Alterations of basilar membrane response phase and velocity after acoustic overstimulation

To investigate the physiology of noise-induced hearing loss, the sound-induced vibrations of the basilar membrane (BM) of the inner ear were measured in living anesthetized guinea pigs before and after intense sound exposure. The vibrations were measured using a laser Doppler velocimeter after placing reflective glass beads on the BM. Pseudo-random noise waveforms containing frequencies between 4 and 24 kHz were used to generate velocity tuning curves. Before overstimulation, sharp response peaks were seen at stimulus frequencies between 15 and 17 kHz, consistent with the expected best frequency of the recording location. The response to low level stimuli lagged the high level ones by up to 90 degrees at the characteristic frequency. Following exposure to loud sound, the BM vibrations showed a pronounced reduction in amplitude, primarily at low stimulus levels, and the best frequency moved to approximately 12 kHz. At higher levels, the reduction was either absent or much smaller. In addition to the amplitude changes, increased phase lags were seen at frequencies near the characteristic frequency. In animals with more severe exposures, response phases were altered also at frequencies showing no change of the amplitude. The phase was independent of stimulus level after severe exposures.