Samuel S. Saunders

Interaction of cisplatin and noise on the peripheral auditory system

The potentiation of cisplatin ototoxicity by noise was explored in the chinchilla. The effects of exposure to cisplatin alone, noise alone or concurrent exposure to both agents were compared in terms of the threshold shift of the auditory evoked potential and the amount of hair cell loss. The combination of cisplatin plus noise produced significantly more hair cell loss and hearing loss at the high frequencies than did either the noise or cisplatin alone when the noise level was 85 dB SPL or higher; no interaction was seen when the noise level was 70 dB SPL. The amount of the interaction, when present, was constant regardless of the noise level. These results indicate that moderate to high levels of noise can exacerbate cisplatin ototoxicity.

Hypersensitivity to electrical stimulation of auditory nuclei follows hearing loss in cats.

The purpose of the study was to determine if permanent, sound-induced hearing loss altered behaviorally measured thresholds for the detection of electrical stimulation applied to auditory nuclei. Electrodes were placed in cochlear nucleus and inferior colliculus in four cats. Behaviorally measured thresholds for the detection of brief trains of electrical pulses were determined before and after a 48 h exposure to a 1 kHz tone of approximately 110 dB SPL. The mean decrease in electrical stimulation threshold as a result of the sound exposure was 10.4 dB. The ongoing electrical activity (in microV, rms) recorded from the electrodes showed a mean 2.2 dB decrease after the sound exposure. In some electrodes, there was partial recovery towards pre-exposure levels for stimulation threshold and for ongoing activity, but typically, the changes persisted until the animals were terminated 30 days later. The magnitudes of the decreases in stimulation threshold and background activity proved not to be highly correlated. The permanent auditory threshold shift across all cats and all frequencies was 19 dB. This mild hearing loss produced a marked alteration in certain characteristics of the central auditory mechanisms.

Discharge patterns of cochlear ganglion neurons in the chicken.

SummaryPhysiological recordings were made of the compound action potential from the round window and single neurons in the cochlear ganglion of normal adult chickens (Gallus domesticus). The compound action potential threshold to tone bursts decreased from approximately 42 dB at 0.25 kHz to 30 dB between 1 and 2 kHz and then increased to 51 dB at 4 kHz. Most of the cochlear ganglion cells had characteristic frequencies below 2 kHz and the thresholds of most neurons were roughly 30–35 dB lower than the compound action potential thresholds. At any given characteristic frequency, thresholds varied by as much as 60 dB and units with the highest thresholds tended to have the lowest spontaneous rates. Spontaneous discharge rates ranged from 0 to 200 spikes/s with a mean rate of 86 spikes/s. Interspike interval histograms of spontaneous activity often contained regular peaks with the time interval between peaks approximately equal to 1/(characteristic frequency). Tuning curves were sharply tuned and V-shaped with approximately equal slopes to the curves above and below characteristic frequency. Q10dB and Q30dB values for the tuning curves increased with characteristic frequency. Post stimulus time histograms showed sustained firing during the stimulus and were characterized by a slight-to-moderate peak at stimulus onset. Most units showed vigorous phase-locking to tones at characteristic frequency although the degree of phase-locking declined sharply with increasing characteristic frequency. Discharge rate-level functions at characteristic frequency had a mean dynamic range of 42 dB and a mean saturation firing rate of 327 spikes/s. In general, the firing patterns of cochlear ganglion neurons are similar in most respects to those reported in other avians, but differ in several important respects from those seen in mammals.

IMPACT NOISE : THE IMPORTANCE OF LEVEL, DURATION, AND REPETITION RATE

The applicability of the equal energy hypothesis (EEH) to impact noise exposures was studied using chinchillas. Hearing thresholds were estimated by recording the evoked potentials from a chronic electrode implanted in the inferior colliculus. The animals were exposed to broadband impacts of 200-ms duration. The study was carried out in two parts. In experiment I, six exposure levels (107, 113, 119, 125, 131, and 137 dB SPL) and three repetition rates (4/s, 1/s and 1/4s) were employed. In the second experiment, the total duration of the exposure as well as the total energy were kept constant by trading level and rate. Results indicate that hearing loss resulting from exposure to impact noise does not conform to the predictions of the EEH. The permanent threshold shift as well as the hair cell loss are more or less equal across the lower peak exposure levels. However, both the hearing loss and the hair cell damage increase for exposures with higher peak levels. Furthermore, hearing loss and cochlear damage are dependent upon the rate of exposure. Thus the amount of hearing loss and hair cell damage appears to depend on the interaction of several factors including peak level, rate, and the susceptibility of the animal.

Psychoacoustics of normal adult chickens: Thresholds and temporal integration

Three adult female chickens were trained to detect pure tones using a positive reinforcement paradigm. A modified perometer estimation-through sequential testing (PEST) threshold determination procedure was used to measure the audiogram between 250 and 5500 Hz, at 0.1-octave steps over most of the range. For the mean audiogram, threshold increased rapidly below 400 Hz and above 4000 Hz. Between these extremes was a broad low threshold area where thresholds varied between 11 and 23 dB, with the lowest thresholds between 1000 and 2000 Hz. Audiograms were similar for all three subjects over the middle of the frequency range, but there was more variability across subjects at the extreme high and low frequencies. After determination of the audiogram, thresholds for 12 frequencies were measured as a function of stimulus duration (ranging from 16 to 750 ms) to determine a threshold-duration function. Average time constants varied from 148 to 348 ms, increasing with increasing stimulus frequency. Average slopes varied from 2.18 dB per doubling of duration to 2.61 dB per doubling of duration, increasing with increasing stimulus frequency. The increases in slope and time constant with stimulus frequency, while consistent, were small compared to the variability of these parameters across subjects.

Effects of noise exposure, race, and years of service on hearing in U.S. Army soldiers

Over the last two decades the U.S. Army has instituted a comprehensive hearing conservation program (HCP) to reduce the prevalence of hearing loss in soldiers and civilian employees. As a component of this program, hearing threshold levels (HTLs) are stored in a central computerized Army-wide hearing conservation data registry. The aim of this study was to analyze the hearing threshold data to compare the hearing loss among soldiers representing different (1) race groups, (2) noise exposure groups, and (3) durations of military service. HTLs were age-corrected using data base B values from IS0 1999 (1990). As may be expected, soldiers exposed to high noise levels had significantly poorer hearing than the group of soldiers with limited noise exposure. On the average, results indicated a significant difference in HTLs among the race groups with black soldiers having the most sensitive hearing and white soldiers having the poorest. Also, subjects with greater durations of military service had the least sensitive hearing. Finally, race and years of service factors were found to interact in their effect on HTL. The findings are discussed in terms of implications of race differences, normative data, and effectiveness of the U.S. Army HCP.

Discharge patterns of chicken cochlear ganglion neurons following kanamycin-induced hair cell loss and regeneration

Hair cells in the basal, high frequency region (>1100 Hz) of the chicken cochlea were destroyed with kanamycin (400 mg/kg/d × 10 d) and allowed to regenerate. Afterwards, single unit recordings were made from cochlear ganglion neurons at various times post-treatment. During the first few weeks post-treatment, only neurons with low characteristic frequencies (<1100 Hz) responded to sound. Despite the fact that the low frequency region of the cochlea was not destroyed, neurons with low characteristic frequencies had elevated thresholds, abnormally broad U-shaped or W-shaped tuning curves and low spontaneous discharge rates. At 2 days post-treatment, the spontaneous discharge rates of some acoustically unresponsive units fluctuated in a rhythmical manner. As recovery time increased, thresholds decreased, tuning curves narrowed and developed a symmetrical V-shape, spontaneous rate increased and neurons with higher characteristic frequencies began to respond to sound. In addition, the proportion of interspike interval histograms with regularly spaced peaks increased. These improvements progressed along a low-to-high characteristic frequency gradient. By 10–20 weeks post-treatment, the thresholds and tuning curves of neurons with characteristic frequencies below 2000 Hz were within normal limits; however, the spontaneous discharge rates of the neurons were still significantly lower than those from normal animals.

Behavioral thresholds for electrical stimulation applied to auditory brainstem nuclei in cat are altered by injurious and noninjurious sound

Each of three young-adult female cats with normal hearing received a total of eight permanent electrodes which were implanted bilaterally in cochlear nucleus (CN) and inferior colliculus (IC). Three experiments were performed using behaviorally measured thresholds for electrical stimulation of CN and IC. In Expt. 1, electrical stimulation thresholds (in dB re 1.0 microA) were obtained in the presence of a continuous tone of moderate intensity and in quiet. In comparison with quiet, electrical stimulation thresholds measured during tone were lower by as much as 15 dB (stimulation hypersensitivity). In Expt. 2, a brief exposure to an intense sound produced a temporary threshold shift (TTS) for acoustic stimuli but only produced small changes in electrical stimulation threshold. The acoustic stimuli used in Expts. 1 and 2 were termed noninjurious since no permanent hearing loss was produced. Expt. 3 employed an exposure to a white noise that resulted in a mean permanent threshold shift (PTS) of 34.1 dB for acoustic stimulation. The PTS was accompanied by a mean stimulation hypersensitivity of 9.6 dB. Comparing Expts. 1 and 3, it was shown that the transient hypersensitivity produced by the noninjurious continuous tone correlated strongly with the permanent hypersensitivity that was produced by the PTS. In regard to the origin of stimulation hypersensitivity, the suggestion is made that it is an indication of a physiological change localizable perhaps in the auditory nuclei of the upper brainstem.

Recovery of thresholds and temporal integration in adult chickens after high-level 525-Hz pure-tone exposure.

Three adult, female chickens, previously trained to detect pure tones using a positive reinforcement paradigm, were exposed to pure tones at 525 Hz at 120-dB sound-pressure level for 48 h. Immediately after exposure, pure-tone thresholds were elevated between 10 and 56 dB compared to pre-exposure values. The configuration of the initial hearing loss was essentially flat between 500 and 2000 Hz, with slightly less loss at lower and higher frequencies. Over the course of 100 to 600 h, depending on subject and test frequency, thresholds returned to preexposure levels. The slopes of the threshold-duration functions measured shortly after exposure were shallower than normal, but returned to the normal slope of approximately 3 dB per doubling of duration as hearing loss resolved to less than 15 to 20 dB. The cochleas from additional chickens examined by scanning electron microscopy (SEM) and fluorescence microscopy between 0 and 1512 h post-exposure showed extensive damage to the tectorial membrane over the middle two-thirds of the cochlea, with mild to moderate hair cell loss in the region of the cochlea associated with the exposure frequency.

Neural correlates of temporal integration in the cochlear nucleus of the chinchilla

Single unit thresholds were measured as a function of stimulus duration for Primary-like and Chopper units in the anteroventral cochlear nucleus (AVCN) of the chinchilla to examine the neural correlates of temporal integration. Thresholds were measured with a two-alternative, forced-choice (2AFC) adaptive tracking procedure. The time constants and the slopes of the threshold-duration functions were estimated by fitting the threshold data with an exponential function and a power law function. The results showed that Primary-like units exhibited greater threshold improvement and a longer time constant than Chopper units. Units with low characteristic frequencies (CF) showed a larger decrease in threshold with increasing duration and a longer time constant than mid-CF or high-CF units. Units with low spontaneous rates (SR) showed a smaller threshold decrease with increasing duration and a shorter time constant than mid-SR or high-SR units. The single unit time constants and the rate of threshold improvement are similar to those measured psychophysically in the chinchilla.