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Auditory Development in the Fetus and Infant

DOI:10.1053/j.nainr.2008.10.010
Authors:
Stanley Norman Graven at University of South Florida
Stanley Norman Graven
Joy Browne at University of Colorado
Joy Browne
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Abstract and Figures

Auditory development in the fetus and infant entails the structural parts of the ears that develop in the first 20 weeks of gestation, and the neurosensory part of the auditory system develops primarily after 20 weeks' gestational age. The auditory system becomes functional at around 25 weeks' gestation. The cochlea of the middle ear and the auditory cortex in the temporal lobe are most important in the development of the auditory system. They are both easily affected by the environment and care practices in the newborn intensive care unit (NICU). The period from 25 weeks' gestation to 5 to 6 months of age is most critical to the development of the neurosensory part of the auditory system. This is the time when the hair cells of the cochlea, the axons of the auditory nerve, and the neurons of the temporal lobe auditory cortex are tuned to receive signals of specific frequencies and intensities. Unlike the visual system, the auditory system requires outside auditory stimulation. This needs to include speech, music, and meaningful sounds from the environment. The preterm as well as the term infant cannot recognize or discriminate meaningful sounds with background noise levels greater than 60 dB. The more intense the background noise, especially low frequency, the fewer specific frequencies (pitch) can be heard and used to tune the hair cells of the cochlea. Continuous exposure to loud background noise in the NICU or home will interfere with auditory development and especially frequency discrimination. The initial stimulation of the auditory system (speech and music) needs to occur in utero or in the NICU to develop tonotopic columns in the auditory cortex and to have the critical tuning of the hair cells of the cochlea occur. The control of outside noise, the exposure to meaningful speech sounds and music, and the protection of sleep and sleep cycles, especially rapid eye movement sleep, are essential for healthy auditory development. The environment and care practices for the fetus in utero or the infant in the NICU are critical factors in the development of the auditory system.
The structure of the human ear. The external ear, especially the prominent auricle, focuses sound into the external auditory meatus. Alternating increases and decreases in air pressure vibrate the tympanum. These vibrations are conveyed across the air-filled middle ear by three tiny, linked bones: the malleus, the incus, and the stapes. Vibration of the stapes stimulates the cochlea, the hearing organ of the inner ear (Reprinted from Kandel ER. Principles of Neural Science, Fourth Edition . New York: McGraw Hill; 2000:591; with permission).
The structure of the human ear. The external ear, especially the prominent auricle, focuses sound into the external auditory meatus. Alternating increases and decreases in air pressure vibrate the tympanum. These vibrations are conveyed across the air-filled middle ear by three tiny, linked bones: the malleus, the incus, and the stapes. Vibration of the stapes stimulates the cochlea, the hearing organ of the inner ear (Reprinted from Kandel ER. Principles of Neural Science, Fourth Edition . New York: McGraw Hill; 2000:591; with permission).
… 
Innervation of the organ of Corti. Most afferent axons end on inner hair cells, each of which constitutes the sole terminus for an average of 10 axons. A few afferent axons of small caliber provide diffuse innervations to the outer hair cells. Efferent axons largely innervate outer hair cells and do so directly. In contrast, efferent innervation of inner hair cells is sparse and is predominantly axoaxonic, at the endings of afferent nerve fibers (Reprinted from Kandel ER. Principles of Neural Science, Fourth Edition . New York: McGraw Hill; 2000:602; with permission).
Innervation of the organ of Corti. Most afferent axons end on inner hair cells, each of which constitutes the sole terminus for an average of 10 axons. A few afferent axons of small caliber provide diffuse innervations to the outer hair cells. Efferent axons largely innervate outer hair cells and do so directly. In contrast, efferent innervation of inner hair cells is sparse and is predominantly axoaxonic, at the endings of afferent nerve fibers (Reprinted from Kandel ER. Principles of Neural Science, Fourth Edition . New York: McGraw Hill; 2000:602; with permission).
… 
The central auditory pathways extend from the cochlear nucleus to the auditory cortex. Postsynaptic neurons in the cochlear nucleus send their axons to other centers in the brain via three main pathways: the dorsal acoustic stria, the intermediate acoustic stria, and the trapezoid body. The first binaural interactions occur in the superior olivary nucleus, which receives input via the trapezoid body. In particular, the medial and lateral divisions of the superior olivary nucleus are involved in the localization of sounds in space. Postsynaptic axons from the superior olivary nucleus, along with the axons from the cochlear nuclei, project to the inferior colliculus in the midbrain via the lateral lemniscus. Each lateral lemniscus contains axons relaying input from both ears. Cells in the colliculus send their axons to the medial geniculate nucleus of the thalamus. The geniculate axons terminate in the primary auditory cortex (Brodmann areas 41 and 42), a part of the superior temporal gyrus (Reprinted from Kandel ER. Principles of Neural Science, Fourth Edition . New York: McGraw Hill; 2000:604; with permission).
The central auditory pathways extend from the cochlear nucleus to the auditory cortex. Postsynaptic neurons in the cochlear nucleus send their axons to other centers in the brain via three main pathways: the dorsal acoustic stria, the intermediate acoustic stria, and the trapezoid body. The first binaural interactions occur in the superior olivary nucleus, which receives input via the trapezoid body. In particular, the medial and lateral divisions of the superior olivary nucleus are involved in the localization of sounds in space. Postsynaptic axons from the superior olivary nucleus, along with the axons from the cochlear nuclei, project to the inferior colliculus in the midbrain via the lateral lemniscus. Each lateral lemniscus contains axons relaying input from both ears. Cells in the colliculus send their axons to the medial geniculate nucleus of the thalamus. The geniculate axons terminate in the primary auditory cortex (Brodmann areas 41 and 42), a part of the superior temporal gyrus (Reprinted from Kandel ER. Principles of Neural Science, Fourth Edition . New York: McGraw Hill; 2000:604; with permission).
… 
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Auditory Development in the Fetus and Infant
Stanley N. Graven, MD and Joy V. Browne, PhD, CNS-BC
Auditory development in the fetus and infant entails the structural parts of the ears that develop in the first
20 weeks of gestation, and the neurosensory part of the auditory system develops primarily after 20 weeks'
gestational age. The auditory system becomes functional at around 25 weeks' gestation. The cochlea of the
middle ear and the auditory cortex in the temporal lobe are most important in the development of the auditory
system. They are both easily affected by the environment and care practices in the newborn intensive care unit
(NICU). The period from 25 weeks' gestation to 5 to 6 months of age is most critical to the development of the
neurosensory part of the auditory system. This is the time when the hair cells of the cochlea, the axons of the
auditory nerve, and the neurons of the temporal lobe auditory cortex are tuned to receive signals of specific
frequencies and intensities. Unlike the visual system, the auditory system requires outside auditory stimulation.
This needs to include speech, music, and meaningful sounds from the environment. The preterm as well as the
term infant cannot recognize or discriminate meaningful sounds with background noise levels greater than 60
dB. The more intense the background noise, especially low frequency, the fewer specific frequencies (pitch) can
be heard and used to tune the hair cells of the cochlea. Continuous exposure to loud background noise in the
NICU or home will interfere with auditory development and especially frequency discrimination. The initial
stimulation of the auditory system (speech and music) needs to occur in utero or in the NICU to develop
tonotopic columns in the auditory cortex and to have the critical tuning of the hair cells of the cochlea occur. The
control of outside noise, the exposure to meaningful speech sounds and music, and the protection of sleep and
sleep cycles, especially rapid eye movement sleep, are essential for healthy auditory development. The
environment and care practices for the fetus in utero or the infant in the NICU are critical factors in the
development of the auditory system.
Keywords: Auditory development; Fetal development; Infant; Hearing; Sound
The human auditory system is unique and different from other
animals. It differs from all others because it develops the
capacity to receive, interpret, and respond to complex language.
It also develops the capacity to hear, discern, and respond to
music. The auditory system supports development of language
as well as musical skills. Unlike the visual system where actual
visual experience begins after birth at term, the auditory system
requires auditory experience with voice and language, music,
and meaningful environmental sounds during the last 10 to
12 weeks of fetal life (28–30 weeks' gestational age) whether in
utero or in a newborn intensive care unit (NICU). Thus,
auditory development and the potential for interference with
auditory development are critical issues for the care of preterm
infants in the NICU or the care of postterm infants in day care or
home environments.
The Structure of the Auditory System
The structure of the external and middle ear is shown in
Fig 1.
1,2
The external ear canal leads to the tympanic
membrane (eardrum). The middle ear contains a chain of
three bones that connect the tympanic membrane to the
cochlea. Vibrations of the tympanic membrane are transmitted
to the cochlea. The cochlea (Fig 1) contains three parallel fluid
chambers. The vibration of the tympanic membrane creates
fluid waves in the cochlea. Within the cochlea, between the
fluid chambers, is the organ of Corti. The organ of Corti
contains the hair cells that have a hair-like projection from
their apex (stereocilia). It is the physical movement of the
stereocilia that is converted into a nerve signal that is then
transmitted through the spiral ganglion and the relay nuclei in
the pons and midbrain to the auditory cortex in the temporal
lobe (Fig 2).
3
The neurons of the temporal lobe connect to
From the Department of Community and Family Health, College of Public
Health, University of South Florida, Tampa, FL; University of Colorado
Denver School of Medicine and the Children's Hospital, Department of
Pediatrics, JFK Partners, Aurora, CO.
Address correspondence to Stanley N. Graven, MD, Department of
Community and Family Health, College of Public Health, University of
South Florida, 3111 E. Fletcher Ave, MDC 100, Tampa, FL 33613.
E-mail: sgraven@health.usf.edu.
© 2008 Published by Elsevier Inc.
1527-3369/08/0804-0278$34.00/0
doi:10.1053/j.nainr.2008.10.010
other parts of the cortex, limbic system (emotions), and
hippocampus (memory and learning).
Development of the Auditory System
The auditory system in the human fetus and infant has its
own developmental sequences. The anatomical or structural
parts of the system develop early. The structural parts of the
cochlea in the middle ear are well formed by 15 weeks'
gestational age and are anatomically functional by 20 weeks'
gestation.
1,2
The somaesthetic (touch), kinesthetic (move-
ment), proprioceptive (position), vestibular (motion-head),
and chemosensory (smell and touch) systems all are both
structurally and functionally operative before 20 weeks'
gestation. The auditory system follows those systems in the
sequence of development.
4
The auditory system becomes functional at around 25 to
29 weeks' gestational age when the ganglion cells of the spiral
nucleus in the cochlea connect inner hair cells to the brain stem
and temporal lobe of the cortex.
2
The earliest evidence of an
auditory evoked response is at 16 weeks' gestational age. At this
age, the ganglion cells in the cochlea are connected to nuclei in
the brainstem that stimulate a physiologic response. At 25 to
26 weeks' gestation, a loud noise in utero or in the NICU will
produce changes in autonomic function. The heart rate, blood
pressure, respiratory pattern, gastrointestinal motility, and
oxygenation can all be affected.
5
The neural connections to the
temporal lobe of the cortex are functional at around 28 to
30 weeks' gestational age. This begins the development of
tonotopic columns in the auditory cortex. They are needed to
receive, recognize, and react to language, music, and mean-
ingful environmental sounds.
The two parts of the auditory system that are most important
in the developmental processes are the cochlea (the receptor
organ) and the auditory cortex. The cochlear nuclei, superior
olivary nuclei, nucleus of the lateral lemniscus, inferior
colliculus, and medial geniculate nucleus all undergo organiza-
tion of the ganglion cells and neurons in response to
stimulation, both endogenous and exogenous (Fig 3).
3
However, they all relate to the signals received from the
neurons of the spiral ganglion and cochlear nuclei of the
cochlea. It is the cochlea and auditory cortex in the temporal
lobe that are most affected by the environment and the care
practices of the NICU.
The Cochlea
The differentiation of the hair cells in the cochlea begins early
in gestation (10–12 weeks). The development of the stereocilia
on the apex of the hair cells follows. It begins first on the inner
hair cells and later on the outer hair cells. The development of
hair cells proceeds from the base of the cochlea to the apical
regions. This is true for both inner and outer hair cells. There are
an excess number of hair cells created early in development, and
some disappear if not connected or used. It is a phenomenon
very similar to the excess ganglion cells of the retina. Nature
starts the infant or fetus with an excess of potential receptors
and connections. The outer hair cells are the last to develop, at
around 22 weeks' gestation and beyond.
2,3
They connect to
some of the neurons of the spiral ganglion, but most receive
feedback connections from the nuclei in the pons and
brainstem. These are feedback connections, many of which
are not functional until near term. The fetus and preterm
infant have limited ability to modulate or reduce an intense
auditory signal.
The movement of the tympanic membrane (eardrum) varies
with the intensity and frequency of the sound stimulus. This is
transmitted to the oval window in the cochlea where a wave is
created in the fluid chamber. The wave causes the basilar
membrane beneath the hair cells to be driven up. The hair cells
are in contact with the tectorial membrane above them. As the
basilar membrane rises, the hair cells are excited. The location of
the rise along the course of the cochlea depends on the sound
frequency or pitch. The extent of the rise in the membrane
depends on the intensity. The higher the intensity, the more the
hair cells are stimulated, resulting in more frequent firing. Each
hair cell has a specific frequency at which it achieves maximum
stimulation. On the average, adjacent hair cells should differ in
their prime or characteristic frequency by only 0.2% (1/30 of
Fig 1. The structure of the human ear. The external
ear, especially the prominent auricle, focuses sound
into the external auditory meatus. Alternating
increases and decreases in air pressure vibrate the
tympanum. These vibrations are conveyed across
the air-filled middle ear by three tiny, linked bones:
the malleus, the incus, and the stapes. Vibration of
the stapes stimulates the cochlea, the hearing organ
of the inner ear (Reprinted from Kandel ER. Princi-
ples of Neural Science, Fourth Edition. New York:
McGraw Hill; 2000:591; with permission).
188 VOLUME 8, NUMBER 4, www.nainr.com
the difference between two piano notes).
6
They are very precise
in the very specific frequency that produces the maximal
response. The hair cells connect to specific cells of the cochlear
nucleus based on the frequency or pitch of the hair cell peak
response. The tuning of the hair cells of the cochlea is facilitated
by the Kölliker organ that resides in the cochlea. It functions
throughout gestation and early infancy but disappears later in
development. Most of the tuning of the hair cells of the cochlea
occurs between 28 weeks' gestational age and early months of
infant life.
Sound energy is actually amplified by the hydrodynamic
and mechanical properties of the cochlea.
6
It is also strongly
affected by the original intensity of the auditory signal. The
greater the intensity of the auditory signal or sound is, the less
the sensitivity is for tuning of the hair cells. The sensitivity of
the basilar membrane to 80-dB stimulation is less than 1% that
for 10-dB stimulation. Although 60 to 80 dB of auditory noise
may not cause damage to adult hair cells or pitch discrimina-
tion, it can severely interfere with the initial tuning of the hair
cells in the fetus or preterm infant. Thus, the environment of
the NICU can profoundly affect the tuning of the hair cells of
the cochlea.
6
More than 90% of the cochlear ganglion cells innervate inner
hair cells. Each axon innervates a single hair cell, but each inner
hair cell directs its output to up to 10 nerve fibers. The neural
information for hearing originates almost entirely from inner
hair cells. At any point along the course of the spiral ganglion in
the cochlea, the neurons respond best to the optimal or prime
frequency of the inner hair cell. Thus, the tonotopic organiza-
tion of the auditory cortex as well as relay nuclei begins with the
postsynaptic site on the inner hair cells.
The acoustic sensitivity of axons in the cochlear nerve
mirrors the innervations pattern of the spiral ganglion cell. Like
the hair cells, each axon has a characteristic frequency of sound
for maximal response. There is a tuning curve for the ganglion
cell nerve fibers, just as there is for hair cells.
The auditory nuclei that are in the pons and midbrain areas
function in sound localization and interaural sound differences.
Because the fetus in utero receives sound by bone conduction,
there is no interaural sound difference until birth at term.
Preterm birth creates interaural sound differences and exposure
to high-frequency sound. Most high-frequency sound is filtered
by the uterus, amniotic fluid, and mother's tissues in utero. In
utero, the hair cells that are tuning to high-frequency sounds are
protected from intense high-frequency sounds, but are exposed
to low-frequency sounds that permit fine tuning of the hair
cells. The hair cells lose their sensitivity to pitch in the face of
intense background sound levels of 60 dB or greater.
6,7
Fig 2. Innervation of the organ of Corti. Most afferent axons end on inner hair cells, each of which constitutes
the sole terminus for an average of 10 axons. A few afferent axons of small caliber provide diffuse innervations
to the outer hair cells. Efferent axons largely innervate outer hair cells and do so directly. In contrast, efferent
innervation of inner hair cells is sparse and is predominantly axoaxonic, at the endings of afferent nerve fibers
(Reprinted from Kandel ER. Principles of Neural Science, Fourth Edition. New York: McGraw Hill; 2000:602;
with permission).
189
NEWBORN &INFANT NURSING REVIEWS,DECEMBER 2008
Fig 3. The central auditory pathways extend from the cochlear nucleus to the auditory cortex. Postsynaptic
neurons in the cochlear nucleus send their axons to other centers in the brain via three main pathways: the
dorsal acoustic stria, the intermediate acoustic stria, and the trapezoid body. The first binaural interactions
occur in the superior olivary nucleus, which receives input via the trapezoid body. In particular, the medial
and lateral divisions of the superior olivary nucleus are involved in the localization of sounds in space.
Postsynaptic axons from the superior olivary nucleus, along with the axons from the cochlear nuclei, project
to the inferior colliculus in the midbrain via the lateral lemniscus. Each lateral lemniscus contains axons
relaying input from both ears. Cells in the colliculus send their axons to the medial geniculate nucleus of the
thalamus. The geniculate axons terminate in the primary auditory cortex (Brodmann areas 41 and 42), a part
of the superior temporal gyrus (Reprinted from Kandel ER. Principles of Neural Science, Fourth Edition. New
York: McGraw Hill; 2000:604; with permission).
190 VOLUME 8, NUMBER 4, www.nainr.com
The auditory cortex is on the outer surface of the temporal
lobe.
3
It develops as an area with tonotopic cell columns or
clusters that represent the characteristic frequencies. The
neurons tuned to the high frequencies are in the caudal region,
and the neurons tuned to the low frequencies are in the rostral
or front end of the auditory cortex. This creates a spread of cell
groupings that are responsive to specific frequencies or pitch.
The auditory cortex is also divided into two types of alternating
zones that are at right angles to the axis of the tonotopic
columns. The first is summation columns that are half of the
zones and are responsive to either ear (EE cells), and the
alternating cortical bands (EI cells) are primarily stimulated by
one ear and inhibited by the other ear. Thus, the auditory cortex
is partitioned into columns that respond to separate frequencies
and from one or both ears.
8
The summation bands respond to
different intensities from one or both ears.
The auditory system must be able to receive and recognize
small differences in frequencies or pitch, differences in intensity
or loudness, interaural differences in sound, sound patterns,
and timing or rhythm. With these capabilities, the human can
use language, hear and feel music, and recognize meaningful
sounds from the environment to avoid danger and manage the
events and activities of daily living.
Processes Involved in Auditory
Development
The building of the human auditory system involves four
basic factors that are essential to the process.
Genetic Endowment, Activity Independent
The basic structures of the auditory system are the result of
cell multiplication, migration, differentiation, and basic cell
position. These are directed by genetic code or genetic
endowment. These events will proceed without stimulation or
outside facilitation. Some gene expression is altered by
environment and outside stimulation; but the basic structure,
cell locations, etc, are the result of genetic code. It is possible to
interfere with genetic processes but not to improve them. In the
case of the auditory system, the structure and shape of the ears,
the middle ear, the basic structure of the cochlea, the nerve
tracks, and the nuclei are also genetically coded.
2,9
The expression of individual genes that direct the
development of the auditory system may be altered by
exposure to factors emanating from the environment. The
expression of any single gene can be altered without changing
the structure of the DNA. This process is termed epigenetics
and is the basis for major genetic research in the past few
years. The alterations in gene expression result from the
exposure to three types of environmental factors. Gene
expression can be altered by chemical or toxic exposure,
nutritional deficiencies or excesses, and intense or constant
abnormal sensory stimulation. These exposures or stimuli not
only affect the mother and her fetus but can also alter the
gene expression in the eggs in the ovary of the female fetus to
transmit the effect to the next generation. Mothers exposed to
diethystilbestrol had both daughters and granddaughters
affected. The development of the auditory system can be
altered by epigenetic processes.
Endogenous Stimulation Dependent
Endogenous stimulation is nerve cell activity that originates
in the brain, sensory organs, or peripheral nerves without
outside stimulation. The first stage of this endogenous activity is
spontaneous irregular firing of ganglion cells of the spiral
nucleus and the cochlear nuclei. This is needed to promote
growth of axons for cell-to-cell connection. In the human, this
starts before the 20th week of gestation. The irregular firing
becomes regular; and with further maturation at around 22
weeks, they become synchronous waves of ganglion cell firing.
This is essential for targeting of axons and midbrain nuclei.
They continue to the temporal lobe of the cerebral cortex by 28
to 29 weeks' gestation. These endogenous stimuli can be
blocked by drugs, alcohol, and toxic chemicals from the
environment. The effect of intense noise or loud sounds on the
endogenous ganglion cell activity is not known.
9
Exogenous or Activity-Dependent Processes
Unlike vision where visual experiences and stimulation are
not needed until after birth at term, the auditory system needs
auditory stimulation as part of development during the last 10
to 12 weeks of fetal life (28–40 weeks' gestational age) and
continuing for several years after birth. Starting at 28 to 29
weeks, the hair cells and their connections in the cochlea are
sufficiently mature to begin tuning for specific sound
frequencies. The hair cells for the lower-frequency sounds are
tuned first. The fetus is protected from most high-frequency
sounds in utero. The internal in utero environment is
sufficiently quiet to permit the recognition and response to
sounds, internal and external. Exposure to outside intense low-
frequency noise (70–80 dB) will block the ability to tune the
hair cells to the very specific prime frequency in utero or in the
NICU. The differences in prime frequencies of adjacent hair cell
should be 0.2%. This requires a very quiet background noise
level either in utero or in the NICU.
6
The fetus is capable of in utero learning such as mother's
voice, simple music, or sounds common to the environment. In
utero learning of sounds, voice, and music has been demon-
strated at as early as 32 weeks' gestational age.
10
Infants in utero
learned mother's voice or a particular melody and were able to
discriminate it from others after birth. The auditory learning
and memory from fetal (or NICU) life must include recognition
of difference in pitch, pattern, intensity, and rhythm. They
cannot discern or respond to harmony or note relationships in
cords. For the fetus to learn to recognize a voice or melody, he
or she needs to have protected sleep cycles with particular
attention to rapid eye movement (REM) sleep. Rapid eye
movement sleep generates the brain waves needed to create
long-term synapses in the auditory cortex and brain stem
nuclei
11
that become the auditory memories.
191NEWBORN &INFANT NURSING REVIEWS,DECEMBER 2008
For in utero learning or NICU learning, a preterm infant
must hear the voice or music when awake or when in quiet
sleep followed by a period of REM sleep. It will take multiple
exposures and multiple cycles with REM sleep. It must occur
with the background noise level at less than 50 dB and no loud
spikes in noise level. In utero, the fetus will primarily hear voice
or music sounds with pitch or frequency around middle C and
below (b300 cps frequency).
11
The exposure to voice, music,
and meaningful sounds between 30 to 40 weeks' gestation is
needed for the fine tuning of the hair cells and their neuron
connection to the spiral ganglion and cochlear nuclei. The fetus
that is exposed to intense (b80 dB) low-frequency sound with
television, boom boxes, machinery, or room noise interspersed
with quiet and absence of voice will arrive at 40 weeks' gestation
with 2 months of language delayed. The infant will be behind in
tuning of hair cell frequency specificity. He or she will not have
developed circuits for recognition of phonemes, speech
patterns, pitch, and special characteristics of mother's voice as
well as other voices close to the infant. If the same tape of music
or voice is played repeatedly, the fetus or infant will habituate to
the tape and not attend to it.
11
All speech, music, and meaningful sounds from the
environment are not only created as memory circuits in the
auditory and language areas of the cortex but have direct
neuroconnections to the limbic system (emotional memories).
Pleasure, joy, fear, sadness, anxiety, or other parts of
emotional memory are recorded and stored as part of
auditory memories but in the limbic system. Even a fetus at
34 or 36 weeks will distinguish different moods or emotional
qualities to speech and music that are retained as part of
accumulated memories.
With in utero or NICU preterm infant learning, the speech
or music must have and repeat some familiar parts; but to
retain interest and expand recognition, new material needs to
be constantly added, and changes must be made. Head phones
should NEVER be used directly on the abdomen of a mother
during pregnancy because in utero sound is nondirectional
and the sound from each earphone is additive. It is easy to
have each earphone at 60 or 70 dB, which is 120 to 140 dB to
the infant. When it is frequencies below middle C, this will
damage and even destroy hair cells with as little as 1 to 4
hours of exposure.
12
Effects of Environment and Sensory Interference
Factors in the environment have a clear impact on auditory
development for the fetus in utero and the infant in an NICU, in
a day care, as well as at home.
In Utero All intense (N60 dB) low-frequency noise should be
avoided and especially after 20 or 22 weeks' gestation. The
fetus in utero, after 28 to 29 weeks, needs exposure to
mother's voice, family voices, music (simple melodies), and
meaningful sounds of the family and environment. The
background noise level needs to be kept to less than 50 dB,
especially in the lower frequencies, for the infant to
discriminate the speech or music.
Newborn Intensive Care Unit The background noise level
should be maintained at or less than an Leq (average sound level, a
weighted, slow response scale overone hour period) of50 dB and
at or less than an L
10
(upper sound level 10% of the time over a
one hour period) of 55 dB. The 1-second maximum should not
exceed 70 dB, a weighted, slow response. This will provide an
environment in which the infant can hear and learn the mother's
voice, music, and meaningful sounds. It is also an environment
with as little disruption of sleep and sleep cycles as possible.
Learning auditory patterns requires REM sleep after 32 weeks'
gestation to create long-term memories. Sleep cycles with REM
sleep, especially protected, are important throughout infancy and
childhood. This applies to home, day care, or NICU.
13
It is important in the care of infants in the NICU to teach and
demonstrate to parents and caregivers the requirements for a
developmentally supported environment, to control back-
ground noise, and to ensure the appropriate auditory experience
(parents' voice, etc). This includes the support for and pro-
tection of REM sleep and sleep cycles. This requires the selection
of and timing of care and care procedures to support the
developmental processes and protect sleep as much as possible.
Early learning of mother's voice and ability to discriminate it
from other voices are important in the attachment process as
well as in providing comfort. The environment of the preterm
and postterm infant is an important factor supporting auditory
language and music learning. It can, with loud, low-frequency
noise or unusual vibration and motion, create significant
interference with healthy auditory development.
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transnatal auditory learning. J Perinatol. 2000;20(8 Pt 2):
S37-S44.
11. Graven S. Sleep and brain development. Clin Perinatol.
2006;33:693-706.
12. Gerhardt KJ, Abrams RM. Fetal exposures to sound and
vibroacoustic stimulation. J Perinatol. 2000;20(8 Pt 2):
S21-S30.
13. Philbin MK, Robertson A, Hall III JW. Recommended
permissible noise criteria for occupied, newly constructed
or renovated hospital nurseries. J Perinatol. 1999;19(8,
Part 1):559-563.
193NEWBORN &INFANT NURSING REVIEWS,DECEMBER 2008
... Despite being marked by its beginning and end points, the neonatal period shouldin many respects -be understood as a direct continuation of intrauterine development. According to knowledge of auditory perception, it is well-established that the fetus can hear and process surrounding stimuli and adequate prenatal auditory stimulation is necessary for normal development of hearing (1,2). ...
... The peripheral part consists of the outer, middle, and inner ear. It participates in capturing and converting an incoming auditory stimulus (mechanical sound waves) into electrical potential, which is transferred to the central auditory system (1). The division of the peripheral system into the outer, middle, and inner ear mostly follows the development of primary germ layers or their derivatives (Figure 2A-D). ...
... The division of the peripheral system into the outer, middle, and inner ear mostly follows the development of primary germ layers or their derivatives (Figure 2A-D). The base of the inner ear forms at the beginning of the fourth gestational week and its development completes in the 20th gestational week (1,10,11). ...
The Development and Neurophysiological Assessment of Newborn Auditory Cognition: A Review of Findings and Their Application
Article
Full-text available
  • Jan 2022
This review article introduces the basic principles of infants' neurophysiology, while summarizing the core knowledge of the anatomical structure of the auditory pathway, and presents previous findings on newborns' neural speech processing and suggests their possible applications for clinical practice. In order to tap into the functioning of the auditory pathway in newborns, recent approaches have employed electrophysiological techniques that measure electrical activity of the brain. The neural processing of an incoming auditory stimulus is objectively reflected by means of auditory event-related potentials. The newborn's nervous system processes the incoming sound, and the associated electrical activity of the brain is measured and extracted as components characterized by amplitude, latency, and polarity. Based on the parameters of event-related potentials, it is possible to assess the maturity of a child's brain, or to identify a pathology that needs to be treated or mitigated. For instance, in children with a cochlear implant, auditory event-related potentials are employed to evaluate an outcome of the implantation procedure and to monitor the development of hearing. Event-related potentials turn out to be an irreplaceable part of neurodevelopmental care for high-risk children e.g., preterm babies, children with learning disabilities, autism and many other risk factors.
... Cross-sectional studies in adults cannot infer a causal link between STS depth asymmetry and functional localization of language-related areas; however, the present longitudinal study may shed more light on the development of this relationship. The fetal auditory system is functional around the 25th weeks of gestational age 26 , and its maturation is subsequently driven by environmental stimuli 27,28 . Functional language areas within the left hemisphere develop successively 29,30 . ...
Fetal temporal sulcus depth asymmetry has prognostic value for language development
Article
Full-text available
  • Jan 2023
In most humans, the superior temporal sulcus (STS) shows a rightward depth asymmetry. This asymmetry can not only be observed in adults, but is already recognizable in the fetal brain. As the STS lies adjacent to brain areas important for language, STS depth asymmetry may represent an anatomical marker for language abilities. This study investigated the prognostic value of STS depth asymmetry in healthy fetuses for later language abilities, language localization, and language-related white matter tracts. Less right lateralization of the fetal STS depth was significantly associated with better verbal abilities, with fetal STS depth asymmetry explaining more than 40% of variance in verbal skills 6–13 years later. Furthermore, less right fetal STS depth asymmetry correlated with increased left language localization during childhood. We hypothesize that earlier and/or more localized fetal development of the left temporal cortex is accompanied by an earlier development of the left STS and is favorable for early language learning. If the findings of this pilot study hold true in larger samples of healthy children and in different clinical populations, fetal STS asymmetry has the potential to become a diagnostic biomarker of the maturity and integrity of neural correlates of language. The analysis of fetal MRI data and follow-up language assessments reveal that temporal sulcus depth asymmetry is a potential biomarker for maturity and integrity of neural correlates of language.
... . Graven und Browne (2008) Eigenschaften der Sprache bereits ab der Geburt vorhanden ist (Sambeth et al., 2008). ...
Der Erwerb prosodischer Grundbausteine im Rahmen der vorsprachlichen Entwicklung - Schreieigenschaften Neugeborener von Müttern mit einer Tonakzentsprache (Japanisch) oder einer tonalen Sprache (Lamnso)
Thesis
  • Jan 2023
Das Ziel der vorliegenden Arbeit war es, intervallartige Strukturen in Melodien von Neugeborenenlauten der ersten Lebenswoche in unterschiedlichen Umgebungssprachen zu identifizieren und quantitativ zu untersuchen. Es wurden Neugeborene von Müttern mit einer Tonakzentsprache (Japanisch) und einer tonalen Sprache (Lamnso) untersucht und die Befunde miteinander verglichen. Die Frage nach einem sprachlichen Einfluss auf die Auftrittshäufigkeit und die Eigenschaften von Melodieintervallen im Weinen standen im Fokus der Arbeit. Dabei sollte auch die Komplexität der Melodieintervalle bezüglich eines sprachlichen Einflusses untersucht werden. Neben diesen Häufigkeitsanalysen wurden auch temporale Eigenschaften der gefundenen Intervalle sowie die Intervallgrößen ermittelt. Nach einer strengen Vorselektion des Gesamtdatenkorpus von 1664 Einzellauten von 40 Probanden (20 Neugeborene der Nso, 20 japanische Neugeborene) wurden 1213 geeignete Melodien auf Intervalle untersucht und die Ergebnisse verglichen. Langfristig sollen so potenzielle Risikomarker zur nicht-invasiven vorsprachlichen Diagnostik von Sprech- und Sprachentwicklungsstörungen gefunden werden. In der Auftrittshäufigkeit von Melodieintervallen zeigten sich keine signifikanten Sprachgruppenunterschiede zwischen japanischen Neugeborenen und den Neugeborenen der Nso. Dies wurde mit einer physiologischen Eigenschaft als Ausdruck der Reife des laryngealen Regelsystems in diesem frühen Alter interpretiert. Der Einfluss der tonalen Sprache zeigte sich aber in der Auftrittshäufigkeit komplexer Intervalle in der Sprachgruppe Lamnso, die in Anwendung eines verallgemeinerten linearen gemischten Modells signifikant größer war als bei den japanischen Neugeborenen. Die Komplexität der Intervalle, die durch den Intervallkomplexitätsindex (ICI) ausgedrückt wurde, zeigte auf Neugeborenenlevel einen signifikanten Unterschied, in der Sprachgruppe Lamnso wurden mehr komplexe Melodieintervalle gefunden. Die temporalen Eigenschaften zeigten teilweise signifikante Unterschiede. Diese betrafen die Längenverhältnisse der Plateaulängen und die Frequenzverhältnisse der Plateaus. Die Frequenzverhältnisse (Intervallgröße) ergaben sehr ähnliche Befunde. Das vorherrschende Melodieintervall im spontanen Weinen der Neugeborenen beider Sprachgruppen war das Einzelintervall der Größe eines Halbtons. Zusammenfassend kann man sagen, dass Melodieintervalle bei gesunden Neugeborenen bereits in der ersten Lebenswoche regelhaft auftreten. Sprachliche Besonderheiten der vokalen Regelleistung scheinen sich in der Komplexität der Melodieintervalle zu zeigen.
... Dalam istilah Arab, kata pendengaran (as-sam'a) diungkap dalam bentuk tunggal (mufrad) karena biasanya objek yang didengar oleh setiap orang baik sendiri maupun kolektif selalu sama dari arah mana pun sumber suara itu berasal (Azkiyani, 2020). Sistem pendengaran manusia mulai berfungsi pada usia kehamilan sekitar ke 25 hingga ke 29 minggu (Stenley Norman Graven, 2008). Pada tahapan ini bayi mulai melakukan respon terhadap suara atau bunyi-bunyi dan dan detak jantung serta getaran dari tubuh ibunya yang mengindikasikan bahwa pendengaran mereka berfungsi dengan baik (Aprilia, 2020) Terbukanya gerbang pendengaran pada saat bayi dilahirkan dalam tradisi Islam dijadikan sebagai momentum awal pembentukan karakter yang baik. ...
Pedagogi Pembentukan Karakter Ulul Albab pada Anak Usia Dini
Article
Full-text available
  • Oct 2022
Penelitian ini dilatarbelakangi oleh permasalahan pendidikan karakter yang hingga kini belum menunjukkan hasil yang memuaskan ditandai dengan munculnya berbagai tindakan destruktif seperti korupsi, kekerasan, kejahatan seksual, narkoba, dan perusakan. Penelitian ini bertujuan mendeskripsikan pola pembentukan karakter ulul albab pada anak usia dini berdasarkan isyarat Al-Qur’an melalui pola pembelajaran pedagogikal sebagai upaya meminimalisir tindakan destruktif. Metode penelitian ini menggunakan studi pustaka sedangkan teknik pengumpulan data menggunakan teknik dokumentasi dengan melacak berbagai referensi kepustakaan sebanyak 50 literatur yang relevan bersumber dari berbagai jurnal, buku dan kitab serta referensi lainnya yang terbit minimal 10 tahun terakhir. Berdasarkan hasil penelitian diperoleh bahwa pembentukan karakter ulul albab pada anak usia dini harus dimulai dengan pembentukan pendengaran, penglihatan, dan hati (nalar fitri) yang didesain secara terstruktur melalui pola pendekatan pedagogis.
... The masking of external sounds by internal sounds, that is the mother's heartbeat, digestion and, importantly, the mother's voice (Querleu et al., 1984), presumably explains why newborns can recognize their father's voice but prefer their mother's voice (Lee and Kisilevsky, 2014). Also, the exposure of preterm babies to outside intense low-frequency noise (50dB-80dB) in the NICU may block the ability to tune hair cells to the very specific prime frequencies of adjacent hair cells (Graven and Browne, 2008). ...
... Dosimeters were calibrated before the recording sessions. Pre-and postcalibration information were according to the American National Standards Institute [38]. The average equivalent continuous sound level (Leq) and the A-weighted sound levels exceeding 10%, 50%, and 90% of the time (L10, L50, and L90, respectively) were calculated for the duration of the recording period in each room. ...
Music Therapy Intervention in an Open Bay Neonatal Intensive Care Unit Room Is Associated with Less Noise and Higher Signal to Noise Ratios: A Case-Control Study
Article
Full-text available
  • Aug 2022
Background: Noise reduction in the Neonatal Intensive Care Unit (NICU) is important for neurodevelopment, but the impact of music therapy on noise is not yet known. Objective: To investigate the effect of music therapy (MT) on noise levels, and whether individual MT (IMT) or environmental MT (EMT) increases meaningful signal-to-noise ratios (SNR). Study design: This case-control study was conducted in a level III NICU. Noise levels were recorded simultaneously from two open bay rooms, with a maximum of 10 infants in each room: one with MT and the other without. MT sessions were carried out for approximately 45 min with either IMT or EMT, implemented according to the Rhythm Breath and Lullaby principles. Noise production data were recorded for 4 h on 26 occasions of EMT and IMT, and analyzed using R version 4.0.2 software. Results: Overall average equivalent continuous noise levels (Leq) were lower in the room with MT as compared to the room without MT (53.1 (3.6) vs. 61.4 (4.7) dBA, p = 0.02, d = 2.1 (CI, 0.82, 3.42). IMT was associated with lower overall Leq levels as compared to EMT (51.2 vs. 56.5 dBA, p = 0.04, d = 1.6 (CI, 0.53, 1.97). The lowest sound levels with MT occurred approximately 60 min after the MT started (46 ± 3.9 dBA), with a gradual increase during the remaining recording time, but still significantly lower compared to the room without MT. The SNR was higher (18.1 vs. 10.3 dBA, p = 0.01, d = 2.8 (CI, 1.3, 3.86)) in the room with MT than in the room without MT. Conclusion: Integrating MT modalities such as IMT and EMT in an open bay NICU room helps reduce noise. Both MT modalities resulted in higher SNR compared to the control room, which may indicate that they are meaningful for the neurodevelopment of preterm infants.
... In addition, audiovisual integration ability is affected by neurodevelopmental disorders such as autism (Wallace & Stevenson, 2014), participants with autism spectrum disorder (ASD) are unable to access information from more than one sense and have difficulties in integrating information into higher levels of processing, so they showed no Colavita effect or rather, a reverse Colavita effect (Moro et al., 2012). Therefore, sensory dominance mainly depends on the extent of reliance on sensory information, which is related to the developmental process of visual and auditory cortices (Graven & Browne, 2008a, 2008bHirst et al., 2018) and individual differences in the ability to assimilate information from more than one sense (Wallace & Stevenson, 2014). ...
Musical training reduces the Colavita visual effect
Article
Full-text available
Although it has been documented that musical training enhances multisensory integration, there is not yet a consensus as to how musical training influences the visual dominance effect in sensory dominance. The present study adopted the Colavita visual dominance paradigm, presenting auditory stimuli concurrent with visual stimuli, to investigate the visual dominance effect between music majors and nonmusic majors and compared the reaction time and response proportion of the two kinds of participants in the bimodal trials. The results showed that the proportion of simultaneous responses in bimodal trials of music majors is higher than that of nonmusic majors; the nonmusic majors show a greater difference between the proportion of "Visual-Auditory" trials and "Auditory-Visual" trials compared with the music majors; the ΔRT of the two responses of the nonsimultaneous bimodal trials of nonmusic majors is longer than that of music majors. The results indicated that musically trained individuals have an enhanced ability to bind visual and auditory information and show a lesser Colavita effect, that is, a reduced visual dominance effect, than their nonmusic major peers. We conclude that musical training extends beyond the field of vision or auditory domain, improves audiovisual integration, and reduces the visual dominance effect.
Deprivation-Induced Plasticity of the Auditory Pathway
Thesis
  • Jan 2022
The human sensory system is a collection of incredibly complex biological networks relying on a host of cell types and connections, the integration of which allows us to perceive, navigate, and engage with our environment. Sustained by delicate balances of inhibitory-excitatory neuronal communication, regions of the peripheral and central nervous systems involved in sensory processing are finely tuned to optimally guide adaptive behaviour. Increased complexity, however, may come at the cost of an increased number of points of failure. In humans, the degeneration of the auditory pathways and resultant hearing loss (e.g., due to natural aging processes, ototoxic exposure, acoustic trauma, etc.) often leads not only to a considerable decrease in well-being and mental health, but also to the emergence of other auditory maladies, such as tinnitus and hyperacusis. Subjective tinnitus refers to the conscious experience of auditory objects (e.g., ringing, buzzing, white noise, beeping, etc.) when no external sound is present. Instead of hearing phantom sounds, people with hyperacusis perceive all external sound as louder, or “turned up”, often to the point of discomfort or pain. Although research in recent decades has led to advances in understanding the neuroscientific principles underlying tinnitus and hyperacusis, the precise neural correlates of these perceptual phenomena as well as their biological generators remain unresolved. Although tinnitus and hyperacusis differ in terms of both experiential symptoms and neural circuitry, the observed upregulation of auditory neural activity in sufferers of these phenomena has led to the development of several theories (e.g., homeostatic plasticity, stochastic resonance, adaptive gain, etc.) which implicate a dynamic central gain mechanism of the auditory pathway. Given the high rates of co-occurrence of tinnitus and hyperacusis with hearing loss, it is thought that a reduction in cochlear input may trigger plastic changes in downstream areas of the auditory pathway, with maladaptive processes resulting in the experience of tinnitus or hyperacusis. However, the diversity of hearing loss types and causes, combined with the complexity of the central auditory pathway, has made such a straightforward model difficult to test and ultimately prove. The aim of the studies included in this thesis is to contribute to the small but growing neuroscientific literature which investigate the adaptive plasticity of the human auditory pathway following a temporary hearing loss. Chapter 1 outlines the biology of the human auditory system and its capabilities, as well as current theories in the neuroscience of tinnitus and hyperacusis. Chapter 2 describes the first study, which investigated adaptive changes in the auditory brainstem following two weeks of bilateral deprivation and their relation to hyperacusis. Chapters 3 and 4 present the second study, which, to the best of my knowledge, is the first magnetoencephalography (MEG) to measure spatiotemporal patterns of cortical activity associated with the development of phantom auditory perception following unilateral deprivation. Finally, Chapter 5 contains a summary and outlook of the current work.
Maternal Voice Exposure and Its Effect on Premature Infants' Feeding Milestones: A Systematic Review
Article
Background: Premature infants experience alterations in maternal stimulation (including auditory sensory alteration such as talking or singing to the infant in the neonatal intensive care unit) due to admission to the neonatal intensive care unit. Because of their physiological and neurobehavioral immaturity, infants are at an increased risk of delays in reaching feeding milestones (a key developmental milestone), which often need to be achieved before discharge. Purpose: This systematic review evaluated the literature regarding the effect of maternal speech on achievement of feeding milestones in premature infants. Data sources: A systematic search of CINAHL, PubMed, Web of Science, and Google Scholar from 2010 to 2021. Study selection: Studies were selected if they examined the effect of maternal voice interventions on premature infants' feeding milestones. Data extraction: Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were used. Results: Six studies were identified. This systematic review of the literature on the effects of maternal voice on feeding milestones in premature infants found equivocal results. Implications for practice: Given the inconsistent results, this systematic review does not support a change in clinical practice. However, encouragement of maternal visits is highly recommended as the additional benefits of the mother's presence may extend beyond exposure to maternal voice. Implications for research: More research is needed including use of more homogenous samples, application of recommended decibel levels, and utilization of an adequately powered randomized controlled trial to further examine the effects of maternal voice on feeding milestones.
Post-Translational Modifications and Age-related Hearing Loss
Article
Post-translational modifications (PTMs) affect nearly all systems of the human body due to their role in protein synthesis and functionality. These reversible and irreversible modifications control the structure, localization, activity, and properties of proteins. For this reason, PTMs are essential in regulating cellular processes and maintaining homeostasis. Diseases such as Alzheimer's, cardiovascular disease, diabetes, cancer, and many others have been linked to dysfunctions of PTMs. Recent research has also shown that irregularities in PTMs can be linked to hearing loss, including age-related hearing loss (ARHL) – the number one communication disorder and one of the top neurodegenerative diseases in our aging population. So far, there has been no FDA approved treatment for ARHL; however, translational studies investigating PTMs involvement in ARHL show promising results. In this review, we summarize key findings for PTMs within the auditory system, the involvement of PTMS with aging and ARHL, and lastly discuss potential treatment options focusing on utilizing PTMs as biomarkers and therapeutic pathway components.
Recommended permissible noise criteria for occupied, newly constructed or renovated hospital nurseries. The Sound Study Group of the National Resource Center
Article
Full-text available
To base permissible noise criteria for occupied, new nurseries on research findings. An interdisciplinary group of clinicians reviewed the literature regarding the effect of sound on the fetus, newborn, and preterm infant and based recommended criteria on the best evidence. An external panel subsequently reviewed the criteria. The recommended criteria: Patient bed areas and the spaces opening onto them shall be designed to produce minimal ambient noise and to contain and absorb much of the transient noise that arises within the nursery. The overall, continuous sound in any bed space or patient care area shall not exceed: (1) an hourly Leq of 50 dB and (2) an hourly L10 of 55 dB, both A-weighted, slow response. The 1-second duration Lmax shall not exceed 70 dB, A-weighted, slow response. The permissible noise criteria will protect sleep, support stable vital signs, and improve speech intelligibility for many infants most of the time.
Physiological Effects of Sound on the Newborn
Article
Full-text available
Excessive sound is an acknowledged problem in neonatal intensive care units (NICUs); however, there is relatively little objective information about the effects of sound on the newborn. The cardiovascular and respiratory systems have been the most extensively studied systems. The patterns of response in these systems may be influenced by a variety of factors, including: the intensity of the sound, the infant's behavioral state, the infant's maturity and postnatal age, and the perinatal history. This article reviews the known cardiovascular, respiratory, and other physiological effects of sound on neonates.
Evidence of Transnatal Auditory Learning
Article
Full-text available
There is converging evidence for fetal retention of auditory experience into early postnatal life, but critical tests with appropriate controls are rare due to methodological hurdles. Research has been conducted on newborn response to naturally occurring stimuli such as heartbeats, intrauterine recordings, pre- and postnatal versions of the maternal voice, father's voice, and unfamiliar voices. Postnatal experience cannot be ruled out as a possible explanation for many results. Only one critical prenatal exposure experiment with postnatal testing has been carried out and published in a peer-reviewed scientific journal. Interpretation of acoustic and linguistic information on intrauterine recordings suggests that the prosodic features of speech (pitch contours, rhythm, and stress) are available to the fetus. This is compatible with newborn responses and may contribute to language acquisition during the first year. There is no sound evidence that providing extra prenatal auditory stimulation benefits the developing child, and there are potential risks.
Recommended Permissible Noise Criteria for Occupied, Newly Constructed or Renovated Hospital Nurseries
Article
To base permissible noise criteria for occupied, new nurseries on research findings. An interdisciplinary group of clinicians reviewed the literature regarding the effect of sound on the fetus, newborn, and preterm infant and based recommended criteria on the best evidence. An external panel subsequently reviewed the criteria. The recommended criteria: Patient bed areas and the spaces opening onto them shall be designed to produce minimal ambient noise and to contain and absorb much of the transient noise that arises within the nursery. The overall, continuous sound in any bed space or patient care area shall not exceed: (1) an hourly Leq of 50 dB and (2) an hourly L10 of 55 dB, both A-weighted, slow response. The 1-second duration Lmax shall not exceed 70 dB, A-weighted, slow response. The permissible noise criteria will protect sleep, support stable vital signs, and improve speech intelligibility for many infants most of the time.
Development of Neurosensory Structures in the Human Cochlea
Article
  • Feb 1992
The present paper summarizes the main stages in the maturation of the cochlear neurosensory structures in humans. New trends in the field of the functional development of the cochlea, as well as some recent experimental data are also discussed.
The Acoustic Environment and Physiological Responses of the Fetus
Article
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.
Fetal Exposures to Sound and Vibroacoustic Stimulation
Article
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 mother's 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.