Kenneth J. Gerhardt

Fetal exposures to sound and vibroacoustic stimulation.

Sounds in the environment of a pregnant woman penetrate the tissues and fluids surrounding the fetal head and stimulate the inner ear through a bone conduction route. The sounds available to the fetus are dominated by low-frequency energy, whereas energy above 0.5 kHz is attenuated by 40 to 50 dB. The fetus easily detects vowels, whereas consonants, which are higher in frequency and less intense than vowels, are largely unavailable. Rhythmic patterns of music are probably detected, but overtones are missing. A newborn human shows preference for his/her mothers voice and to musical pieces to which he/she was previously exposed, indicating a capacity to learn while in utero. Intense, sustained noises or impulses produce changes in the hearing of the fetus and damage inner and outer hair cells within the cochlea. The damage occurs in the region of the inner ear that is stimulated by low-frequency sound energy.

Sound environment of the fetal sheep

The internal sound pressure levels within the intact amnion of pregnant ewes surgically implanted with a hydrophone was determined during conditions of quiet and during sound field exposures to broadband and octave-band noise. Measurements were made of sound pressures outside and inside the ewe, and sound attenuation through maternal tissues and fluids was calculated. Sound pressures generated by low frequencies (less than 0.25 kHz) were 2 to 5 dB greater inside than outside the ewe. Above 0.25 kHz, sound attenuation increased at a rate of 6 dB per octave. For 4.0 kHz, sound attenuation averaged 20 dB. The sound pressure recorded at different locations within the amnion with respect to the sound source varied by up to 6 dB. The internal noise floor in the absence of externally generated sounds was as low as 50 dB (spectrum level) above 0.2 kHz. Thus the fetus is developing in an environment that is rich with internal and external sounds.

Effect of stress on cochlear glucocorticoid protein: Acoustic stress

Levels of glucocorticoid (GR) receptor protein were determined by a quantitative enzyme-linked immunosorbent assay (ELISA) technique in inner ear tissue of rats exposed daily to 85 dB SPL white noise for 4 hours on 3 consecutive days. GR levels in spiral ligament and organ of Corti tissues were detected using a monoclonal antibody to the GR receptor, BuGR2. A non-significant 13% decrease in GR levels of spiral ligament tissues was observed in the noise exposed animals relative to untreated animals. A statistically significant decrease of 27% in GR protein levels was seen in the organ of Corti region (P < 0.03), however. There was a concomitant increase of serum corticosterone levels (P < 0.03) in noise exposed animals as opposed to those of controls. These results indicate a tissue specific response of GR receptor to acoustic stress. Inner ear GR protein therefore may be a useful marker in determining the effect of stress on the inner ear. Finally, such data may be applicable to support the hypothesis that stress is an etiological agent in Ménières disease.

The acoustic environment and physiological responses of the fetus.

The acoustic environment of the fetus is composed of continuous cardiovascular, respiratory, and intestinal sounds that are punctuated by isolated, shorter bursts during maternal body movements and vocalizations. The distribution of sounds is confined to frequencies below 300 Hz. Additionally, vibrations on the external surface of the maternal abdomen can induce sounds inside the uterus. The half-round sound pressure contours in the abdomen during vibroacoustic stimulation differ from the circular distribution of contours resulting from airborne sound pressure exposure. The static and dynamic forces of the vibrator and the vibrator distance from the target are also factors in sound transmission. Responses to sound are best described in animals and include changes in behavioral state, brain bloodflow, auditory brainstem response, and local cerebral glucose utilization along the central auditory pathway.

Cochlear microphonics recorded from fetal and newborn sheep

PURPOSE Sounds present within the uterus stimulate the fetal inner ear and central auditory pathway. This study was undertaken to determine the efficiency of transmission of exogenous airborne stimuli to the fetal inner ear. In this way, we may quantify the extent to which the fetal auditory system is isolated from sounds produced outside the mother. MATERIALS AND METHODS Cochlear microphonics were recorded from fetal and newborn sheep to evaluate the extent to which the fetus is isolated from sounds exogenous to the ewe. Electrodes were surgically placed in contact with the round window membrane in nine near-term fetal sheep. Cochlear microphonics were recorded in response to 1/3 octave-band noises (0.125 to 2.0 kHz) delivered through a loudspeaker 1.8 m from one side of the pregnant ewe. Sound pressure levels generated by the noises were simultaneously recorded ex utero with a microphone and in utero with a hydrophone previously sutured to the fetal neck. After cochlear microphonic amplitudes were recorded, the fetus was delivered through an abdominal incision. Recordings were repeated from the newborn lamb. Fetal sound isolation was calculated as the difference between the sound pressure levels that were necessary to evoke equal cochlear microphonic amplitudes from the fetus and from the newborn lamb. RESULTS The sound attenuation observed was variable for all frequencies. The fetus was isolated from external sounds by 11.1 dB for 0.125 kHz, 19.8 dB for 0.25 kHz, 35.3 dB for 0.5 kHz, 38.2 dB for 1.0 kHz, and 45.0 dB for 2.0 kHz. CONCLUSIONS Other investigators have demonstrated that the immature auditory system is more susceptible to damage produced by noise exposure than is the mature auditory system. Low-frequency noise produces damaged cells that later in life code higher frequencies. A possibility of fetal hearing loss produced by intense noise exposure needs more careful evaluation.

Fetal hearing: Characterization of the stimulus and response

Before sounds originating outside the abdomen of pregnant women can reach the inner ear of the fetus, they must first pass through the tissues and fluids surrounding the fetal head. Low-frequency sound energy easily penetrates to the fetal head, less than 5 dB attenuation for frequencies below 500 Hz, whereas higher frequencies are attenuated by up to 20 to 30 dB. The sound energy in amniotic fluid stimulates fetal hearing through a bone conduction route rather than through the external and middle ear systems. During passage through the bones of the skull, sound energy is slightly diminished for frequencies less than 250 Hz (10 to 20 dB), yet significantly reduced for frequencies from 500 to 2,000 Hz (40 to 50 dB). Thus, the fetus in utero can easily detect low-frequency sound energy (< 500 Hz) produced at levels that are comfortably loud for its mother, but probably cannot detect acoustic energy at frequencies higher than 500 Hz.

The perception of speech sounds recorded within the uterus of a pregnant sheep

The intelligibility of speech stimuli recorded within the uterus of a pregnant sheep was determined perceptually using a group of untrained judges. The intrauterine sound environment of the ewe was intended to simulate that of a pregnant woman. Two separate lists, one of meaningful and one of nonmeaningful speech stimuli, were delivered through a loudspeaker to the side of the ewe and were simultaneously recorded with an air microphone located 15 cm from the flank and with a hydrophone previously sutured to the neck of the fetus. Perceptual test tapes generated from these recordings were played to 102 judges. The intelligibility of the phonemes recorded in the air was significantly greater than the intelligibility of phonemes recorded from the uterus. A male talkers voice was more intelligible than a female talkers voice when recorded from within the uterus, but not so when recorded in the air. An analysis of the feature information transmission from recordings inside and outside the uterus revealed that voicing information is better transmitted in utero than place or manner information.

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.

Cocaine alters behavioral states in fetal sheep.

Intrauterine cocaine exposure causes subtle neurologic abnormalities in human newborn infants; however, the mechanism for these abnormalities is not known. To investigate whether cocaine alters fetal behavioral state, the electrocortical, electro-ocular and neck muscle electrical activity was monitored in 7 chronically instrumented fetal sheep before and during both saline and cocaine HCl infusions directly to the fetus. Saline infusion to the fetus had no effect on the percentage of time spent in rapid eye movement sleep compared to no infusion (37.5 +/- 11.6% vs 46.3 +/- 4.6%, mean +/- SD, P greater than 0.1). Cocaine infusion directly to the fetus had no effect on fetal arterial pO2, but did increase mean arterial pressure from 53.6 +/- 15 mmHg to 61.0 +/- 21 mmHg (P less than 0.01). In addition, during cocaine infusion, the percentage of time spent in rapid eye movement sleep dropped to 3.9 +/- 5.1% (P less than 0.0001) and the average duration of rapid eye movement epochs decreased from 10.1 +/- 3.0 min precocaine infusion to 1.9 +/- 2.6 min during cocaine infusion (P less than 0.02). The influence of cocaine was noted in a frequency analysis of the electrocorticogram. The amplitude of the energy centered at 1 Hz during cocaine infusion (73.8 +/- 4.0 dB) was greater than the amplitude during rapid eye movement sleep (65.5 +/- 4.7 dB) and less than the amplitude during non-rapid eye movement periods (79.9 +/- 4.5 dB) (P = 0.01). Cocaine appears to alter fetal behavioral state directly and this may play a role in the abnormal behavior in newborn infants exposed to cocaine in utero.

Combined effects of adrenalectomy and noise exposure on compound action potentials, endocochlear potentials and endolymphatic potassium concentrations

The effects of removal of endogenous corticosteroids via bilateral adrenalectomy in combination with noise exposure (30 min at 100 dB) were determined by recording compound action potential (CAP) and endocochlear potentials (EP), and by measuring potassium concentrations (K+e) within the endolymph. Thirty-eight Long-Evans rats were divided into groups according to experimental treatments: adrenalectomy (ADX) or non-ADX and noise exposure or non-noise exposure. CAP thresholds, EP and K+e values were subjected to repeated-measures analysis of variance with group and time as factors classifying the measurements. Noise exposure resulted in significant elevations of CAP thresholds in both the ADX and non-ADX animals, but had no effect on either EP or endolymphatic K+e. Recovery was noted during all post-exposure measurement periods and was significantly faster for ADX animals. EP and K+e did not change during or after noise exposure. ADX animals showed a non-significant reduction of EP and a statistically significant increase of K+e during all measurement periods as compared to non-ADX animals.