Abstract:
Threshold shifts (TS) which decrease over several days of an interrupted noise exposure, despite continuing exposure, have given rise to the notion of a ``toughened'' cochlea. The ``toughening'' phenomena was explored in chinchillas, as the noise peak SPL was varied from 109 to 127 dB. Exposure: A narrow-band (400 Hz), 1 kHz center frequency (CF) impact, presented 1/s; 6 h/day over 20 days or 24 h/day for 5 days. Pairs of exposures with equal peaks had equal energy. Results: Initial TSs of 40 dB developed at 1 kHz and increased very little as impact intensities increased while 3 to 4 octaves above the impact CF, initial TSs increased by as much as 60 dB. ``Toughening,'' which amounted to 20--30 dB, was typically greatest 2 or 3 oct above the impact CF and tended to shift toward the higher audiometric test frequencies as impact intensity increased. There were no statistically significant differences in PTS or sensory cell losses between the two sets of equal energy exposures. These results suggest that the TTS and PTS components of TS have different substrates and that the decrement in TS, identified as ``toughening,'' is a modulatable component of TTS. [Research supported by NIOSH Grant No. R01 OH02317.]