The effects of interruption on squirrel monkey temporary threshold shift to a 96-hour noise exposure.

6 squirrel monkeys (Saimiri scuireus ) were exposed to an octave band of noise with a center frequency of 500 Hz under both continuous and interrupted conditions. Continuous exposures lasted for 4, 8, 16, 24, 48, 72 and 96 h, with complete recovery of hearing before the next exposure. The interrupted exposure was 96 h, long with 5-min interruptions so that temporary threshold shift (TTS) could be measured at 2, 4, 8, 12, 16, 24, 36, 48, 60, 72 and 84 h of exposure. There were no differences in TTS from 8 to 96 h of exposure between the two conditions. However, at 4 h of exposure, the interrupted exposure showed significantly less TTS (p less than 0.05) than the continuous exposure. The only difference between the exposures was a 5-min interruption at 2 h of exposure during the interrupted exposure. Thus, the interrupted-exposure method is valid for making generalizations about continuous exposures of 8 h or longer. The results also confirmed our previous finding that there was no asymptotic threshold shift for the squirrel monkey although we extended the exposure time to 96 h in this experiment. The similarity of human and squirrel monkey TTS growth functions is consistent with our earlier results and supports the suitability of the squirrel monkey as an animal model for noise-induced hearing loss in humans.