Effects of intense noise on fetal sheep hearing mechanism

The auditory brain-stem response was recorded from in utero fetal sheep from 111 to 136 days gestational age. Fetuses were prepared for chronic recordings. The auditory brain-stem response was evoked by clicks and tone bursts of 500, 1000, and 2000 Hz. Pregnant ewes were exposed to an intense broadband noise of 120 decibels sound-pressure level for 16 h while carrying fetuses at gestational ages of either 113 days (early noise-exposed group) or 130 days (late noise-exposed group). A third group of animals was nonexposed. Auditory brain-stem response threshold and latency increases were identified immediately after the noise exposure for the late noise-exposed group. Although there was no immediate effect on the auditory brain-stem response for the early noise-exposed group, when auditory brain-stem responses were elicited from these animals at later gestational ages, higher thresholds were identified in the noise-exposed group than in the nonexposed animals. Cochlear reconstructions and cytocochleograms completed from serial celloidin sections obtained from both noise-exposed and nonexposed fetuses had a similar appearance. Results showed that exogenous sounds can penetrate the uterus and that these sounds result in alterations of the fetal auditory brain-stem response. Light microscopy showed no sensory cell differences between the noise-exposed and the nonexposed animals. [Work supported by U.S. Army.]