Anticipatory middle-ear reflex activity from noisy toys

Human middle-ear muscles rarely contract in anticipation of acoustic impulses: Implications for hearing risk assessments

The current study addressed the existence of an anticipatory middle-ear muscle contraction (MEMC) as a protective mechanism found in recent damage-risk criteria for impulse noise exposure. Specifically, the experiments reported here tested instances when an exposed individual was aware of and could anticipate the arrival of an acoustic impulse. In order to detect MEMCs in human subjects, a laser-Doppler vibrometer (LDV) was used to measure tympanic membrane (TM) motion in response to a probe tone. Here we directly measured the time course and relative magnitude changes of TM velocity in response to an acoustic reflex-eliciting (i.e. MEMC eliciting) impulse in 59 subjects with clinically assessable MEMCs. After verifying the presence of the MEMC, we used a classical conditioning paradigm pairing reflex-eliciting acoustic impulses (unconditioned stimulus, UCS) with various preceding stimuli (conditioned stimulus, CS). Changes in the time-course of the MEMC following conditioning were considered evidence of MEMC conditioning, and any indication of an MEMC prior to the onset of the acoustic elicitor was considered an anticipatory response. Nine subjects did not produce a MEMC measurable via LDV. For those subjects with an observable MEMC (n = 50), 48 subjects (96%) did not show evidence of an anticipatory response after conditioning, whereas only 2 subjects (4%) did. These findings reveal that MEMCs are not readily conditioned in most individuals, suggesting that anticipatory MEMCs are not prevalent within the general population. The prevalence of anticipatory MEMCs does not appear to be sufficient to justify inclusion as a protective mechanism in auditory injury risk assessments.

Detecting incipient inner-ear damage from impulse noise with otoacoustic emissions

Audiometric thresholds and otoacoustic emissions (OAEs) were measured in 285 U.S. Marine Corps recruits before and three weeks after exposure to impulse-noise sources from weapons' fire and simulated artillery, and in 32 non-noise-exposed controls. At pre-test, audiometric thresholds for all ears were <or=25 dB HL from 0.5 to 3 kHz and <or=30 dB HL at 4 kHz. Ears with low-level or absent OAEs at pre-test were more likely to be classified with significant threshold shifts (STSs) at post-test. A subgroup of 60 noise-exposed volunteers with complete data sets for both ears showed significant decreases in OAE amplitude but no change in audiometric thresholds. STSs and significant emission shifts (SESs) between 2 and 4 kHz in individual ears were identified using criteria based on the standard error of measurement from the control group. There was essentially no association between the occurrence of STS and SES. There were more SESs than STSs, and the group of SES ears had more STS ears than the group of no-SES ears. The increased sensitivity of OAEs in comparison to audiometric thresholds was shown in all analyses, and low-level OAEs indicate an increased risk of future hearing loss by as much as ninefold.

Validation of the auditory hazard assessment algorithm for the human with impulse noise data

Predicting auditory hazard from intense acoustic impulses, such as weapons fire or airbags, has been an intractable problem. The U.S. Army developed a theoretically based mathematical model of the ear designed to predict such hazards [the Auditory Hazard Assessment Algorithm for the Human (AHAAH)]. To validate it as a predictor of hazard, data from the literature (wave forms and changes in hearing sensitivity) were processed with the model in order to predict the onset of unacceptable threshold shift (25 dB or more) in the 95th percentile human ear. For comparison, alternate standards MIL-STD-1747D and A-weighted energy were also used to compute hazards for the same data. The primary dataset was that of the US Army's "Albuquerque studies" (53 different cases) and other impulses from the literature (19 additional predictions). The AHAAH model predicted correctly in over 95% of the cases, the MIL-STD-1474D was correct in 42% of the cases, and A-weighted energy was correct in 25% of the cases. Errors for all methods tended to be in the direction of overprediction of hazard. In addition to greatly increased accuracy, the AHAAH model also has the advantage of being theoretically based and including novel diagnostic features.

Predicting mechanical damage to the organ of Corti

The potential for pharmacological intervention to ameliorate the effects of exposure to intense auditory stimulation is a truly exciting possibility. In theory, the effects of intense stimulation could be primarily a function of mechanical stress and its sequelae or possibly metabolic exhaustion. Conceivably, specific pharmacological therapies might be more effective following different types of insult, depending on the loss mechanism(s) involved. The Auditory Hazard Assessment Algorithm for the Human (AHAAH), a first-principles mathematical model for the ear, has been developed specifically to predict hazard at high intensities based on basilar membrane displacement. Validation studies have proven it to be accurate in rating risk for the human ear. AHAAH is available for download on the Internet. In the present context it was used to propose analytic stimuli that would help to elucidate the loss mechanisms and also to identify exposures for the clinician that should be considered as sufficiently hazardous to warrant potential pharmacological intervention either before or after the exposure.

Temporary threshold shift after exposure to predicted and unpredicted noise

In order to investigate if anticipation of noise could influence individual susceptibility to temporary threshold shift (TTS), an experimental study was carried out comprising 10 voluntary normal-hearing subjects. The noise stimulus consisted of noise bursts centered at 4000 Hz with a duration of one second. Initially, each subject was exposed in a test situation where the noise was presented instantly upon the subject's response (producer). The subject was instructed to respond by pressing a button as fast as possible after an optical stimulation. The rate was approximately 1 noise burst/6 sec and the total number of bursts were 100. The intervals between bursts were randomized. Each individual's exposure time pattern was recorded and used for the second test situation. Here the first exposure was reproduced, but each noise burst presented without notice (consumer). Each subject participated in 5 producer sessions and in 5 consumer sessions. Evaluation of TTS measurements revealed a maximum TTS at 6000 Hz. The mean TTS for all subjects in the two different situations, showed no significant difference and only one subject demonstrated a tendency toward a greater TTS susceptibility in the 'consumer' sessions.