broad range of test frequencies. These data show: (1) consistent outer hair cell losses with less than 5 dB permanent threshold shifts (PTS) across all the test frequencies; (2) the first approximately 30 dB of PTS is established by losses of primarily outer hair cells; (3) in regions of the cochlea that transduce frequencies higher than or equal to 2 kHz, the three rows of outer hair cells show the same degree of loss for a given PTS, while in the 0.5 to 1.0 kHz region of the cochlea, the third row of outer hair cells (OHC) consistently shows less loss than do rows one and two; (4) appreciable inner hair cell (IHC) loss does not begin to appear until PTS exceeds approximately 30 dB; (5) in the virtual absence of OHC, hearing thresholds are least sensitive to IHC loss in the octave band centered at 4 kHz, i.e., the 4 kHz region can be as functional as other areas of the cochlea in spite of a greater amount of damage. The quantitative relation between cell loss and PTS varies as a function of test frequency in an orderly fashion.

Evoked‐response audibility curve of the chinchilla

The audibility curve of the chinchilla was measured on 76 animals using the auditory evoked response (AER). The standard behavioral measures of sensitivity and the AER measures are shown to agree within an average of 5 dB through the 0.25 to 8 kHz range. Furthermore, the AER is shown to reflect temporary and permanent partial losses of hearing due to noise exposure. It is concluded that the AER is a useful index of auditory sensitivity for the brief tones of the order of 20‐msec duration.

Blast overpressure induced structural and functional changes in the auditory system

Blast overpressure of sufficient intensity can produce injury to various organ systems. Unprotected ears result in the auditory system being the most susceptible. The injuries to the auditory system include: rupture of the tympanic membrane, dislocation or fracture of the ossicular chain, and damage to the sensory structures on the basilar membrane. All these injuries can be characterized as a form of mechanical damage to the affected structure. Injury to the sensory structures on the basilar membrane leads to temporary and permanent loss of hearing sensitivity. The temporary component of the hearing loss shows a time course after removal from the noise which frequently will include an initial increase in hearing loss followed by a recovery period during which threshold may return to preexposure levels or stabilize at a higher level which represents a permanent loss of hearing sensitivity. This type of recovery function suggests that there are damage processes which continue after the traumatic event and that intervention might mitigate some of the damage and hearing loss.

Basic and applied aspects of noise-induced hearing loss

Anatomical Bases of Noise Induced Hearing Loss.- Morphology of Stereocilia on Cochlear Cells after Noise Exposure.- Mechanical Changes in the Stereocilia following Overstimulation: Observations and Possible Mechanisms.- The Morphology of Sterocilia and Their Coss-Links in Relation to Noise Damage in the Guinea Pig.- Synaptology of the Cochlea: Different Types of Synapse, Putative Neurotransmitters and Physiopathological Implications.- The Morphology of the Normal and Pathological Cell Membrane and Junctional Complexes of the Cochlea.- Mechanically Induced Morphological Changes in the Organ of Corti.- The Application of Morphometric and Stereological Principles to Epithelial Tissue: Theoretical and Practical Considerations.- Morphometric Methods for the Evaluation of the Cochlear Microvasculature.- Physiological Changes with Noise-Induced Hearing Loss.- Mechanical Correlates of Noise Trauma in the Mammalian Cochlea.- Auditory Sensitivity, Auditory Fatigue and Cochlear Mechanics.- The Response of Mammalian Cochlear Hair Cells to Acoustic Overstimulation.- Structure-Function Correlation in Noise-Damaged Ears: A Light and Electron-Microscopic Study.- Psychophysical and Physiological Aspects of Auditory Temporal Processing in Listeners with Noise-Induced Sensorineural Hearing Loss.- Increase in Central Auditory Responsiveness During Continuous Tone Stimulation or Following Hearing Loss.- Adjustments in Coronary Blood with Noise Stimulation.- Critical Periods of Susceptibility to Noise-Induced Hearing Loss.- The Acoustic Reflex in Industrial Impact Noise.- Noise History, Audiometric Profile and Acoustic Reflex Responsivity.- Stimulation of the Middle Ear Acoustic Reflex Applied to Damage-Risk for Hearing Loss Produced by Burst Fire.- Psychoacoustic Performance Changes with Noise-Induced Hearing Loss.- Changes in Auditory Threshold During and After Long Duration Noise Exposure: Species Differences.- The Curious Half-Octave Shift: Evidence for a Basalward Migration of the Traveling-Wave Envelope with Increasing Intensity.- Human Noise Experiments Using a Temporary Threshold Shift Model.- The Relationship Between Speech Perception and Psychoacoustical Measurements in Noise-Induced Hearing Loss Subjects.- Speech Perception in Individuals with Noise-Induced Hearing Loss and its Implication for Hearing Loss Criteria.- The Perception of Synthetic Speech in Noise.- Concept - Reference Coherence in Speech Perception: Consequences for Native and Second Language Speech Comprehension in Noise.- Impulse Noise/Blast Wave Effects.- A Parametric Evaluation of the Equal Energy Hypothesis.- Impulse Noise Hazard as a Fuction of Level and Spectral Distribution.- Experimental Studies of Impulse Noise.- The Role of Peak Pressure in Determining the Auditory Hazard of Impulse Noise.- Effects of Weapon Noise on Hearing.- Critical Peak Level for Impulse Noise Hazard: Permanent Hearing Threshold Shifts in Military Drill Squads Following Known Variation of Impulse Noise Exposure.- Can TTS be an Indicator for Individual Susceptibility to PTS?.- Field Studies on Impluse Noise Annoyance in the Environment of Garrison Firing Range.- The Results of Long-Term Field Studies on Acoustic Traumata in Military Personnel.- Effects of Blast Waves on Nonauditory Epithelial Tissue.- Nonauditory Effects of Repeated Exposures to Intense Impulse Noise.- Experimental and Analytical Studies of Blast Wave Effects on Major Organ Systems of the Body.- Complex and Interactin Effects of Noise.- Hearing in Fishermen and Coastguards.- Interactions Between Different Classes of Noise.- Some Issues Associated with Interactions Between Ototoxic Drugs and Exposure to Intense Sounds.- Hearing and Endocrine function.- A Pathway for the Interaction of Stress and Noise Influences on Hearing.- Implications for Noise Standards.- The Effects of Age, Otological Factors and Occupational Noise Exposure on Hearing Threshold Levels of Various Populations.- Current Perspectives on Issues in Personal Hearing Protection.- Hearing Conservation and Impulse Noise in the British Army.- Mathematical Simulation of the Cochlear Mechanism Applied to Damage-Risk Criteria for Impulse Noise.- Acoustic Reflex and Exchange Rate for White Noise Short Stimuli.- The Proposed ISO Standard Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment.- Presidents Farewell Address.- French Abstracts.- Contributors.

Gap detection by the chinchilla

Five monaural chinchillas were trained with a method of shock-avoidance conditioning to respond to silent intervals, or gaps, in an otherwise noise. The noise was low-pass filtered at either 10 or 6 kHz and presented at six intensities ranging between 23- and 77- dB sound-pressure level (SPL). Gap detection thresholds were determined according to the method of constant stimuli. For both noise bands, gap thresholds were approximately 3 ms at the highest intensity levels and increased to approximately 6 ms at the lowest level. The results obtained from the chinchilla are in general agreement with those obtained from man.

Response patterns of auditory nerve fibers during temporary threshold shift

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

Discharge patterns in the cochlear nucleus of the chinchilla following noise induced asymptotic threshold shift.

SummaryChinchillas were exposed to an 86 dB SPL octave band of noise centered at 4.0 kHz for 3.5–5 days. The noise elevated the hearing thresholds between 4.0 and 16.0 kHz to between 60 and 75 dB SPL. Measurements from single neurons in the cochlear nucleus revealed abnormalities in the response properties of neurons with characteristic frequencies (CF) above 2.0 kHz. Units above 2.0 kHz had elevated thresholds (between 50 and 90 dB SPL) and broad tuning curves due to a greater loss in sensitivity near CF than at lower frequencies. The tuning curve Q10dB values for high frequency neurons were generally less than 3.0 and approached the Q10dB values for basilar membrane displacement. Spontaneous activity rates in units above 2.0 kHz were also low. In a few units, the threshold for single tone inhibition was significantly lower than that for excitation; the best inhibitory frequencies were always below 2.0 kHz. Two-tone inhibition was present in both low and high threshold neurons, but its strength was not assessed. Cochleagrams obtained 12 hours postexposure revealed discrete hair cell lesions in the basal third of the cochlea. The locations of the lesions were consistent with the frequencies of maximum hearing loss. The behavioral thresholds and the thresholds at CF of the most sensitive units were within 10–15 dB of each other. The results indicate that intense sounds reduce the sensitivity, frequency selectivity and spontaneous activity of units in the cochlear nucleus. The findings are similar to those obtained in auditory nerve fibers with ototoxic drugs and hypoxia.

Impulse noise: some definitions, physical acoustics and other considerations.

An overview of the impulse noise (blast wave) stimulus is presented with an emphasis on examining those parameters that have been traditionally used to quantify the stimulus for the purpose of understanding its effects on hearing.