Distribution of focal lesions in the chinchilla organ of Corti following exposure to a 4-kHz or a 0.5-kHz octave band of noise

Effect of shock wave power spectrum on the inner ear pathophysiology in blast-induced hearing loss

Blast exposure can induce various types of hearing impairment, including permanent hearing loss, tinnitus, and hyperacusis. Herein, we conducted a detailed investigation of the cochlear pathophysiology in blast-induced hearing loss in mice using two blasts with different characteristics: a low-frequency dominant blast generated by a shock tube and a high-frequency dominant shock wave generated by laser irradiation (laser-induced shock wave). The pattern of sensorineural hearing loss (SNHL) was low-frequency- and high-frequency-dominant in response to the low- and high-frequency blasts, respectively. Pathological examination revealed that cochlear synaptopathy was the most frequent cochlear pathology after blast exposure, which involved synapse loss in the inner hair cells without hair cell loss, depending on the power spectrum of the blast. This pathological change completely reflected the physiological analysis of wave I amplitude using auditory brainstem responses. Stereociliary bundle disruption in the outer hair cells was also dependent on the blast’s power spectrum. Therefore, we demonstrated that the dominant frequency of the blast power spectrum was the principal factor determining the region of cochlear damage. We believe that the presenting models would be valuable both in blast research and the investigation of various types of hearing loss whose pathogenesis involves cochlear synaptopathy.

Paired measurements of cochlear function and hair cell count in Dutch-belted rabbits with noise-induced hearing loss

The effects of noise-induced hearing loss have yet to be studied for the Dutch-belted strain of rabbits, which is the only strain that has been used in studies of the central auditory system. We measured auditory brainstem responses (ABRs), 2f₁-f₂ distortion product otoacoustic emissions (DPOAEs), and counts of cochlear inner and outer hair cells (IHCs and OHCs, respectively) from confocal images of Myo7a-stained cochlear whole-mounts in unexposed and noise-overexposed, Dutch-belted, male and female rabbits in order to characterize cochlear function and structure under normal-hearing and hearing-loss conditions. Using an octave-band noise exposure centered at 750 Hz presented under isoflurane anesthesia, we found that a sound level of 133 dB SPL for 60 min was minimally sufficient to produce permanent ABR threshold shifts. Overexposure durations of 60 and 90 min caused median click-evoked ABR threshold shifts of 10 and 50 dB, respectively. Susceptibility to overexposure was highly variable across ears, but less variable across test frequencies within the same ear. ABR and DPOAE threshold shifts were smaller, on average, and more variable in male than female ears. Similarly, post-exposure survival of OHCs was higher, on average, and more variable in male than female ears. We paired post-exposure ABR and DPOAE threshold shift data with hair cell count data measured in the same ear at the same frequency and cochlear frequency location. ABR and DPOAE threshold shifts exhibited critical values of 46 and 18 dB, respectively, below which the majority of OHCs and IHCs survived and above which OHCs were wiped out while IHC survival was variable. Our data may be of use to researchers who wish to use Dutch-belted rabbits as a model for the effects of noise-induced hearing loss on the central auditory system.

Decreased sound tolerance: hyperacusis, misophonia, diplacousis, and polyacousis

Definitions, potential mechanisms, and treatments for decreased sound tolerance, hyperacusis, misophonia, and diplacousis are presented with an emphasis on the associated physiologic and neurophysiological processes and principles. A distinction is made between subjects who experience these conditions versus patients who suffer from them. The role of the limbic and autonomic nervous systems and other brain systems involved in cases of bothersome decreased sound tolerance is stressed. The neurophysiological model of tinnitus is outlined with respect to how it may contribute to our understanding of these phenomena and their treatment.

Effects of sensorineural hearing loss on temporal coding of harmonic and inharmonic tone complexes in the auditory nerve

Listeners with sensorineural hearing loss (SNHL) often show poorer thresholds for fundamental-frequency (F0) discrimination and poorer discrimination between harmonic and frequency-shifted (inharmonic) complex tones, than normal-hearing (NH) listeners-especially when these tones contain resolved or partially resolved components. It has been suggested that these perceptual deficits reflect reduced access to temporal-fine-structure (TFS) information and could be due to degraded phase locking in the auditory nerve (AN) with SNHL. In the present study, TFS and temporal-envelope (ENV) cues in single AN-fiber responses to band-pass-filtered harmonic and inharmonic complex tones were -measured in chinchillas with either normal-hearing or noise-induced SNHL. The stimuli were comparable to those used in recent psychophysical studies of F0 and harmonic/inharmonic discrimination. As in those studies, the rank of the center component was manipulated to produce -different resolvability conditions, different phase relationships (cosine and random phase) were tested, and background noise was present. Neural TFS and ENV cues were quantified using cross-correlation coefficients computed using shuffled cross correlograms between neural responses to REF (harmonic) and TEST (F0- or frequency-shifted) stimuli. In animals with SNHL, AN-fiber tuning curves showed elevated thresholds, broadened tuning, best-frequency shifts, and downward shifts in the dominant TFS response component; however, no significant degradation in the ability of AN fibers to encode TFS or ENV cues was found. Consistent with optimal-observer analyses, the results indicate that TFS and ENV cues depended only on the relevant frequency shift in Hz and thus were not degraded because phase locking remained intact. These results suggest that perceptual "TFS-processing" deficits do not simply reflect degraded phase locking at the level of the AN. To the extent that performance in F0- and harmonic/inharmonic discrimination tasks depend on TFS cues, it is likely through a more complicated (suboptimal) decoding mechanism, which may involve "spatiotemporal" (place-time) neural representations.

Structural and functional effects of acoustic exposure in goldfish: evidence for tonotopy in the teleost saccule

BACKGROUND

Mammalian and avian auditory hair cells display tonotopic mapping of frequency along the length of the cochlea and basilar papilla. It is not known whether the auditory hair cells of fishes possess a similar tonotopic organization in the saccule, which is thought to be the primary auditory receptor in teleosts. To investigate this question, we determined the location of hair cell damage in the saccules of goldfish (Carassius auratus) following exposure to specific frequencies. Subjects were divided into six groups of six fish each (five treatment groups plus control). The treatment groups were each exposed to one of five tones: 100, 400, 800, 2000, and 4000 Hz at 176 dB re 1 μPa root mean squared (RMS) for 48 hours. The saccules of each fish were dissected and labeled with phalloidin in order to visualize hair cell bundles. The hair cell bundles were counted at 19 specific locations in each saccule to determine the extent and location of hair cell damage. In addition to quantification of anatomical injury, hearing tests (using auditory evoked potentials) were performed on each fish immediately following sound exposure. Threshold shifts were calculated by subtracting control thresholds from post-sound exposure thresholds.

RESULTS

All sound-exposed fish exhibited significant hair cell and hearing loss following sound exposure. The location of hair cell loss varied along the length of the saccule in a graded manner with the frequency of sound exposure, with lower and higher frequencies damaging the more caudal and rostral regions of the saccule, respectively. Similarly, fish exposed to lower frequency tones exhibited greater threshold shifts at lower frequencies, while high-frequency tone exposure led to hearing loss at higher frequencies. In general, both hair cell and hearing loss declined as a function of increasing frequency of exposure tone, and there was a significant linear relationship between hair cell loss and hearing loss.

CONCLUSIONS

The pattern of hair cell loss as a function of exposure tone frequency and saccular rostral-caudal location is similar to the pattern of hearing loss as a function of exposure tone frequency and hearing threshold frequency. This data suggest that the frequency analysis ability of goldfish is at least partially driven by peripheral tonotopy in the saccule.

Relation of focal hair-cell lesions to noise-exposure parameters from a 4- or a 0.5-kHz octave band of noise

In a previous study, we examined the relation between total energy in a noise exposure and the percentage losses of outer (OHC) and inner (IHC) hair cells in the basal and apical halves of 607 chinchilla cochleae [Harding, G.W., Bohne, B.A., 2004a. Noise-induced hair-cell loss and total exposure energy: analysis of a large data set. J. Acoust. Soc. Am. 115, 2207-2220]. The animals had been exposed continuously to either a 4-kHz octave band of noise (OBN) at 47-108 dB SPL for 0.5h-36 d, or a 0.5-kHz OBN at 65-128 dB SPL for 3.5h-433 d. Interrupted exposures were also employed with both OBNs. Post-exposure recovery times ranged from 0 to 913 days. Cluster analysis was used to separate the data into three magnitudes of damage. The data were also separated into recovery times of 0 days (acute) and >0 days (chronic) and the apical and basal halves of the organ of Corti (OC). A substantial part of these hair-cell losses occurred in focal lesions (i.e., >or=50% loss of IHCs, OHCs or both over a distance of >or=0.03 mm). This aspect of the damage from noise was not included in the previous analysis. The present analysis describes, within the same three clusters, the apex-to-base distribution of 1820 focal lesions found in 468 of 660 (71%) noise-exposed cochleae. In these cochleae, OC length in mm was converted to percent distance from the apex. The lesion data were analyzed for location in percent distance from the apex and size (mm) of the lesions. In 55 of 140 (39%) non-noise-exposed, control OCs, there were 186 focal hair-cell lesions, the characteristics of which were also determined. Focal lesions with hair-cell loss >or=50% involved predominantly OHCs, IHCs only, or both OHCs and IHCs (i.e., combined OHC-IHC lesions). The predominantly OHC and combined lesions were pooled together for the analysis. The distributions of lesion location (in percent distance from the apex), weighted by lesion size (in percent of OC length) were tallied in 2%-distance bins. In controls, focal lesions were uniformly distributed from apex to base and 70% of them were pure IHC lesions. In cochleae exposed to the 4-kHz OBN, lesions were distributed throughout the basal half of the OC. In cochleae exposed to the 0.5-kHz OBN, lesions occurred in both halves of the OC. With continuous exposures, 74% of the lesions were predominantly OHC or combined lesions. With interrupted exposures, 52% of the lesions were OHC or combined lesions. Lesion size was generally larger in the chronic compared to acute cochleae with similar exposures. There was a minimum total energy at which focal lesions began to appear and slightly higher energies resulted in nearly all exposed cochleae having focal lesions.