Disruption of Lateral Olivocochlear Neurons via a Dopaminergic Neurotoxin Depresses Sound-Evoked Auditory Nerve Activity

We applied the dopaminergic (DA) neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the guinea pig cochlear perilymph. Immunolabeling of lateral olivocochlear (LOC) neurons using antibodies against synaptophysin was reduced after the MPTP treatment. In contrast, labeling of the medial olivocochlear innervation remained intact. As after brainstem lesions of the lateral superior olive (LSO), the site of origin of the LOC neurons, the main effect of disrupting LOC innervation of the cochlea via MPTP was a depression of the amplitude of the compound action potential (CAP). CAP amplitude depression was similar to that produced by LSO lesions. Latency of the N1 component of the CAP, and distortion product otoacoustic emission amplitude and adaptation were unchanged by the MPTP treatment. This technique for selectively lesioning descending LOC efferents provides a new opportunity for examining LOC modulation of afferent activity and behavioral measures of perception.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41386/1/10162_2004_Article_2429.pd

Disruption of Lateral Efferent Pathways: Functional Changes in Auditory Evoked Responses

The functional consequences of selectively lesioning the lateral olivocochlear efferent system in guinea pigs were studied. The lateral superior olive (LSO) contains the cell bodies of lateral olivocochlear neurons. Melittin, a cytotoxic chemical, was injected into the brain stem using stereotaxic coordinates and near-field evoked potentials to target the LSO. Brain stem histology revealed discrete damage to the LSO following the injections. Functional consequences of this damage were reflected in depressed amplitude of the compound action potential of the eighth nerve (CAP) following the lesion. Threshold sensitivity and N1 latencies were relatively unchanged. Onset adaptation of the cubic distortion product otoacoustic emission (DPOAE) was evident, suggesting a reasonably intact medial efferent system. The present results provide the first report of functional changes induced by isolated manipulation of the lateral efferent pathway. They also confirm the suggestion that changes in single-unit auditory nerve activity after cutting the olivocochlear bundle are probably a consequence of disrupting the more lateral of the two olivocochlear efferent pathways.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41379/1/10162_2002_Article_3018.pd

Variation in Music Player Listening Level as a Function of Campus Location

There has been significant discussion in the literature regarding music player use by adolescents and young adults, including whether device use is driving an increase in hearing loss in these populations. While many studies report relatively safe preferred listening levels, some studies with college student participants have reported listening habits that may put individuals at risk for noise-induced hearing loss (NIHL) if those listening habits continue over the long term. The goal of the current investigation was to extend listening level data collection sites from urban city settings studied by others to a more rural campus setting. This was a prospective study. Participants were 138 students on the University of Florida campus (94 males, 44 females), 18 years or older (mean = 21 years; range: 18-33 years). In this investigation, the current output level (listening level) was measured from personal listening devices used by students as they passed by a recruiting table located in one of three areas of the University of Florida campus. One location was in an open-air campus square; the other two locations were outside the campus recreation building ("gym") and outside the undergraduate library, with participants recruited as they exited the gym or library buildings. After providing written informed consent, participants completed a survey that included questions about demographics and typical listening habits (hours per day, days per week). The output level on their device was then measured using a "Jolene" mannequin. Average listening levels for participants at the three locations were as follows: gym: 85.9 ± 1.4 dBA; campus square: 83.3 ± 2.0 dBA; library: 76.9 ± 1.3 dBA. After adjusting to free-field equivalent level, average listening levels were gym: 79.7 ± 1.4 dBA; campus square: 76.9 ± 2.1 dBA; library: 70.4 ± 1.4 dBA. There were no statistically significant differences between male and female listeners, and there were no reliable differences as a function of race. After accounting for daily and weekly use patterns, 5% were deemed at risk based on the criteria put forward by the Occupational Safety and Health Administration, and 9% were deemed at risk based on the guidance provided by the National Institute for Occupational Safety and Health. Some 5-10% of the participants were deemed at risk; this finding is consistent with other studies using similar methods. It is possible that the same listeners would have selected different listening levels in other noise backgrounds, however. This unknown variable makes it difficult to estimate risk with a single listening level measurement

Hidden Hearing Loss? No Effect of Common Recreational Noise Exposure on Cochlear Nerve Response Amplitude in Humans

This study tested hypothesized relationships between noise exposure and auditory deficits. Both retrospective assessment of potential associations between noise exposure history and performance on an audiologic test battery and prospective assessment of potential changes in performance after new recreational noise exposure were completed.Methods: 32 participants (13M, 19F) with normal hearing (25-dB HL or better, 0.25–8 kHz) were asked to participate in 3 pre- and post-exposure sessions including: otoscopy, tympanometry, distortion product otoacoustic emissions (DPOAEs) (f2 frequencies 1–8 kHz), pure-tone audiometry (0.25–8 kHz), Words-in-Noise (WIN) test, and electrocochleography (eCochG) measurements at 70, 80, and 90-dB nHL (click and 2–4 kHz tone-bursts). The first session was used to collect baseline data, the second session was collected the day after a loud recreational event, and the third session was collected 1-week later. Of the 32 participants, 26 completed all 3 sessions.Results: The retrospective analysis did not reveal statistically significant relationships between noise exposure history and any auditory deficits. The day after new exposure, there was a statistically significant correlation between noise “dose” and WIN performance overall, and within the 4-dB signal-to-babble ratio. In contrast, there were no statistically significant correlations between noise dose and changes in threshold, DPOAE amplitude, or AP amplitude the day after new noise exposure. Additional analyses revealed a statistically significant relationship between TTS and DPOAE amplitude at 6 kHz, with temporarily decreased DPOAE amplitude observed with increasing TTS.Conclusions: There was no evidence of auditory deficits as a function of previous noise exposure history, and no permanent changes in audiometric, electrophysiologic, or functional measures after new recreational noise exposure. There were very few participants with TTS the day after exposure - a test time selected to be consistent with previous animal studies. The largest observed TTS was approximately 20-dB. The observed pattern of small TTS suggests little risk of synaptopathy from common recreational noise exposure, and that we should not expect to observe changes in evoked potentials for this reason. No such changes were observed in this study. These data do not support suggestions that common, recreational noise exposure is likely to result in “hidden hearing loss”