Effects of click intensity and frequency on the brain-stem auditory evoked potentials in the common marmoset (Callithrix jacchus)

Brain-stem auditory evoked potentials (BAEPs) were recorded in 20 common marmosets (Callithrix jacchus) to investigate the effects of click frequency up to 99 kHz, in consideration of the higher hearing range of the marmoset, and intensity on wave forms and peak latencies. According to the results of BAEP recordings at frequencies of 4, 32, and 99 kHz, the number of components recorded was affected by the stimulus intensity and the clicks at an intensity of 80 dB peak equivalent sound pressure level (pe SPL) had the maximum number of clear components. Therefore, it was indicated that click stimulations at an intensity of 80 dB pe SPL over a broad range of frequencies appears to be useful for recording the maximum number of components in marmosets and may increase the information obtainable from BAEPs. BAEP latencies were prolonged as the stimulus intensity decreased from 100 to 50 dB pe SPL. The effects of stimulus frequency on the wave latencies and amplitudes in response to 80 dB pe SPL at frequencies between 0.5 and 99 kHz revealed various changes: the amplitude of wave I increased at 16 and 32 kHz, but that of waves III and V increased at 4-8 and 64-99 kHz. These increases in amplitudes of the waves may correlate with higher synchronous activity of the peripheral or central auditory pathways.     

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.     

Interactions of gonadal steroids and pesticides (DDT, DDE) on gonaduct growth in larval tiger salamanders, Ambystoma tigrinum

The nervous system is dependent upon thyroid hormones for normal development, and we previously reported that developmental Aroclor 1254 (A1254) exposure caused hypothyroxinemia, hearing loss and other behavioral changes in rats. (Goldey et al., 1995a; Herr et al., 1996). The hypothesis that A1254-induced hypothyroxinemia may have contributed to the observed functional changes was tested in primiparous Long-Evans rats given daily oral doses of corn oil (control) or 8 mg/kg of Aroclor 1254 from gestation day (GD) 6 through postnatal day (PND) 21. In addition, from PND 4 to PND 21, all pups in one-half of the litters received daily, subcutaneous injections of saline or 100 micrograms/kg thyroxine (T4), to yield four groups of litters: corn oil plus saline (CO-S),. corn oil plus T4 (CO-T4), Aroclor 1254 plus saline (PCB-S), and Aroclor 1254 plus T4 (PCB-T4). We measured thyroid hormone concentrations (T4 and T3) in serum collected from 7-, 14-, and 21-day-old pups. The kinetics of the injected T4 were also monitored in the CO-T4 and PCB-T4 groups on PND 7 and 21 by measuring T4 and T3 at 1, 3, 5, 8, and 24 h after injection. Circulating T4 concentrations were dramatically depleted in the PCB-S group relative to CO-S. The kinetics study indicated that T4 therapy raised circulating T4 concentrations following in the PCB-T4 pups to near CO-S concentrations, but only for approximately 6 h postinjection, and T4 concentrations fell precipitously thereafter to near PCB-S concentrations. In accord with previous studies, PCB-S pups showed early eye opening, an effect which was exacerbated by T4 injection (in both the CO-T4 and the PCB-T4 groups). Motor activity (figure-eight maze) testing also replicated our finding of an age-dependent, transient reduction in motor activity on PND 15 that was significantly attenuated in the PCB-T4 group. Similarly, we again found reduced acoustic startle amplitudes on PND 23 and low-frequency (1 kHz) hearing loss in animals tested as adults (the latter determined by reflex modification audiometry). Importantly, the hearing loss at 1 kHz in PCB-exposed animals was significantly attenuated by T4 replacement therapy. These data suggest the hypothesis that hypothyroxinemia is involved in PCB-induced alterations in motor and auditory function, while other effects (e.g., eye opening) appear to have a different mechanism of action.     

Effects of bilateral lesions of auditory cortex in mice on the acoustic startle response

The acoustic startle response (ASR) was used to investigate the effects of auditory cortical lesions on a brain stem-mediated auditory behavior. The ASRs were obtained longitudinally from young adult C57BL/6J mice before bilateral ablation of auditory cortex, 1 day after ablation, and 1 month later. Control mice received lesions of nonauditory cortex. For some mice, averaged brain stem-evoked responses (ABR) were obtained, and these indicated no effects of lesions on auditory sensitivity. One month after surgery, mice with auditory cortex ablations were statistically indistinguishable from controls on all suprathreshold measures of ASR. However, 1 day after ablation of auditory cortex, experimental animals (but not controls) exhibited a change in ASR amplitude (but not threshold or latency). When a noise burst of 80 dB SPL was used to elicit the ASR, the amplitude was diminished, but with a 110 dB stimulus, amplitude was enhanced. The findings can be interpreted in one of two ways: temporary interference with modulation of the ASR normally performed by auditory cortex; or a general effect of auditory cortex ablation on brain stem auditory circuits not specific to the ASR. In any event, if auditory cortex plays a modulatory role with regard to the ASR, it is apparently nonessential and/or readily compensated for after ablation.     

Otoacoustic emissions evoked by electrical stimulation

Otoacoustic emissions evoked by electrical stimulation of the cochlea were measured in guinea pigs. Alternating current with acoustic frequencies was delivered directly to the scala tympani and the vestibuli of the basal turn by using a constant current stimulator made in our laboratory. The amplitude of the electrically evoked otoacoustic emmisson (EEOAE), which was 21.0 dB SPL in response to 2 kHz electrical stimuli of 120 microArms, was decreased gradually with anoxia of the animal and was reduced to below the noise level by extirpation of the ossiculum. The EEOAEs rose with a delay of 100 microseconds from the beginning of the stimulus, and reached maximum amplitude within three cycles. The output of EEOAE was magnified in a linear manner as the stimulus current increased from 10 to 200 microArms. The averages and standard deviations of the EEOAE output at the stimulus level of 170 microArms in six animals were 20.5 +/- 4.1 dB SPL for 1 kHz, 23.3 +/- 4.8 dB SPL for 2 kHz, 10.5 +/- 6.0 dB SPL for 3 kHz, 17.1 +/- 4.7 dB SPL for 4 kHz, 13.6 +/- 4.4 dB for 5 kHz and 18.3 +/- 4.8 dB SPL for 6 kHz. Measurement of EEOAE, in which stable responses could be obtained with simple and easy preparation, was considered a potential procedure for assessing the electromotility of the cochlear outer hair cells in vivo.     

Interaction between drug discriminative stimuli and exteroceptive, sensory signals.

Interactions between drug discriminative stimuli based on 5.6 and 10 mg/kg sodium pentobarbital (ip) and exteroceptive stimuli (visual and auditory) were studied in 27 male Sprague-Dawley rats in a T-maze. In 3 groups, visual stimuli (light vs dark) were differentially paired with drug stimuli; the 4th group discriminated combinations of tonal frequencies (1 kHz or 10 kHz) and the presence or absence of pentobarbital (10 mg/kg). In general, visual stimuli controlled choice behavior (left or right turn) to a greater extent than did the drug training stimuli, whereas the auditory stimuli exerted no apparent control over the pentobarbital stimulus in Group 4. Tests with higher doses (13.75 and 17.5 mg/kg) indicated augmented stimulus control by the drug dimension in 2 groups (Group 1, 10 mg/kg pentobarbital vs saline; Group 2, 5.6 mg/kg vs 10 mg/kg pentobarbital) but not in the 3rd group (5.6 mg/kg pentobarbital vs saline) in the "conflict" situation, in which the exteroceptive conditions signaled one response whereas the drug stimulus signaled the opposite response. Discrimination training with only one of the stimulus dimensions resulted in stimulus control in the following order: 10 mg/kg vs saline?>?5.6 mg/kg vs saline?>?1 kHz vs 10 kHz. This indicates that the auditory stimuli were of marginal significance. It is concluded drugs can compete with exteroceptive, visual stimuli for associative strength. (23 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

2f1-f2 distortion product otoacoustic emissions in White Leghorn chickens (Gallus domesticus): effects of frequency ratio and relative level

The effects of primary tone frequency ratio (f2/f1 ratio) and relative level (L2/L1) on the amplitude of the cubic difference tone (CDT: 2f1-f2) distortion product otoacoustic emissions (DPOAEs) were investigated in adult White Leghorn chickens (Gallus domesticus). In experiment 1, 9 f2/f1 ratios ranging from 1.05 to 1.8 were investigated. Measurements were obtained from both ears of 4 chickens at 7 f1 frequencies ranging from 0.8 to 4.0 kHz. The primary tones were equal in level, and varied from 20 to 80 dB SPL. The mean CDT amplitude increased with increasing primary tone level once the measurement noise floor was exceeded. The input/ output functions assumed one of two shapes: one in which there was a systematic increase in DPOAE amplitude with increasing primary tone level, and the other in which there was a plateau in the input/output function near 65-70 dB SPL. At the highest primary tone level (80 dB SPL), there was a decrease in the CDT amplitude with increasing f2/f1 ratio. At high primary tone levels, the f2/f1 ratio which produced the largest CDT was 1.05 or 1.1, while at lower primary tone levels the largest CDT occurred at f2/f1 ratios of 1.2-1.3. In experiment 2, L2 was held constant at 70 dB SPL, and L1 varied from 50 to 80 dB SPL. For f1 frequencies of 0.8 and 3.2 kHz, there was an increase in the CDT amplitude with increasing L1, followed by an asymptote at higher levels. In contrast, for 1.6 and 2.0 kHz f1 frequencies, the amplitude increased, plateaued and then increased again at higher levels. Informal measurements suggest that spontaneous otoacoustic emissions (SOAEs) are rarely seen in chickens. However, a reliable SOAE was observed in 1 chicken, which could be suppressed by external sounds and anoxia.     

Influence of spontaneous otoacoustic emissions on distortion product otoacoustic emission amplitudes

Although the influence of the levels and ratios of the primary stimulus on the amplitude of distortion product otoacoustic emissions (DPOAEs) has been studied intensely, the influence of the presence of spontaneous otoacoustic emissions (SOAEs) has been investigated less thoroughly. The present investigation analysed whether the unilateral presence of 58 SOAEs in 43 normal-hearing adults was related to larger DPOAEs in the ear with SOAEs compared to the contralateral ear having no SOAEs. The study was designed such that the only factor that could influence the amplitude of DPOAEs was the presence of SOAEs. Input/output (I/O) functions were collected in response to primary tones that were presented in 5-dB steps from 70 to 40 dB SPL at the frequency of the unilaterally recorded SOAE of each subject. The primary outcome was the demonstration of statistically significant (P < 0.05) larger DPOAEs in ears exhibiting SOAEs than in ears without measurable SOAEs, except at the highest stimulus level of 70 dB SPL. These results suggest that SOAEs play an additive role in the measurement of DPOAEs. The enhancing effect of the unilateral presence of SOAEs on DPOAEs was statistically significant for 65 dB SPL and lower levels of primary tones. The authors speculate that passive cochlear properties begin to participate in the generation of DPOAEs at primary-stimulus levels greater than 65 dB SPL.     

Repeated exposure of adult rats to Aroclor 1254 causes brain region-specific changes in intracellular Ca2+ buffering and protein kinase C activity in the absence of changes in tyrosine hydroxylase

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants, some of which may be neurotoxic. In vitro studies from this laboratory indicated that noncoplanar PCBs perturbed intracellular signal transduction mechanisms including Ca2+ homeostasis, receptor-mediated inositol phosphate production, and translocation of protein kinase C (PKC). In the present study, we examined the effects of PCBs in vivo by dosing adult male Long-Evans rats orally with Aroclor 1254 (0, 10, or 30 mg/kg/day; 5 days/week for 4 weeks) in corn oil. At 24 h after the last dose, rats were tested for motor activity in a photocell device for 30 min. Immediately, the rats were euthanized, blood was collected for thyroid hormone analysis, and brains were removed, dissected into regions (cerebellum, frontal cortex, and striatum), and subcellular fractions were obtained for neurochemical analysis. Following Aroclor 1254 treatment, body weight gain in the high-dose group was significantly lower than the control and low-dose groups. Horizontal motor activity was significantly lower in rats dosed with 30 mg/kg Aroclor 1254. Ca2+ buffering by microsomes was significantly lower in all three brain regions from the 30 mg/kg group. In the same dose group, mitochondrial Ca2+ buffering was affected in cerebellum but not in cortex or striatum. Similarly, total cerebellar PKC activity was decreased significantly while membrane-bound PKC activity was significantly elevated at 10 and 30 mg/kg. PKC activity was not altered either in cortex or the striatum. Neurotransmitter levels in striatum or cortex were slightly altered in PCB-exposed rats compared to controls. Furthermore, repeated oral administration of Aroclor 1254 to rats did not significantly alter forebrain tyrosine hydroxylase immunoreactivity or enzymatic activity. Circulating T4 (total and free) concentrations were severely depressed at both doses in Aroclor 1254-exposed rats compared to control rats, suggesting a severe hypothyroid state. These results indicate that (1) in vivo exposure to a PCB mixture can produce changes in second messenger systems that are similar to those observed after in vitro exposure of neuronal cell cultures; (2) second messenger systems seem to be more sensitive than alterations in neurotransmitter levels or tyrosine hydroxylase involved in dopamine synthesis during repeated exposure to PCBs; and (3) the observed motor activity changes were independent of changes in striatal dopamine levels.     

Remote masking in normal-hearing and noise-exposed chinchillas

Remote masking (RM), the phenomenon whereby an intense high-frequency masking noise elevates thresholds for low-frequency signals, has been shown to be sensitive to various types of hearing loss in humans. We performed two experiments to evaluate the chinchilla as a model of RM and to examine changes in RM associated with temporary threshold shifts (TTSs) induced by low-frequency noise exposure. Thresholds for 0.5-, 1- and 2-kHz tones were measured in quiet, then in the presence of a narrow-band (300-Hz-wide) masking noise centered at 3 kHz. In Experiment I, effective masking was measured as a function of masker level, from 48 to 98 dB sound pressure level (SPL; referenced to 20 microPa), to determine threshold and rate of growth of RM in the chinchilla. In Experiment II, RM was measured before, during and after exposure to a low-frequency noise known to produce TTSs in chinchillas (i.e., a 0.5-kHz octave band noise at 90 dB SPL for 6 h/day for 10 days). The results show that normal-hearing chinchillas have the same pattern of RM as humans, and that a noise exposure that produces TTSs also produces rapid and significant changes in RM.     

Prevention of hearing loss in experimental pneumococcal meningitis by administration of dexamethasone and ketorolac

Pneumococcal meningitis remains a significant cause of morbidity, particularly sensorineural hearing loss. Recent literature has suggested that a vigorous host immune response to Streptococcus [corrected] pneumoniae is responsible for much of the neurologic sequelae, including deafness, after bacterial meningitis. This study used a rabbit model of hearing loss in experimental pneumococcal meningitis to evaluate the therapeutic effect of two anti-inflammatory agents, dexamethasone and ketorolac, coadministered with ampicillin. Both adjunctive drugs minimized or prevented sensorineural hearing loss compared with placebo. Dexamethasone, administered 10 min before ampicillin, was particularly effective in minimizing mean hearing threshold change compared with placebo for both clicks (dexamethasone: 6.7-dB sound pressure level [SPL] vs. placebo: 33. 4-dB SPL, P=.0078) and 10-kHz tone bursts (dexamethasone: 8.4-dB SPL vs. placebo: 53.4-dB SPL, P=.0003). These findings support the beneficial role of anti-inflammatory agents in reducing the incidence of hearing loss from pneumococcal meningitis, especially if therapy is instituted early in the course of infection.     

Transient-evoked and 2F1-F2 distortion product oto-acoustic emissions in dogs: preliminary findings

Transient (click)-evoked oto-acoustic emissions (TEOAEs) and distortion product oto-acoustic emissions (DPOAEs) were recorded in a feasibility study in 7 healthy mixed-breed dogs using the ILO 92 OAE analyser (Otodynamics, Hartfield, UK). Five dogs were found to have normal hearing in both ears and 2 dogs in the left ear only following otoscopy, tympanometry and auditory brainstem response audiometry. Twelve sets of TEOAEs (click-evoked) to 80 dB peSPL click stimulus and 9 sets of DPOAEs (2F1-F2) to 8 different stimulus levels of the primary tones (L1/L2) were collected at 11 test frequencies (F2) in these normal-hearing dogs. TEOAEs were successfully recorded in 11 of the 12 ears using the default user setting and in all 12 ears using the quickscreen program. DPOAEs were successfully recorded in all 9 ears tested. While the TEOAEs parameters matched those for humans, the average signal-to-noise ratio of DPOAEs was considerably higher in the dogs. Stimulus levels at 55/55, 55/45 and 55/35 dB SPL were demonstrated to produce DPOAEs that seem to reflect the active dynamic status of the outer hair cell system. Postmortem DPOAEs at these stimulus levels and TEOAEs at 80 Db peSPL could not be elicited 5 min following euthanasia of dogs. However, DPOAEs could still be recorded albeit with reduced amplitude at stimulus levels where L1 > 55 dB SPL. The results suggest that TEOAEs and DPOAEs in dogs have the potential to provide valuable insights into their mechanisms of generation, and the specific role and behaviour of outer hair cells of the cochlea in certain pathological conditions, particularly in drug-induced ototoxicity, in humans.     

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.     

Level-dependent representation of stimulus frequency in cat primary auditory cortex

The tonotopicity of the cat's primary auditory cortex (AI) is thought to provide the framework for frequency-specific processing in that field. This study was designed to assess this postulate by examining the spatial distribution of neurons within AI that are activated by a single tonal frequency delivered to the contralateral ear. Distributions obtained at each of several stimulus levels were then compared to assess the influence of stimulus amplitude on the spatial representation of a given stimulus frequency in AI. Data were obtained from 308 single units in AI of four adult, barbiturate-anesthetized cats, using extracellular recording methods. Stimuli were 40-ms tone pulses presented through calibrated, sealed stimulating systems. In each animal, the CF (stimulus frequency to which the unit is most sensitive), threshold at CF, response/level function at CF, and binaural interactions were determined for isolated neurons (usually one per track) in 60-90 electrode tracks. For each unit, regardless of its CF, responses to 40 repetitions of contralateral tones of a single frequency, presented at each of four or five sound pressure levels (SPLs) in the range from 10 to 80 dB were obtained. Different test frequencies were used in each of four cats (1.6, 8.0, 11.0, and 16.0 kHz). For tones of each SPL, we generated maps of the response rates across the cortical surface. These maps were then superimposed on the more traditional maps of threshold CF. All units whose CF was equal to the test frequency could be driven at some SPL, given an appropriate monaural or binaural configuration of the stimulus. There was a clear spatial segregation of neurons according to the shapes of their CF tone response/level functions. Patches of cortex, often occupying more than 2 mm2, seemed to contain only monotonic or only nonmonotonic units. In three cortices, a patch of nonmonotonic cells was bounded ventrally by a patch of monotonic cells, and in one of these cases, a second patch of monotonic cells was found dorsal to the nonmonotonic patch. Contralateral tones of any given SPL evoked excitatory responses in discontinuous cortical territories. At low SPLs (10, 20 dB), small foci of activity occurred along the isofrequency line representing the test frequency. Many of these cells had nonmonotonic response/level functions. (ABSTRACT TRUNCATED AT 400 WORDS)     

Interleukin 8 can affect inner ear function

The chemokine interleukin 8 (IL-8) was instilled into the round window niche of rats through a small perforation in the tympanic membrane in order to study its effect on inner ear function by electrophysiological and morphological techniques. The frequency-specific auditory brainstem response (ABR) was recorded at the frequencies 4, 8, 10, 12, 16 and 20 kHz just before and 1, 2, 5 and 14 days after instilling IL-8 to ascertain the hearing level during each interval. Morphological examination by light microscopy was performed during the same interval following the instillation of IL-8. On day 1, the rise in ABR threshold was within 5 dB SPL (non-significant elevation). However, a significant threshold elevation (above 5 dB SPL) occurred in high-frequency areas (16 and 20 kHz) on day 2, and in middle frequency areas (10 and 12 kHz) on day 5 with sensorineural hearing loss type intensity-latency curves. By day 14, the elevated thresholds had returned to pre-instillation levels. In the lowest areas (4 and 8 kHz), no significant threshold elevation was detected at any time during the observation period. By light microscopy, on day 1, clusters of inflammatory cells (predominantly neutrophils) were observed just outside the round window membrane (RWM), while only a few neutrophils were detected in the cochlea. These cells were still present outside the RWM on day 2. The neutrophils had disappeared by day 5 and only macrophages were present on the middle ear side of the RWM. However, throughout the observation period, the organ of Corti and stria vascularis appeared to be intact. These results suggest that IL-8 in the middle ear cavity is able to influence inner ear function.     

Outer hair cell activity is not required for the generation of the forward masking curve

Forward masking of the auditory brainstem response (ABR) was achieved by increasing the time interval from 0 to 12 ms between the masker offset and the probe onset. The forward masking response demonstrated a near linear function with an approximate 3.0-dB increase in masking threshold for every millisecond interval increase in the control guinea pig. The slope of the masking curve at selected frequencies together with the quantification of hair cell loss through the analysis of cochlear surface morphology was studied before and after chemical insult. The intracochlear infusion of sodium salicylate caused an approximately 45-dB threshold shift of the ABR whereas the slope of the forward masking curve was not significantly different from the control values at the tested frequencies (1, 4, and 8 kHz). Systemic kanamycin administration (400 mg/kg body weight for 9 consecutive days) caused a permanent ABR threshold shift of 43-63 dB at 1, 4, and 8 kHz. The slope of the forward masking curve was not significantly different at 1 kHz despite significant outer hair cell loss. The slope of the forward masking curve at 4 and 8 kHz showed significant reductions at the time intervals between 0 and 4 ms. Analysis of the kanamycin-treated cochleae revealed not only significant outer hair cell loss throughout the cochlea but significant inner hair cell and inner pillar cell loss in the basal end of the cochlea. The results suggest that the outer hair cells are not needed for maintaining a normal forward masking curve, whereas the slope of the forward masking curve is sensitive to alterations induced to either the inner hair cells or the inner pillar cells.     

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)     

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.     

Frequency selectivity on aspirin-induced hearing loss in rats with auditory stimulus-induced conditioned suppression

The conditioned suppression technique was employed to examine the acute effects of aspirin on auditory function in rats. Lever pressing behavior for water reinforcement was suppressed in the presence of an auditory stimulus that had been previously paired with electric shocks. A single intravenous injection of aspirin at a dose of 225 mg/kg caused an erroneous lever pressing response in the broad sound intensities of 2 kHz tone stimulus during the conditioned stimulus period. A statistically significant increase in the threshold for 2 kHz was found 1 to 72 hr after dosing but not for 4, 8 and 10 kHz. These results suggest that the hearing for low sound frequency in rats is vulnerable to the effects of aspirin. This paradigm in rats may be useful to further assess the different outer hair cells along the cochlear duct and provide an additional evidence for the aspirin ototoxicity research.     

Health administration functions in occupational health

Study of optimum seeking method with orthogonal test for auditory P300 measuring was carried out in healthy adolescents. The result showed that the optimal conbination of parameters was as follows: stimulus sound was Logon, recording electrode site Cz, analysis time 750 ms, average 100, stimulus rate 1 c/s, target stimulus (TS) probability 10%, bandpass filter 1-50 Hz, non-target stimulus (NTS) freqency 1 kHz, target stimulus (TS) frequency 2 kHz, stimuli intensities 110 dB peSPL. The normal values of auditory P300, obtained from 30 healthy adolescents, including latencies and amplitudes, were measured under optimal parameters. P300 latency was about 310 ms, RT about 258 ms, these two values present a significant linear correlation.