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Dear List,
In case it might be useful to anyone else, please see my response to Jan's question below.
Best,
Brian
Brian B. Monson, PhD
Research Fellow | Department of Pediatric Newborn Medicine
Brigham and Women's Hospital | Harvard Medical School
Dear Jan,
Fetal hearing is certainly a complicated issue. I've seen several individuals at conferences refer to the intrauterine environment as simply a 500-Hz low-pass filter, which doesn't appear to be entirely accurate. Another point sometimes brought up is
that, to a first approximation, intrauterine hearing should mimic underwater hearing, which stimulates the inner ear primarily via bone conduction (Hollien, 1973, JASA, 53:1288-1295). However, I think we need to be cautious about bone conduction for children
and adults vs. a fetus whose bones (including the ossicles) aren't completely ossified. On the other hand, inner ear stimulation could occur entirely via fluid rather than bone (Perez et al, 2011, Hearing Research, 280:82-85).
To your specific question, the middle ear of the fetus is fluid filled, which will likely dampen whatever mechanical forces the non-ossified ossicles might provide. Further complicating the process, however, the (incompressible) fluid in the middle ear
would also provide another direct acoustic pathway from the tympanic membrane to both the oval window and round window. Theoretically this pathway is also present via the airspace when the middle ear is filled with air, but I suppose it typically gets ignored
because the ossicles would dominate oval window actuation. That might not be true of cartilaginous ossicles in a fluid-filled middle ear space.
In any event, fetuses have been reported to develop sensitivity to low frequencies first, followed by high frequencies (Hepper & Shahidullah, 1994, "The development of fetal hearing"), so the question of what the cochlea picks up is highly dependent on
the gestation time point of interest. Gerhardt & Abrams have a review on fetal hearing that you might already have, but I'm attaching just in case.
I hope this is helpful. I'd be interested to hear any input you might get from others.
Best,
Brian
Brian B. Monson, PhD
Research Fellow | Department of Pediatric Newborn Medicine
Brigham and Women's Hospital | Harvard Medical School
Dear List,
I was looking around for literature describing the intra-uterine sound environment of unborn fetuses and came across this paper
which suggests that sound in the uterus - if measured with a hydrophone - is essentially unattenuated relative to the source of air born sound and shows apparently no low-passing. That would suggest that sound in utero is essentially crystal clear and
not "muffled" as one might perhaps intuitively suspect. However, as I was pondering this I wondered: what about the middle ear of unborn children? Are they filled with (amniotic?) fluid? Or with air? And does it matter? How good could we expect the cochlea
in the unborn to be at picking up sound from the amniotic fluid, and does that depend on whether the middle ear is fluid or air filled?
Best wishes,
Jan
--
Prof Jan Schnupp
University of Oxford
Dept. of Physiology, Anatomy and Genetics
Sherrington Building - Parks Road
Oxford OX1 3PT - UK
+44-1865-282012
http://jan.schnupp.net
<Gerhardt_2000_Fetal Exposures to Sound and Vibroacoustic Stimulation.pdf>
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