Download Frequency analysis in the ear of the cicada Tettigetta josei

Biophysics of hearing in the male cicada Tettigetta josei. How does the mechanics of
the eardrum contribute to frequency analysis?
Axel Michelsen and Paulo Fonseca (Lisbon)
Measurements of the amplitude and phase of the vibration velocity were performed by means
of laser vibrometry in the frequency range 1 to 25 kHz at many positions on the eardrum, which
has an area of c. 2 mm2. Special attention was given to the vibrations of the ridge (a sclerotized
part of the eardrum) and the apodeme (a rod connecting the ridge with receptor cells inside an
auditory capsule, which is located at some distance from the eardrum). The eardrum vibrates in
its basic mode, whereas the ridge has a complicated mechanical behaviour with large variations
of both amplitude and phase. In addition, a travelling wave is found in the eardrum, but not in
the ridge. The bending vibrations travelling along the apodeme to the receptor cells have
simple amplitude and phase spectra, but the vibration amplitude decreases 15-20 dB from the
ridge to the entrance of the auditory capsule, which is about halfway to a crescent-like structure, to which the majority of the receptor cells attach. The signal transmission through the
apodeme also seems to include rotational vibrations. In addition, by using a point probe nanosensor as a go-between we could measure vibrations in the plane of the ear drum and directed
to the apodeme, which thus seems to vibrate with three degrees of freedom. This explains why
the receptor cells attach at all possible angles to the apodeme. We speculate that the lateral
movements of the apodeme may be restricted near its center in a manner similar to how an oar
hole restricts an oar. If true, the receptor cells may, after all, be subjected to fairly large vibrations.
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