The Pathway Enabling External Sounds to Reach and Excite the Fetal Inner Ear

Abstract

The human fetus in utero is able to respond to sounds in the amniotic fluid enveloping the fetus after about 20 weeks gestation. The pathway by which sound reaches and activates the fetal inner ear is not entirely known. It has been suggested that in this total fluid environment, the tympanic membrane and the round window membrane become ‘transparent’ to the sound field, enabling the sounds to reach the inner ear directly through the tympanic membrane and the round window membrane. It is also possible that sounds reach the inner ear by means of tympanic membrane – ossicular chain – stapes footplate conduction (as in normal air conduction). There is also evidence that sounds reach the fetal inner ear by bone conduction. Several animal and human models of the fetus in utero were studied here in order to investigate the pathway enabling sounds to reach and activate the fetal inner ear. This included studying the auditory responses to sound stimuli of animals and humans under water. It was clearly shown in all the models that the dominant mechanism was bone conduction, with little if any contribution from the external and middle ears. Based on earlier experiments on the mechanism and pathway of bone conduction, the results of this study lead to the suggestion that the skull bone vibrations induced by the sound field in the amniotic fluid enveloping the fetus probably give rise to a sound field within the fetal cranial cavity (brain and CSF) which reaches the fetal inner ear through fluid communication channels connecting the cranial cavity and the inner ear.

Inner ear ossification and mineralization kinetics in human embryonic development - microtomographic and histomorphological study

“…The human fetus responds to sounds after about 21–23 wks 25, 40, 41 of gestational age, concomitant with a complete bony covering of the cochlea by the inner cortical bone as seen on 2D  μ CT images. Studies on cochlear hearing physiology have focused more on the relationship between the stiffness of the basilar membrane supporting the organ of Corti and frequency perception than on the bony coverage of the related part of the cochlea.…”

“…The progressive filling-up with mineralized matrix is concomitant with the development of the auditory system. This suggests a role for the middle ear ossicles in sound perception by the fetus during the third trimester of pregnancy, even though transmission of skull vibrations to the cochlear fluids has been shown to be the dominant mechanism of sound perception in utero 18, 25, 26 .…”

Maternal sounds elicit lower heart rate in preterm newborns in the first month of life

“…As an indication of this, newborn infants show a clear preference for their mother’s voice shortly after birth [2,3]. The fetus, however, does not begin its auditory experience by hearing sounds, but rather by sensing them through the bones of the skull [4]. Hearing begins at approximately 25–26 weeks of gestational age (GA) as cochlear hair cells first translate acoustic vibrations and then airborne sound stimulation into coded electrical signals that are sent to the brainstem for additional processing [5].…”

Near‐term fetuses process temporal features of speech

“…The general conclusion was that the fetus hears through bone conduction, and thus that the middle ear was non‐functional in utero . Other authors reached the same conclusion (Sohmer, Perez, Sichel, Priner & Freeman, 2001). The characteristics of speech sounds from the CM recorded in utero and ex utero were also examined (Huang, Gerhardt, Griffiths & Abrams, 2002; Smith, Gerhardt, Griffiths, Huang & Abrams, 2003).…”