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. 2011 Oct;280(1-2):236-44.
doi: 10.1016/j.heares.2011.06.002. Epub 2011 Jun 15.

Auditory brainstem responses predict auditory nerve fiber thresholds and frequency selectivity in hearing impaired chinchillas

Kenneth S Henry  1 , Sushrut Kale, Ryan E Scheidt, Michael G Heinz
Affiliations

Auditory brainstem responses predict auditory nerve fiber thresholds and frequency selectivity in hearing impaired chinchillas

Kenneth S Henry et al. Hear Res. 2011 Oct.

Abstract

Noninvasive auditory brainstem responses (ABRs) are commonly used to assess cochlear pathology in both clinical and research environments. In the current study, we evaluated the relationship between ABR characteristics and more direct measures of cochlear function. We recorded ABRs and auditory nerve (AN) single-unit responses in seven chinchillas with noise-induced hearing loss. ABRs were recorded for 1-8 kHz tone burst stimuli both before and several weeks after 4 h of exposure to a 115 dB SPL, 50 Hz band of noise with a center frequency of 2 kHz. Shifts in ABR characteristics (threshold, wave I amplitude, and wave I latency) following hearing loss were compared to AN-fiber tuning curve properties (threshold and frequency selectivity) in the same animals. As expected, noise exposure generally resulted in an increase in ABR threshold and decrease in wave I amplitude at equal SPL. Wave I amplitude at equal sensation level (SL), however, was similar before and after noise exposure. In addition, noise exposure resulted in decreases in ABR wave I latency at equal SL and, to a lesser extent, at equal SPL. The shifts in ABR characteristics were significantly related to AN-fiber tuning curve properties in the same animal at the same frequency. Larger shifts in ABR thresholds and ABR wave I amplitude at equal SPL were associated with greater AN threshold elevation. Larger reductions in ABR wave I latency at equal SL, on the other hand, were associated with greater loss of AN frequency selectivity. This result is consistent with linear systems theory, which predicts shorter time delays for broader peripheral frequency tuning. Taken together with other studies, our results affirm that ABR thresholds and wave I amplitude provide useful estimates of cochlear sensitivity. Furthermore, comparisons of ABR wave I latency to normative data at the same SL may prove useful for detecting and characterizing loss of cochlear frequency selectivity.

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Figures

Fig. 1
Fig. 1
(A) ABRs to a 4 kHz tone burst at various SPLs before noise induced hearing loss and (B) ABRs after hearing loss (middle and lower traces) compared to a pre exposure control (top trace). All ABRs were recorded from the same experimental animal. Comparisons after hearing loss are made at both similar stimulus SPL (~50 dB SPL; middle trace) and similar SL (~30 dB SL; lower trace). The ABR threshold, which we quantified using a cross-correlation algorithm (see text), increased from 18.9 dB SPL before hearing loss to 42.9 dB SPL after hearing loss. “+” symbols indicate ABR wave I. Wave I amplitude was measured relative to the subsequent trough (“−” symbols). Wave I latency was measured relative to stimulus arrival at the ear canal (dotted vertical line).
Fig. 2
Fig. 2
(A) Thresholds and (B) frequency selectivity (Q10) of 253 AN fibers from 9 control animals and 233 fibers from 11 animals with noise-induced hearing loss including the 7 animals in the present study (adapted from Kale and Heinz, 2010; see also Scheidt et al., 2010). Values are plotted as a function of CF. The trend lines in panel A denote the mean thresholds of each population of fibers, calculated using a local regression model with a smoothing parameter of 0.5 (LOES procedure; SAS). The trend line in panel B denotes the median Q10 of the unimpaired fiber population (from Kale and Heinz, 2010).
Fig. 3
Fig. 3
(A) ABR thresholds of chinchillas as a function of stimulus frequency before and several weeks after noise exposure. (B) ABR threshold shifts following noise exposure. Thick horizontal bars are least squares means and vertical bars indicate ±2 standard errors; other symbols in panel B represent individual animals.
Fig. 4
Fig. 4
ABR wave I amplitude as a function of stimulus intensity before and after noise exposure. Stimulus frequency is indicated at the top of each panel. Trend lines were calculated using a multiple regression analysis.
Fig. 5
Fig. 5
Shifts in ABR wave I amplitude following noise induced hearing loss as a function of stimulus frequency. Thick horizontal bars are least squares means and vertical bars indicate ±2 standard errors; other symbols represent individual animals.
Fig. 6
Fig. 6
ABR wave I latency as a function of stimulus SPL before and after noise exposure. Stimulus frequency is indicated at the top of each panel. Trend lines were calculated using a multiple regression analysis.
Fig. 7
Fig. 7
ABR wave I latency as a function of stimulus SL before and after noise exposure. Stimulus frequency is indicated at the top of each panel. Trend lines were calculated using a multiple regression analysis.
Fig. 8
Fig. 8
Shifts in ABR wave I latency following noise exposure as a function of stimulus frequency. Latency shifts are shown at both (A) equal SPL and (B) equal SL. Thick horizontal bars are least squares means and vertical bars indicate ±2 standard errors; other symbols represent individual animals.
Fig. 9
Fig. 9
AN fiber thresholds following noise induced hearing loss as a function of (A) ABR threshold shifts and (B) ABR wave I amplitude shifts in the same frequency band. Frequency bands are indicated in the legend. AN thresholds are expressed in dB relative to the median threshold of a large population of unimpaired fibers with CFs ranging from 0.84–9.52 kHz (20.1 dB SPL; from Kale and Heinz, 2010; note that the median threshold does not vary appreciably over this CF range; see also Fig. 2A). The lower, middle, and upper dashed horizontal lines indicate the 5th, 50th, and 95th percentiles, respectively, of the unimpaired population. Trend lines indicate predicted mean values of the statistical model.
Fig. 10
Fig. 10
The frequency selectivity of AN fibers following noise induced hearing loss as a function of ABR wave I latency shifts at (A) equal SPL and (B) equal SL in the same frequency band. Frequency bands are indicated in the legend. Frequency selectivity was normalized by taking the base 10 logarithm of the observed Q10 value over the median Q10 value of unimpaired fibers with the same CF (from Kale and Heinz, 2010; see also Fig. 2B). The lower, middle, and upper dashed horizontal lines indicate the 5th, 50th, and 95th percentiles, respectively, of unimpaired fibers. Trend lines indicate predicted mean values of the statistical model.
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