AUDITORY SYSTEM : THE RUFFLING EFFECT OF RUMBLE
Barely perceptible
low-frequency signals nevertheless activate measurable responses in our
auditory circuits. Neurobiologists at Ludwig-Maximilians-Universitaet (LMU) in
Munich have now characterized the remarkable impact of low-frequency sounds on
the inner ear
The human auditory
system appears to be poorly adapted to the perception of low-frequency sound
waves, as hearing thresholds become markedly higher for frequencies lower than
about 250 Hz. Yet sensory cells do react to pressure waves with frequencies
below 100 Hz, as revealed by the fact that such signals actually evoke
detectable micromechanical responses in nerve cells in the inner ear, as LMU
neurobiologists now report in the journal Royal Society Open Science.
Sources of low-frequency
signals are a prominent feature of technologically advanced societies like our
own. Wind turbines, air-conditioning systems and heat pumps, for instance, can
generate such sounds. Hearing thresholds in this region of the acoustic
spectrum vary from one person to the next. "But the assumption that the
ear is unresponsive to low-frequency sounds because these are seldom
consciously perceived is actually quite false. The ear indeed reacts to very
low-frequency signals," says Dr. Markus Drexl of LMU. In collaboration
with researchers led by Professor Benedikt Grothe (Head of the Division of
Neurobiology in LMU's Department of Biology II) and a team based at Munich
University Medical Center, Drexl has carried out a laboratory study which shows
that low-frequency sounds, though virtually imperceptible, actually have a
surprisingly strong effect on sensory cells in the inner ear.
Low-frequency hum
stimulates the cochlea The new study is based on data collected from 21
experimental subjects with normal hearing, whose ears were exposed to a 30-Hz
tone for 90 seconds at a sound-pressure level equivalent to 80 decibels. To
determine how the inner ear responded to the signal, the researchers took
advantage of a phenomenon referred to as spontaneous otoacoustic emissions
(SOAEs). SOAEs are scarcely perceptible acoustic signals which are produced by
the inner ear in the absence of overt stimulation, and can be detected with a
sensitive microphone inserted in the ear canal.
"It turns out
that low-frequency sounds have a clearly definable modulatory influence on
spontaneous otoacoustic emissions," says Drexl. Following exposure to the
30-Hz signal for 90 seconds, the subjects' SOAEs exhibited slow oscillations in
frequency and level, which persisted for up to 120 seconds. "Strikingly,
the effect of the low-frequency stimulus on the cochlea persists for longer
than the duration of the stimulus itself," Drexl points out. Further
experiments will probe the possibility that this phenomenon may be linked to
noise-induced auditory damage, one of the most common causes of hearing
impairment in industrialized countries.
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