Manfred Kössl, Doreen Möckel, Melanie Weber and Ernst-August Seyfarth
(2008): Otoacoustic emissions from insect ears: evidence of active hearing?
J. Comp. Physiol. A 194 (7), 597-609
Abstract: Sensitive hearing organs often employ nonlinear mechanical sound
processing which generates distortion-product otoacoustic emissions (DPOAE).
Such emissions are also recordable from tympanal organs of insects. In
vertebrates (including humans), otoacoustic emissions are considered
by-products of active sound amplification through specialized sensory
receptor cells in the inner ear. Force generated by these cells primarily
augments the displacement amplitude of the basilar membrane and thus
increases auditory sensitivity. As in vertebrates, the emissions from insect
ears are based on nonlinear mechanical properties of the sense organ.
Apparently, to achieve maximum sensitivity, convergent evolutionary
principles have been realized in the micromechanics of these hearing
organs?although vertebrates and insects possess quite different types of
receptor cells in their ears. Just as in vertebrates, otoacoustic emissions
from insects ears are vulnerable and depend on an intact metabolism, but so
far in tympanal organs, it is not clear if auditory nonlinearity is achieved
by active motility of the sensory neurons or if passive cellular
characteristics cause the nonlinear behavior. In the antennal ears of flies
and mosquitoes, however, active vibrations of the flagellum have been
demonstrated. Our review concentrates on experiments studying the tympanal
organs of grasshoppers and moths; we show that their otoacoustic emissions
are produced in a frequency-specific way and can be modified by electrical
stimulation of the sensory cells. Even the simple ears of notodontid moths
produce distinct emissions, although they have just one auditory neuron. At
present it is still uncertain, both in vertebrates and in insects, if the
nonlinear amplification so essential for sensitive sound processing is
primarily due to motility of the somata of specialized sensory cells or to
active movement of their (stereo-)cilia. We anticipate that further
experiments with the relatively simple ears of insects will help answer
these questions.
URL: http://www.springerlink.com/content/ag65616076505281/
For reprints please contact Ernst-August Seyfarth (Email:
Christopher Bergevin, Dennis M. Freeman, James C. Saunders and Christopher
A. Shera (2008): Otoacoustic emissions in humans, birds, lizards, and frogs:
evidence for multiple generation mechanisms. J. Comp. Physiol. A 194 (7),
665-683.
Abstract: Many non-mammalian ears lack physiological features considered
integral to the generation of otoacoustic emissions in mammals, including
basilar-membrane traveling waves and hair-cell somatic motility. To help
elucidate the mechanisms of emission generation, this study systematically
measured and compared evoked emissions in all four classes of tetrapod
vertebrates using identical stimulus paradigms. Overall emission levels are
largest in the lizard and frog species studied and smallest in the chicken.
Emission levels in humans, the only examined species with somatic hair cell
motility, were intermediate. Both geckos and frogs exhibit substantially
higher levels of high-order intermodulation distortion. Stimulus frequency
emission phase-gradient delays are longest in humans but are at least 1 ms
in all species. Comparisons between stimulus-frequency emission and
distortion-product emission phase gradients for low stimulus levels indicate
that representatives from all classes except frog show evidence for two
distinct generation mechanisms analogous to the reflection- and
distortion-source (i.e., place- and wave-fixed) mechanisms evident in
mammals. Despite morphological differences, the results suggest the role of
a scaling-symmetric traveling wave in chicken emission generation, similar
to that in mammals, and perhaps some analog in the gecko.
URL: http://www.springerlink.com/content/981814l70xmk0551/
For reprints please contact Christopher Bergevin (Email:
Kind regards
Sonja
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Dr. Sonja Amoser
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