William A. Ahroon

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.

Complex noise exposures: An energy analysis

Industrial noise environments usually present a complex stimulus to the exposed individual. These environments often contain mixtures of multiply reflected impact noises and a relatively Gaussian broadband noise. Noise exposure standards do not consider the possibility of interactions between the two classes of noise that can exacerbate the amount of hearing trauma. This paper presents the results of a large series of experiments designed to document the hazard posed to hearing from complex noise exposures. Twenty-three groups of chinchillas with 5 to 11 animals per group (total N = 135) were exposed for 5 days to either octave bands of noise, impacts alone, or combinations of impact and octave bands of noise. Evoked potential measures of hearing thresholds and cochleograms were used to quantify the noise-induced trauma. The results show that, for sound exposure levels (SEL) which produce less than approximately 10 dB PTS (permanent threshold shift) or 5% total sensory cell loss, equal-energy exposures tend to produce equivalent effects on hearing. However, there is a range of at least 10 dB in the SEL parameter where hearing loss from equal-energy exposures at a particular SEL can be exacerbated by increasing the repetition rate of the impacts or by the addition of a Gaussian low-level noise. The exacerbation of trauma from the addition of a Gaussian continuous noise is dependent upon the spectrum of that noise.

The energy spectrum of an impulse: Its relation to hearing loss

Permanent threshold shifts obtained from 242 chinchillas that were exposed to various impulse noise paradigms have been related to the energy spectra of the impulses. The impulses were generated by three different shock tubes that produced impulse noise spectra whose A-weighted energies showed peaks at 0.25, 1, and 2 kHz. The results show that there is an increasing susceptibility to NIPTS as the audiometric test frequency increases from 0.5 to 16 kHz. This increase in susceptibility to NIPTS is further accentuated by approximately 5 to 10 dB for impulses whose spectra peak at 2 kHz.

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.

Comparison of Auditory-Evoked Potentials and Behavioral Thresholds in the Normal and Noise-Exposed Chinchilla

Auditory sensitivity was tested in three monaural chinchillas using standard techniques and the auditory-evoked potential technique (AEP). Hearing was measured at octave steps from 500 to 8000 Hz before, 1 day after, and 30 days after exposure to a simultaneous combination of 50 impulses (A duration = 30 microseconds) presented at a rate of of 1/min and at 158 dB pe SPL and continuous noise (1 h of 95 dB 2-4 kHz octave band of noise). The two independent assessments of auditory sensitivity showed good agreement and the results support the use of AEP testing in experimental animals.

Threshold recovery functions following impulse noise trauma

An analysis of the pure-tone threshold recovery functions obtained from 118 chinchillas exposed to high-level impulse noise showed that there are at least three distinctly different types of recovery function: type I--a recovery function for which the initial threshold shift recovers monotonically with increasing postexposure time; type II--a delayed recovery; i.e., for a period as long as 6 h following removal from noise, the pure-tone threshold remains elevated and stable before thresholds begin to follow a monotonic course of recovery; and type III--the growth function; i.e., over a period of at least 6 h following removal from the noise, pure-tone thresholds continue to get worse before they begin to follow a monotonic course of recovery. There is more permanent threshold shift (PTS), more sensory cell loss, and predictions of PTS and cell loss based upon initial measures of threshold shift are less accurate at those frequencies characterized by a type III recovery process than at those frequencies characterized by a type I recovery process.

Interrupted noise exposures: Threshold shift dynamics and permanent effects

A parametric study of the reduction of threshold shift (toughening phenomena) that takes place during the course of an interrupted noise exposure is described. 266 chinchillas randomly assigned to one of 32 experimental groups were exposed to one of the following: a 400-Hz narrow-band impact noise having a center frequency of 0.5, 1.0, 2.0, 4.0, or 8.0 kHz and peak sound-pressure levels of 109, 115, 121, or 127 dB. The impacts were presented for 5 d, 24 h/d or for 20 d, 6 h/d. corresponding pairs of exposures had equal energy. Group mean noise effects were estimated from pure-tone threshold obtained form inferior colliculus evoked potentials and from surface preparation histology. The threshold shift (TS) toughening phenomena is shown to occur in response to all stimuli that produce a TS and at all audiometric test frequencies. The amount of toughening, which is limited to less than 35 dB, varies with noise frequency and intensity. Based on group mean data the auditory system is not protected from the permanent effects of an interrupted noise exposure as a result of the toughening effect but rather differences in permanent effects between the 5- and 20-d exposures are attributed to the spreading of the exposure energy over an extended period of time.

An isohazard function for impulse noise

Existing criteria for safe exposure to impulse noise do not consider the frequency spectrum of an impulse as a variable in the evaluation of the hazards to the auditory system. This study was designed to determine the relative potential that impulsive energy concentrated at different frequencies has in causing auditory system trauma. One hundred and thirty (130) chinchillas, divided into 22 groups of 5 to 7 animals, were used. Pre‐ and postexposure audiograms were measured on each animal using avoidance conditioning procedures. Quantitative histology was used to determine the extent and pattern of the sensory cell damage (cochleograms). Noise exposure employed seven different computer‐generated narrow‐band impulses (approximately 400‐Hz bandwidth) having center frequencies located at 0.260, 0.775, 1.025, 1.350, 2.075, 2.450, and 3.550 kHz, presented at two to four different intensities. An isohazard weighting function derived from the audiometric and histological data demonstrates that equivalent amounts ...

Noise and vibration interactions: Effects on hearing

There is the suggestion in the literature that vibration may potentiate the effects of noise and may thus increase the risk of hearing loss in a variety of exposure situations. However, in human experimental studies, which, by necessity, are limited to low levels of exposure, the effects measured are relatively small. A very limited number of animal studies have also shown an enhanced noise-induced hearing loss in the presence of vibration, but the scope of these studies is limited. The animal studies (chinchilla) that form the basis of this report were performed using a 30-Hz, 3g rms and a 20-Hz, 1.3g rms cage vibration separately and in combination with continuous noise (95-dB, 0.5-kHz octave band) and impact noise (113, 119, or 125 dB peak SPL) exposure paradigms. All exposures lasted for 5 days. The impact noise exposures were designed to have approximately equal total energy. Temporary and permanent threshold shifts were measured using evoked potentials, and sensory cell loss was measured using surface preparation histology. The results obtained from some of the noise/vibration paradigms showed that such exposures can alter some of the dependent measures of hearing. This effect was statistically significant only for the stronger vibration exposure conditions and was evident primarily in the extent of the outer hair cell losses and in the shape of the PTS audiogram.

Evoked potentials: computer-automated threshold-tracking procedure using an objective detection criterion.

A computer-automated, threshold-tracking procedure was developed for measuring evoked response thresholds on-line. The procedure utilizes an objective detection criterion (correlation) to evaluate whether a response is absent or present. Sound intensity is adjusted according to a modified PEST (parameter estimation by sequential testing) procedure in order to estimate threshold. The computer-automated procedures were used to obtain objective estimates of evoked response thresholds in normal and hearing-impaired chinchillas. The evoked response waveforms stored in the computer were also used to obtain visual judgments of threshold. The objective thresholds determined by the computer were virtually identical to the visual detection thresholds. Thus, the computer-automated procedure provides a reliable, objective, and efficient method of estimating evoked response thresholds.