D. Henderson

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.

Comparison of psychophysical and evoked-potential tuning curves in the chinchilla

Frequency selectivity was examined in normal-hearing chinchillas using psychophysical and evoked-potential tuning curves. The acoustic conditions and masking procedures used for the evoked-potential and psychophysical studies were nearly identical. Frequency selectivities as measured by psychophysical and physiologic techniques were quite similar across different probe frequencies. The results suggest that the relatively efficient evoked-potential procedure may be substituted for the time-consuming psychophysical paradigm. Furthermore, the results are consistent with the view that tuning takes place primarily at the auditory periphery.

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.

Gap detection in hearing‐impaired chinchillas

Auditory temporal resolution is known to deteriorate with sensorineural hearing loss; however, there is considerable intersubject variability in human studies. The purpose of the present study was to obtain measures of temporal resolution in the chinchilla as the degree of noise-induced hearing loss was systematically varied. Gap-detection thresholds, a measure of temporal resolution, were evaluated at four levels of noise-induced asymptotic threshold shift (ATS). Gap thresholds were normal when the pure-tone thresholds were elevated approximately 15 dB. With a hearing loss of approximately 30 dB, the gap thresholds were longer than normal if compared at the same sound pressure level, but within normal limits if compared at the same sensation level. When the hearing loss exceeded 40 dB, gap thresholds were longer than normal both in terms of sound pressure level and sensation level. These results show that there is an orderly breakdown in temporal resolution as the degree of noise-induced ATS increases. The results are related to neural data and models of temporal resolution.

Neural correlates of sensorineural hearing loss.

Sensorineural hearing loss is characterized by a relatively well defined set of audiological signs and symptoms such as elevated thresholds, abnormally rapid loudness growth, subjective tinnitus, poor speech discrimination, and a reduction in temporal summation of acoustic energy. Knowledge of the underlying neural mechanisms responsible for some of these auditory distortions has progressed substantially within the past 10 yrs as a result of physiological studies on hearing-impaired animals. Some of the important neurophysiological changes relevant to sensorineural hearing loss are reviewed. One important effect associated with sensorineural hearing loss is the broadening of the cochlear filtering mechanism which may influence loudness growth and the perception of complex sounds. The neurophysiological results may also provide new insights in interpreting traditional audiological data and help in developing more refined tests for fitting hearing aids or differentiating patients with sensorineural hearing loss.

Auditory fatigue: retrocochlear components

Changes in auditory sensitivity were measured at the VII nerve, cochlear nucleus, and inferior colliculus after a fatiguing sound exposure. Losses in sensitivity progressively increased from peripheral to central auditory sites. The results suggest that there is a retrocochlear component to auditory fatigue when it is induced by low-level sounds of short duration.

Evoked response forward masking patterns in chinchillas with temporary hearing loss

Evoked response forward masking data were measured from the inferior colliculus of the chinchilla before and during a temporary threshold shift. The hearing loss was induced by a 2 kHz pure tone of 85 dB SPL presented from 5-8 days. The exposure elevated thresholds by approximately 35 dB at the mid frequencies, but had no effect on low frequency hearing. The exposure also altered the time course of the evoked response forward masking data. Time constants fitting the forward masking data increased by up to a factor of three at the frequency with the greatest loss, but remained within normal limits at the low frequencies where hearing was normal. The increase in the forward masking time constants became most noticeable once the hearing loss exceeded 25 dB. These physiological results are consistent with psychophysical forward masking data from hearing impaired listeners.

Single auditory nerve fiber activity and behavioral thresholds in pathological cochleas

Four chinchillas were exposed to 4 kHz CF octave band of noise at 86 dB for five days. After a recovery period of approximately six months, behavioral audiograms were obtained and auditory nerve fiber activity was recorded. The animals were sacrificed and the cochleas embedded in Araldite resin to obtain surface preparations and 1μ sections of the Organ of Corti. Permanent threshold shifts ranged from 5 to 20 dB between 4 and 8 kHz. VIII nerve recordings showed single fiber thresholds were elevated up to 70 dB between 4 and 14 kHz, while higher and lower CF fibers were essentially normal. Spontaneous activity in some of the animals was equal to or greater than normal. Cochleagrams all showed narrow lesions of inner and/or outer hair cells over approximately a 1‐mm distance. A comparison of the three realms of data revealed the following: (1) The greatest loss in sensitivity from the noise exposure was seen in the single nerve fiber thresholds while the smallest change was seen in the behavioral threshold ...

Effects of hearing loss on the detection of amplitude modulated noise

Hearing loss is known to influence auditory temporal processing; there is a decrease in the time constant of temporal integration for tone bursts however, the same hearing loss increases the time constant for gap detection and forward masking. The detection of amplitude modulated signals, another aspect of temporal processing, was studied in three monaural chinchillas using conditioned avoidance procedures. Pre‐exposure pure tone thresholds were measured from 0.25 to 16 kHz and modulation thresholds (0.002 to 4.096 kHz) were obtained using sinusoidally amplitude modulated broad band noise having an SPL of 53 dB. Chinchillas were initially exposed for 7 days to an octave band of noise centered at 8 kHz. The animals developed a high frequency loss of 50–70 dB and the amplitude modulation function showed a decline in sensitivity. Then the noise exposure band was widened over the next 3 weeks. The hearing loss progressively increased toward the low frequencies; however, the amplitude modulation function remai...

Auditory nerve fiber activity, behavioral thresholds and cochlear histology following noise induced asymptotic threshold shift

Chinchillas were exposed three to five days to a 95‐dB SPL octave band of noise centered at 0.5 kHz. During the exposure, the behaviorally measured thresholds reached an asymptotic level that was between 40–60 dB SPL from 0.5–16.0 kHz. During a 27–15 h period following the exposure, single auditory nerve fibers and the whole nerve action potential (AP) were studied with clicks and tone bursts. Single neuron thresholds were generally 40–60 dB above normal values and were in general agreement with the behavioral audiogram. Fiber tuning curves were broader than normal with reduced slopes. Some units had a region of increased sensitivity below CF. Click thresholds for the units were generally 40–60 dB above normal levels. The visual detection level for AP was also 40–60 dB above its normal value and consistent with the single neuron data. The cochleagrams obtained 15 h after the exposure generally showed a peak outer hair cell loss (<50%) near the apex with smaller losses (<20%) at more basal locations. [Rese...