PP1321 - Intensity Increment Detection in Chinchillas with Selective-Induced Inner Hair Cell Loss

The objective of this study was to evaluate the effects of carboplatin-induced selective inner hair cell (IHC) loss on intensity increment detection (IID) in the chinchilla animal model. Studies have shown that carboplatin-induced selective IHC loss does not impact audiometric thresholds, but may effect temporal processing or intensity coding tasks, such as IID. These deficits may underlie poor speech understanding and hearing-in-noise performance. In this study, we measured IID thresholds in chinchillas pre- and post-carboplatin treatment. Our results showed that IID thresholds increased following IHC loss at both high and moderate presentation levels.


Summary:

Objective and Rationale:
Pure tone thresholds are the most widely used clinical measure of hearing sensitivity. However, pure tone thresholds may not be sensitive in detecting loss of IHC. Previous studies that evaluated the effects of selective IHC loss within the chinchilla found that, while pure tone thresholds and temporal integration tasks are not affected, gap detection ability worsens. These deficits in gap detection suggest that IHC loss may impact intensity coding. Temporal processing and intensity coding deficits can lead to poor speech understanding and poor performance in challenging listening environments. To evaluate this, chinchillas were trained on an intensity increment detection (IID) task. This task was used measure the ability to detect small intensity changes within narrowband noise stimuli. The overall goal of this study was to evaluate the effects of carboplatin-induced IHC loss on intensity discrimination performance in the chinchilla animal model. IID thresholds were obtained at 2 intensity levels, high (70 dB SPL), and moderate (50 dB SPL). We hypothesized that IID thresholds would be elevated following carboplatin-induced selective IHC loss, suggesting that IID tasks may be valuable in detecting cochlear pathology.

Design:
This study included 8 healthy, young-adult chinchillas between the ages of 2-5 years. Pure-tone thresholds were obtained at 1, 2, 4, 8, and 12 kHz using a modified methods of limits. Thresholds were measured by decreasing stimuli intensity by 10 dB following correct responses and increasing by 5 dB after no response. IID was measured by presenting continuous carrier narrowband noise (bandwidth = 10 dB), at intensity levels of 50 and 70 dB SPL. Intensity level was then increased by a set dB, as the acoustic stimuli level. Overall intensity increase was decreased by 1 dB upon correct responses and increased by 0.5 dB after no response. Distortion product otoacoustic emissions (DPOAE) were also obtained as a measure of outer hair cell (OHC) integrity. Following baseline testing, subjects received a dose of carboplatin (75 mg/kg) known to produce 60-80% selective IHC loss and preserve OHC function. Post treatment testing was performed 21 days after treatment was administered.

Results:
Pure tone thresholds and DPOAE amplitudes were not significantly impacted following carboplatin. IID thresholds demonstrated a significant increase at both the high and moderate intensity levels. This suggests that IHCs may play a role in intensity coding and IID could be used as a measure of cochlear function.

Conclusions:
The results of this study indicate that intensity coding is impacted by IHC loss. Furthermore, both tested intensities showed significant impairment following selective IHC damage, suggesting that temporal processing deficits are present despite varying levels of audibility. This may contribute to impaired speech in noise performance in individuals with otherwise normal audiometric hearing thresholds. Significant differences across both intensities indicate that IID may be used in the future as a reliable measure of IHC pathology when hearing sensitivity is not impacted.