Xinyan Huang

Fetal sheep in utero hear through bone conduction

PURPOSE Although the air-conduction pathway is the principal mode of sound transmission to the inner ear, this may not be true for the fetus in utero. The fetus detects and responds to sounds in the maternal environment. Exogenous sounds can reach the fetal inner ear through the ear canal and middle ear system, bone conduction, or both. This study was designed to compare the effectiveness of these two routes of sound transmission by recording cochlear microphonic potentials from the fetus in utero in response to airborne sounds. MATERIALS AND METHODS Cochlear microphonics (CMs) recorded from one round window (RW) of fetal sheep in utero were obtained in three conditions: (1) head uncovered; (2) head covered with a neoprene hood; and (3) head covered with a neoprene hood fashioned with a hole that permitted the pinna and ear canal to be exposed. Tone bursts (0.5, 1.0, and 2.0 kHz) were delivered through a loudspeaker at high intensities (100 to 135 dB sound pressure level) to the flank of the ewe. CMs were detected with indwelling electrodes, amplified, and averaged. CM input-output functions were obtained from the fetus in each of the three conditions described above. RESULTS CMs recorded with the head uncovered were more sensitive than were the CMs recorded with the hood in place. There was no difference in sensitivity between the condition during which the head was completely covered and the condition in which the pinna and ear canal are exposed. CONCLUSION The principal mode of sound transmission into the fetal inner ear is through bone conduction.

Effects of intense noise exposure on fetal sheep auditory brain stem response and inner ear histology.

OBJECTIVE To evaluate, in two separate experiments, the effects of intense noise exposures delivered to fetal sheep in utero during a time of rapid auditory development. DESIGN In the first experiment, auditory brain stem response (ABR) thresholds to clicks and tone bursts were recorded from chronically instrumented fetal sheep in utero before and after exposure of pregnant ewes to intense broadband noise. A single 16 hr exposure was delivered at 113 days gestational age, a time when the ABR is just emerging. Thresholds were compared with an age-matched, nonexposed control group. In the second experiment, fetal sheep at the same gestational age were exposed four times to broadband noise and their cochleae were harvested 20 days later for histological analysis by the use of scanning electron microscopy. Comparisons were made with an age-matched, nonexposed control group. RESULTS Experiment One: ABR thresholds recorded between 10 to 20 days after the exposure were not as sensitive as thresholds obtained from control fetuses. There was a tendency for thresholds to 0.5 kHz tone bursts to be more affected than thresholds to clicks. Experiment Two: Scanning electron microscopy of the organ of Corti from fetuses exposed to noise from 111 to 114 days gestational age revealed significant damage to inner and outer hair cells in the middle and apical turns of cochleae. Similar hair cell damage was not present in control fetuses. CONCLUSIONS Intense exogenous noise penetrated the uterus of pregnant sheep and resulted in elevations in ABR thresholds 2 to 3 wk after exposure. In fetuses repeatedly exposed to noise, the middle and apical turns of the cochlea showed greater hair cell damage than found at the same locations in control cochlea. The basal turn of the cochlea was not damaged.

Temporary threshold shifts induced by low-pass and high-pass filtered noises in fetal sheep in utero

Auditory brainstem responses (ABRs) were obtained from nine late gestational age fetal sheep in utero before and after a 16-h exposure to low-pass (cut-off frequency 1.0 kHz) and high-pass (cut-off frequency 1.0 kHz) noises (approximately 120 dB sound pressure level, recorded in air). Bone-conduction ABRs were elicited by broadband clicks and 0.5, 1.0 and 2.0 kHz tone bursts. Following low-pass noise exposure, ABR thresholds and wave IV latencies increased significantly for 0.5 and 1.0 kHz tone bursts. The high-pass noise exposure produced significant shifts in ABR thresholds and wave IV latencies only for the 1.0 kHz tone bursts. These findings confirm previous reports of low-frequency sound transmission into the fetal inner ear.

Effects of impulse noise on the hearing of fetal sheep in utero

Knowledge of the transmission of exogenous sounds to the fetal head and the effects that these sounds have on fetal hearing is incomplete. The purposes of this study were to measure the transmission of impulse noise into the uterus and to evaluate the effects of impulse noise delivered to pregnant sheep on the hearing of the fetus in utero. A shock tube produced impulses that averaged 169.7 dB peak sound pressure level (pSPL) in air. In the uterus, the pSPL varied as a function of fetal head location. When the fetal head was against the abdominal wall, peak levels were within 2 dB of airborne levels and the stimulus resembled a Friedlander wave. When the fetal head was deep within the uterus, the duration of the impulse increased and the peak amplitude decreased. In some instances the decrease in pSPL exceeded 10 dB. Slight elevations of evoked potential thresholds were noted but only for low‐frequency stimuli. The integrity of hair cells from these animals was assessed using scanning electron microscopy.

Effects of impulse noise stimulation on electrocorticogram and heart rate.

It is now recognized that high-level impulse noises penetrate the uterus of pregnant sheep, elevate thresholds of fetal auditory-evoked potential and produce damage to fetal inner ear hair cells. However, little is known about functional effects of airborne impulse noise on the late-term fetus. In the present study, the effects of a series of 20 impulses on the behavioral state of 6 fetal sheep were tested. Noise impulses produced 169.3 peak sound pressure level (pSPL) in air. Peak levels recorded near the fetal head averaged 161.1 dB. Impulses delivered to the fetus during periods of NREM sleep resulted in a decrease in average fetal heart rate (FHR) from 185 ± 22 beats/min (bpm) before stimulation to 174 ± 23 bpm 2–5 s after stimulation (p < 0.05). During REM sleep, an FHR acceleration occurred (before stimulation: 177 ± 24 bpm, after stimulation: 189 ± 31 bpm; p < 0.05). Impulse exposure during NREM sleep resulted in reductions in delta-, theta- and alpha-band powers. As a consequence, total power decreased from 100 to 72 ± 16% (p < 0.05). During REM sleep, stimulation provoked a short decrease in total band power from 100 to 73 ± 20% and a similar decrease in the theta- and beta-band powers. The results indicated that impulse noise evoked short-term alterations in FHR and cortical activity. These changes were mediated by auditory brain stem activation that led to cortical desynchronization during both NREM and REM sleep in late-term fetal sheep.

Transmission of continuous and impulse noises to the fetus in utero

Sounds which originate outside the abdomen of pregnant women reach the fetal inner ears after being filtered by the materials surrounding the fetal head and by its skull. In sheep, exogeneous sounds are low‐pass filtered from 0.25 to 4.0 kHz at a rate of 6 dB/octave before reaching the head. There is little difference in sound pressures for low‐frequency signals when recorded inside and outside the abdomen. The transmission route for fetal stimulation is via bone conduction with low‐frequency sounds (<0.25 kHz) being attenuated by about 10 dB and higher‐frequency sounds (0.5–2.0 kHz) attenuated by up to 45 dB. The amplitude and waveform of an impulse is influenced markedly by the location of the recording hydrophone within the uterus. Close to the intra‐abdominal wall, the impulse is best characterized as a simple Friedlander wave (duration <1.0 ms) with a peak sound pressure level (pSPL) that is approximately 5 dB less than that recorded in air. With the hydrophone located deep inside the uterus, the pSPL is reduced by over 15 dB and the waveform resembles that of an impact with a decay time of greater than 12 ms. Fetal cochlear microphonic resembles the waveform of the impulse when it is recorded with a hydrophone located by the fetal head.

Effects of intense noise exposure on the auditory brain‐stem response and inner ear histology of fetal sheep

Past evidence from investigations in which an intense noise was delivered to the fetal hearing mechanism has been contradictory. Some investigations suggest the possibility of a critical period during auditory development when exposure to intense noise may adversely affect auditory function. Other studies have reported no effect of intense noise on the auditory brainstem response of fetal sheep. In one recent experiment, auditory brain‐stem response thresholds were recorded in utero from fetal sheep prior to and following an intense broadband noise exposure delivered at a developmental period when the ABR was emerging. The ABR thresholds recorded from the noise‐exposed fetuses, when measured over developmental time, were not as sensitive as thresholds obtained from age‐matched control fetuses. In a related experiment, sheep were exposed four times to an intense broadband noise and the cochleae were harvested 20 days later for histological analysis. Histological comparisons revealed significant inner and o...