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Re: bone conduction

----- Original Message -----
From: Christian Kaernbach <chris@PSYCHOLOGIE.UNI-LEIPZIG.DE>
Sent: Tuesday, November 28, 2000 9:32 AM
Subject: bone conduction

> 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.

No, it was suggested by the authors themselves. See the final sentences in
each of the two abstracts. The details are in the full papers, of course.

> 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.

The full paper does. Freeman et al. (2000) carried out just this experiment
with one rat and with one guinea pig using a Bruel and Kjaer accelerometer
(Type 4393). Results: no bone vibration, when the vibrator was applied to
the brain. [Reversing the instruments, however, showed the propagation of
vibration from bone to brain.]

> These findings would be even more intriguing if it could be shown that
> no bone vibration took place.

Exactly this was shown.

> 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?

Yes, there are. Freeman et al. (2000) carried out just these experiments. In
six fat sand rats, which are particularly suited for such a preparation,
completely immobilized the stapes (confirmed postmortem). In three of these
animals also the round window was immobilized. In the former case hearing
thresholds even improved (5-10 dB), in the latter case they worsened (25-30
dB), when the vibrator was applied to the brain. Thus it was demonstrated
that "bulk flow" of cochlear fluids is not necessary for hair cell

[By the way, reading the full papers should be recommended. They are well
written throughout.]

> Christian Kaernbach
> Institut fuer Allgemeine Psychologie
> Universitaet Leipzig
> Germany

Martin Braun
Neuroscience of Music
Gansbyn 14
S-671 95 Klässbol