Developmental exposure to Aroclor 1254 produces low-frequency alterations in adult rat brainstem auditory evoked responses

Developmental exposure of Long-Evans rats to 0, 1, 4, or 8 mg/kg/day Aroclor 1254 (A1254) from Gestational Day 6 through Postnatal Day 21 produces an elevated behavioral threshold for a 1-kHz tone. Brainstem auditory evoked responses (BAERs) were assessed in a subset of these animals (about 1 year old) using filtered clicks at 1 (65 and 80 dB SPL), 4 (60 and 80 dB SPL), 16 (40 and 80 dB SPL), and 32 (40 and 80 dB SPL) kHz. Aroclor 1254 decreased BAER amplitudes at 1 and 4 kHz, but not at 16 or 32 kHz. A dose-related decrease in the baseline-to-peak P1A amplitude was observed for the 1-kHz (80-dB) stimulus. Doses of 1, 4, or 8 mg/kg/day A1254 decreased the peak-to-peak amplitude of both P1AN1 and P1BN1 for a 1-kHz (80-dB) stimulus. Doses of 4 and 8 mg/kg/day A1254 decreased the peak-to-peak amplitude of N1P2 and P2N2 for a 4-kHz (60-dB) or 1-kHz (80-dB) stimulus. At 8 mg/kg/day, A1254 also increased the latency of peak P4 at 1 kHz (65 dB). The decreases in peak P1A amplitudes are consistent with a dysfunction of the cochlea and/or auditory nerve. Together, the data confirm that developmental exposure of rats to A1254 produces a permanent low- to mid-frequency auditory dysfunction and suggest a cochlear and/or auditory nerve site of action.     

Changes in distortion product otoacoustic emissions during prolonged noise exposure

The aim of this study was to evaluate the reduction in 2f1-f2 distortion product otoacoustic emission (DPOAE) amplitude resulting from prolonged noise exposures. A group of five chinchillas was exposed continuously to an octave-band noise centered at 4.0 kHz for a total of 42 days, 6 days at each of seven exposure levels. Exposure level increased in 8-dB steps from 48 to 96 dB SPL. DPOAE input-output (I/O) functions were measured at octave intervals over a range of primary tone f2 frequencies between 1.2 and 9.6 kHz. Measurements were obtained (1) pre-exposure, (2) during days 3-6 of each 6-day exposure, and (3) 4 weeks after the final exposure. Continuous noise exposure caused a reduction in DPOAE amplitude that was greatest at f2 frequencies within and above (3.4-6.8 kHz) the octave-band noise exposure. For these f2 frequencies, DPOAE amplitudes decreased as exposure level increased up to approximately 72-80 dB SPL; higher exposure levels failed to cause any further reduction in DPOAE amplitude. The noise level at which DPOAE amplitude began to decrease was approximately 50 dB SPL. Above this critical level, DPOAE amplitude decreased 1.3 dB for every dB increase in noise level up to approximately 75 dB SPL.     

R-phenylisopropyladenosine attenuates noise-induced hearing loss in the chinchilla

Reactive oxygen species, which are cytotoxic to living tissues, are thought to be partly responsible for noise-induced hearing loss. In this study R-phenylisopropyladenosine (R-PIA), a stable non-hydrolyzable adenosine analogue which has been found effective in upregulating antioxidant enzyme activity levels, was topologically applied to the round window of the right ears of chinchillas. Physiological saline was applied to the round window of the left ears (control). The animals were then exposed to a 4 kHz octave band noise at 105 dB SPL for 4 h. Inferior colliculus evoked potential thresholds and distortion product otoacoustic emissions (DPOAE) were measured and hair cell damage was documented. The mean threshold shifts immediately after the noise exposure were 70-90 dB at frequencies between 2 and 16 kHz. There were no significant differences in threshold shifts at this point between the R-PIA-treated and control ears. By 4 days after noise exposure, however, the R-PIA-treated ears showed 20-30 dB more recovery than saline-treated ears at frequencies between 4 and 16 kHz. More importantly, threshold measurements made 20 days after noise exposure showed 10-15 dB less permanent threshold shifts in R-PIA-treated ears. The amplitudes of DPOAE also recovered to a greater extent and outer hair cell losses were less severe in the R-PIA-treated ears. The results suggest that administration of R-PIA facilitates the recovery process of the outer hair cell after noise exposure.     

Infants' detection of increments in low- and high-frequency noise

A visually reinforced operant paradigm was employed to examine the relationship between the difference limen (DL) for intensity and level of the standard during infancy. In Experiment 1, 7-month-old infants and adults detected increments in continuous noise presented via headphones at each of four levels ranging from 28 to 58 dB SPL. Noise stimuli were 2-octave bands centered at either 400 or 4000 Hz, and increments were 10 and 100 msec in duration. Infants' DLs were significantly larger than those of adult subjects and significantly larger for low- than for high-frequency stimuli. For the high-frequency noise band, infants' DLs were generally consistent with Weber's law, remaining essentially constant for standards higher than 28 dB SPL (3 dB SL) for 100-msec increments and 38 dB SPL (13 dB SL) for 10-msec increments. For low-frequency noise, infants' absolute thresholds were exceptionally high, and sensation levels of the standards were too low to adequately describe the relationship. In Experiment 2, 7-month-old infants detected 10- and 100-msec increments in 400-Hz noise stimuli presented in sound field. Infants' low-frequency DLs were large at low intensities and decreased with increases in level of the standard up to at least 30 dB SL. For both low- and high-frequency noise, the difference between DLs for 10- and 100-msec increments tended to be large at low levels of the standard and to decrease at higher levels. These results suggest that the relationship between the DL and level of the standard varies with both stimulus frequency and duration during infancy. However, stimulus-dependent immaturities in increment detection may be most evident at levels within approximately 30 dB of absolute threshold.     

Effect of low-frequency gain reduction on speech recognition and its relation to upward spread of masking

Speech recognition was measured in listeners with normal hearing and in listeners with sensorineural hearing loss under conditions that simulated hearing aid processing in a low-pass and speech-shaped background noise. Differing amounts of low-frequency gain reduction were applied during a high-frequency monosyllable test and a sentence level test to simulate the frequency responses of some commercial hearing aids. The results showed an improvement in speech recognition with low-frequency gain reduction in the low-pass noise, but not in the speech-shaped background noise. Masking patterns also were obtained with the two background noises at 70 and 80 dB SPL to compare with the speech results. There was no correlation observed between the masking results and the improvement in speech recognition with low-frequency gain reduction.     

Blood pressure measurement. Criteria employed in scientific articles published in Brazilian journals]

OBJECT: To investigate whether hand-arm vibration and noise have a combined effect on temporary threshold shift (TTS) of hearing among healthy subjects. METHOD AND DESIGN: Nineteen healthy subjects with an average age of 25.7 (SD 7.7) years were exposed to vibration (30 m/s2, 60 Hz), noise [90 dB(A)] and both, respectively. The subject's right hand was placed on the plate of a vibrator and the right ear exposed to noise via headphones. Subjects were exposed to vibration and/or noise for 3 min and after a 1-min pause the exposure was repeated five times. Hearing thresholds at 1, 4 and 6 kHz were measured during the time periods before, between (during pauses) and after exposure. RESULTS: Exposure to vibration alone caused almost no hearing threshold changes at every frequency tested. But exposure to noise or a combination of vibration and noise caused a significant increase in TTSs at 4 and 6 kHz. Moreover, exposure to a combination of vibration and noise caused significantly higher TTSs than exposure to noise at 4 and 6 kHz. CONCLUSION: The present results demonstrate the combined effects of hand-arm vibration and noise can enhance exposure to hand-arm vibration and noise can enhance the TTS of hearing more than noise exposure, though hand-arm vibration alone may hardly affect TTS.     

Remote masking generated by high-frequency two-tone complexes

Auditory masking generated by two-tone complexes centered around 7 000 Hz was measured in 10 young adults with normal hearing sensitivity as a function of the frequency separation (deltaf) and SPL of the masker's components. Remote masking (1) was evident for test signals in the frequency region corresponding to the masker's deltaf; (2) increased with masker SPL, but at a rate less than that usually observed when lower frequency bands of noise are used as maskers, and (3) was relatively constant in magnitude for a given SPL as a function of the masker's deltaf. The masking produced in low-frequency regions by high-frequency two-tone complexes adds support to the hypothesis that remote masking is primarily a result of aural distortion.     

Effects of stapedius-muscle contractions on the masking of auditory-nerve responses

There has been little exploration of the mechanisms by which stapedius muscle contractions reduce the masking of responses to high-frequency sounds by low-frequency sounds. To fill this gap in knowledge, controlled stapedius contractions were elicited with direct shocks in anesthetized cats, and measurements were made of the effects of these contractions on the masking of single auditory-nerve fibers and on the attenuation of middle-ear transmission. The results show that the stapedius-induced reductions of masking can be much larger than the attenuations of low-frequency sound. With a 300-Hz band of masking noise centered at 500 Hz, and signal tones at 6 or 8 kHz, unmasking effects over 40 dB were observed for sounds 100 dB SPL or less. The data suggest that much larger unmasking might occur. The observed unmasking can be explained completely by a linear stapedius-induced attenuation of sound transmission through the middle ear and a nonlinear growth rate of masking for auditory-nerve fibers. No central effects are required. It is argued that the reduction of the upward spread of masking is probably one of the most important functions of the stapedius muscle.     

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.     

István Csapody

This pilot study compared the susceptibility of the infant (48 hr) and adult (120 days) guinea pig to the effects of noise. Subjects were exposed to a narrow band of noise (center frequency 4 kHz) at an intensity of 115 dB sound pressure level (SPL) for 1 hr. Postexposure thresholds were obtained by a conditioned suppression technique. Results indicated that the infant animals displayed a mean hearing threshold of 25 dB SPL that significantly differed from the adult mean threshold of 7.5 dB SPL.     

Hemodynamic effects of short-term noise exposure--comparison of steady state and intermittent noise at several sound pressure levels

The purpose of the present study was to investigate the extent of blood pressure elevation during noise exposure, to elucidate the underlying hemodynamic mechanisms and to assess baroreceptor cardiac reflex sensitivity in connection with blood pressure elevation. Twenty-two young normotensive males participated in the experiment and underwent six noise exposure conditions of 20 min each: steady state and intermittent pink noises of 80 dB (sound pressure level (SPL)), 90 dB (SPL) and 100 dB (SPL). The results indicate that elevations in mean arterial pressure, as well as diastolic and systolic blood pressure, were significant or almost significant in the intermittent 100 dB (SPL) and 90 dB (SPL) conditions. Habituation occurred particularly with the steady state noises. In at least the intermittent 100 dB (SPL) condition, an increase in peripheral vascular resistance was the underlying hemodynamic mechanism of blood pressure elevation. Decreases in cardiac output and stroke volume were also associated with the peripheral vasoconstriction. Baroreceptor reflex sensitivity was maintained near the baseline level for all of the noise exposure conditions. Therefore, reflex sensitivity may not have been suppressed even in the intermittent 100 dB (SPL) condition during which blood pressure elevations occurred.     

Low-frequency suppression of auditory nerve responses to characteristic frequency tones

The effects of low-frequency (50, 100, 200 and 400 Hz) 'suppressor' tones on responses to moderate-level characteristic frequency (CF) tones were measured in chinchilla auditory nerve fibers. Two-tone interactions were evident at suppressor intensities of 70-100 dB SPL. In this range, the average response rate decreased as a function of increasing suppressor level and the instantaneous response rate was modulated periodically. At suppression threshold, the phase of suppression typically coincided with basilar membrane displacement toward scala tympani, regardless of CF. At higher suppressor levels, two suppression maxima coexisted, synchronous with peak basilar membrane displacement toward scala tympani and scala vestibuli. Modulation and rate-suppression thresholds did not vary as a function of spontaneous activity and were only minimally correlated with fiber sensitivity. Except for fibers with CF < 1 kHz, modulation and rate-suppression thresholds were lower than rate and phase-locking thresholds for the suppressor tones presented alone. In the case of high-CF fibers with low spontaneous activity, excitation thresholds could exceed suppression thresholds by more than 30 dB. The strength of modulation decreased systematically with increasing suppressor frequency. For a given suppressor frequency, modulation was strongest in high-CF fibers and weakest in low-CF fibers. The present findings strongly support the notion that low-frequency suppression in auditory nerve fibers largely reflects an underlying basilar membrane phenomenon closely related to compressive non-linearity.     

Epidemiologic analysis and clinical course of 84 consecutive cases of pemphigus in eastern Sicily

The effects of intense sound exposure on neural activity in the dorsal cochlear nucleus (DCN) were studied in the rat. Seventeen anesthetized adult rats were exposed to a 10-kHz tone at 125-130 dB SPL for 4 h. Fourteen unexposed rats served as controls. Spontaneous activity (SA) and neural thresholds at the characteristic frequency were measured in three rows of 8-12 sites along the mediolateral, tonotopic, axis of the DCN surface 27-61 days after exposure. The results showed that intense tone exposure induced chronic increases in SA. This hyperactivity was found to be distributed broadly across the DCN with an emphasis around the 10-kHz locus and was associated with shifted response thresholds. These findings demonstrate the usefulness of the rat for studies of physiological phenomena related to noise-induced tinnitus and hearing loss.     

Acoustic startle threshold of the albino rat (Rattus norvegicus).

The startle threshold of the albino Sprague-Dawley rat runs parallel to the curve of the hearing threshold. The difference between the startle and hearing threshold is 87 dB (SPL) at a background noise level of 75 dB (SPL). At 110 dB (SPL), the threshold has a range from 2 kHz to 50 kHz with a minimum at 10 kHz and a second minimum at 40 kHz. Amplitude and latency of the startle response are not only dependent on the sensation level of the acoustic stimulus but also on the frequency. At threshold, only the head movement component of the startle response is elicited. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of vestibular nerve section on cytochrome oxidase activity in the vestibular ganglion cells of the squirrel monkey

Estimates of behavioral thresholds of infants are elevated relative to those of adults. Explanations for the differences include auditory sensory factors and non-sensory factors, but no direct estimates of the relative contributions of these two factors have been made. In this investigation, thresholds in quiet and in increasing levels of a masking noise for a 1 kHz tone, in infants 8 to 11 months old and in adults, were determined. The infant-adult differences in unmasked threshold was compared to the infant-adult difference in an estimate of the minimum masking level (MML) that was derived from the masking data. The intensity level of a masking noise at which masking begins is assumed to be independent of the non-sensory factors that impact on the threshold value itself. Therefore, it is reasoned that the infant-adult difference in MML reflects more closely differences in auditory sensory factors than does the infant-adult difference in unmasked threshold. In the region of 1 kHz, the infant-adult difference in behavioral threshold was 12 dB and the infant-adult difference in MML was 8 dB. Therefore, according to our assumptions, 8 dB of the infant-adult difference in unmasked threshold is accounted for by sensory factors and the remaining 4 dB must be attributable to non-sensory factors.     

Effects of bilateral auditory cortical lesions on gap-detection thresholds in the ferret (Mustela putorius).

Ferrets were tested for their ability to detect temporal gaps in noise before and after bilateral lesions of the primary auditory cortex. Thresholds for gap detection were determined first for normal animals with band-pass noises at various center frequencies (0.5 to 32 kHz) and at 8 kHz with various sound pressure levels (-10 to 70 dB). Gap-detection ability improved steadily as sound pressure increased up to 70 dB. No systematic relation was found between threshold and center frequency. To determine the effects of brain damage, ferrets were tested with 8-kHz band-pass noise at 70 dBSPL. After bilateral lesions of auditory cortex, ferrets were still capable of detecting gaps, but the mean threshold was elevated from 10.1 to 20.1 ms. The data demonstrate that auditory cortex is important for perceptual tasks requiring fine temporal resolution. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Recovery of the human compound action potential following prior stimulation

The recovery from prior stimulation of the compound action potential (CAP) was measured using a forward masking stimulus paradigm in four normal-hearing, human subjects. The CAP was recorded using a wick electrode placed on the tympanic membrane. The effects of a 4000-Hz, 97-dB SPL conditioning stimulus on CAP amplitude in response to a 4000-Hz probe were measured as a function of conditioner-probe interval for three probe levels. The normalized probe response amplitude was completely recovered to the control values at an average conditioner-probe interval of 1359 ms, similar to that observed in chinchilla (Relkin, E.M., Doucet, J.R., Sterns, A., 1995. Recovery of the compound action potential following prior stimulation: evidence for a slow component that reflects recovery of low spontaneous-rate auditory neurons, Hear. Res. 83, 183-189). The present results are interpreted as a consequence of the slow recovery of low spontaneous-rate (SR), high threshold neurons from prior stimulation (Relkin, E.M., Doucet, J.R., 1991. Recovery from prior stimulation. I: Relationship to spontaneous firing rates of primary auditory neurons. Hear. Res. 55, 215-222) and may provide indirect physiological evidence for the existence of a class of low-SR auditory neurons in humans.     

Temporal integration at 6 kHz as a function of masker bandwidth

Thresholds were measured for a 6-kHz sinusoidal signal presented within a 500-ms masker. The masker was either a bandpass Gaussian noise of varying bandwidth, or a sinusoid of the same frequency as the signal. The spectrum level of the noise masker was kept constant at 20 dB SPL, and the level of the sinusoidal masker was 40 dB SPL. Thresholds for signal durations between 2 and 300 ms were measured for masker bandwidths ranging from 60 to 12,000 Hz. The masker was spectrally centered around 6 kHz. For masker bandwidths less than 600 Hz, the slope of the temporal integration function decreased with decreasing masker bandwidth. The results are not consistent with current models of temporal integration or temporal resolution. It is suggested that the results at narrow bandwidths can be understood in terms of changes in the power spectrum of the stimulus envelope or modulation spectrum. According to this view, the onset and offset ramps of the signal introduce detectable high-frequency components into the modulation spectrum, which provide a salient cue in narrowband maskers. For broadband maskers, these high-frequency components are masked by the inherent rapid fluctuations in the masker envelope. Additionally, for signal durations between 7 and 80 ms, signal thresholds decreased by up to 5 dB as the masker bandwidth increased from 1200 to 12,000 Hz. The mechanisms underlying this effect are not yet fully understood.     

The effects of decreased audibility produced by high-pass noise masking on cortical event-related potentials to speech sounds/ba/and/da

This study investigated the effects of decreased audibility produced by high-pass noise masking on cortical event-related potentials (ERPs) N1, N2, and P3 to the speech sounds /ba/and/da/presented at 65 and 80 dB SPL. Normal-hearing subjects pressed a button in response to the deviant sound in an oddball paradigm. Broadband masking noise was presented at an intensity sufficient to completely mask the response to the 65-dB SPL speech sounds, and subsequently high-pass filtered at 4000, 2000, 1000, 500, and 250 Hz. With high-pass masking noise, pure-tone behavioral thresholds increased by an average of 38 dB at the high-pass cutoff and by 50 dB one octave above the cutoff frequency. Results show that as the cutoff frequency of the high-pass masker was lowered, ERP latencies to speech sounds increased and amplitudes decreased. The cutoff frequency where these changes first occurred and the rate of the change differed for N1 compared to N2, P3, and the behavioral measures. N1 showed gradual changes as the masker cutoff frequency was lowered. N2, P3, and behavioral measures showed marked changes below a masker cutoff of 2000 Hz. These results indicate that the decreased audibility resulting from the noise masking affects the various ERP components in a differential manner. N1 is related to the presence of audible stimulus energy, being present whether audible stimuli are discriminable or not. In contrast, N2 and P3 were absent when the stimuli were audible but not discriminable (i.e., when the second formant transitions were masked), reflecting stimulus discrimination. These data have implications regarding the effects of decreased audibility on cortical processing of speech sounds and for the study of cortical ERPs in populations with hearing impairment.     

Loudness scaling in rats and chinchillas.

In 2 experiments, 10 male chinchillas and 24 male Sprague-Dawley-derived rats were trained on operant discriminations in which the discriminative stimuli were 2 different sound pressure levels of a 4-kHz tone. Two or more of these 2-intensity discriminations were used at each of 3 levels of discriminability: high, medium, and low. For any given level, each of the stimulus pairs used differed in decibel separation but were similar in loudness-unit differences calculated from a power function. Different groups of Ss trained on stimuli separated by equal numbers of loudness units produced equivalent performances at each of the 3 levels of discriminability. It is concluded that loudness growth for both of these species, as for humans, is well described by a power function (S. S. Stevens's 1961 law). For the chinchilla the exponent is .25, and for the rat it is .35. (25 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)