bone conduction

[Christian Kaernbach <chris@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

The results by Freeman et al. (2000) and Sohmer et al. (2000) cited by
Martin Braun constitute intriguing evidence that bone conduction occurs
when a bone vibrator is applied to the brain of animals with parts of
the skull removed, to the fontanels of neonates, or to the eye. These
findings are about the start of the transmission pathway: it needs not
be bone. How about the end of the transmission pathway? Braun speculates
that the waves enter through the vestibular or cochlear aqueduct. It
could, however, be bone. Were there made measurements with bone
microphones close to the receiving ear to prove that no bone vibration
took place? The abstracts don't tell.

These findings would be even more intriguing if it could be shown that
no bone vibration took place. Maybe this could then be seen as a proof
of aqueduct transmission. In order to prove that no extra fluid movement
took place (more than the minimal movement needed to generate a pressure
wave) it could be a good idea to monitor what happens to the round
window (and/or the tympanic membrane).

And why not block the round window? No extra displacement of the fluid
would take place (as a control, the oval window impedance should change
dramatically). If hearing thresholds (and frequency separation) would
remain unaltered after round window blocking, this would constitute a
proof of pressure waves being perfectly sufficient and the traveling
wave being an epiphenomenon. If thresholds would rise or the frequency
tuning would go down, this would show the improving effect of traveling
waves. Are there any data on that?

Christian Kaernbach
Institut fuer Allgemeine Psychologie
Universitaet Leipzig
Germany