Assessing Noise Induced Changes in Forward Masking of Wave I Latency

Abstract

Exposure to loud noise causes trauma to the outer hair cells (OHC), which improve hearing sensitivity and frequency discrimination. The damage done can be permanent causing a significant hearing loss. Detecting noise-induced hearing loss (NIHL) early is crucial for hearing conservation programs. The current study was undertaken as part of a larger study that looks at forward masking (FWM) of the rat auditory brainstem response (ABR). FWM causes a change in ABR response by delivering a masker sound earlier in time than the probe signal. Death of OHC creates marked changes in FWM patterns. The current study assessed FWM of the latency of wave I of an ABR wave and how it changes with NIHL due to damage to the OHC. Nine Sprague-Dawley rats were tested before and after extreme noise exposure. Probe signals at 7 kHz were used to elicit the ABR. A FWM tone was played for 40 ms followed by silence for 5 ms then a 1 ms tone burst. The Sprague-Dawley rats were then exposed to a noise band ranging from 5-10 kHz at a level of 115 dB for 90 minutes. A 21 day rest period followed to ensure permanent hearing loss. The resulting ABR waves were analyzed to calculate wave I latencies. The results demonstrated that the FWM induced a prolongation of latency in an intensity-dependent fashion. An on-frequency FWM induced linear prolongation of latency, while the off-frequency FWM required much higher levels to induce prolonged latencies. Noise exposure rendered the off-frequency masking much more linear, and similar to the on-frequency masking effects. The work suggests that latency of the ABR wave I is affected by FWM, and behaves in a manner similar to ABR amplitudes for both on- and off-frequency maskers, and before and after noise exposure.