Acoustic-reflex activity and behavioral thresholds following exposure to noise

The relationships between properties of the acoustic reflex and temporary threshold shift (TTS) were examined in eight subjects exposed to a 95 dB SPL, 1.0-kHz octave-band noise for 4 h. The specific pattern of TTS obtained from this exposure was consistent with the expected sensitivity changes along the cochlear partition. Maximum decrease in behavioral thresholds was noted at 1.4 kHz with significant TTS also occurring at 2.0 and 4.0 kHz. Behavioral threshold at 0.5 kHz, one octave below the center frequency of exposure, was not affected. Significant reflex threshold shift (RTS) occurred at 1.4 and 2.0 kHz. In addition, RTS was noted at 0.5 kHz in the absence of any change in behavioral sensitivity at that frequency. Magnitude of the acoustic reflex was examined from threshold to 10-dB sensation level. A systematic reduction in magnitude was noted during the noise exposure for 1.4 and 2.0 kHz. The same pattern did not emerge for 0.5 kHz. Magnitude of the acoustic reflex pre-exposure was significantly correlated to TTS at 1.4 kHz. Changes in acoustic reflex thresholds and magnitudes followed the same time course as changes in behavioral thresholds during the growth and recovery periods.