New evidence: Also mammals hear without traveling wave

[Martin Braun <nombraun@xxxxxxxxxxxxxxx>]

Subject:    New evidence: Also mammals hear without traveling wave

Findings:
"Bone conduction" can occur without bones. It actually is tissue - and fluid
conduction.

Implications:
In common with all other classes of vertebrates, also mammals hear by direct
hair cell resonance and not by means of a traveling wave transmission.

1) Freeman et al. (2000) and Sohmer et al. (2000) reported a series of
radically new experiments on bone conduction in the hearing of mammals,
including humans (see abstracts below).

2) Their results are bound to change our views on bone conduction, as
pointed out by the authors themselves.

3) Their results, however, are also bound to change the views of some of us
on inner ear function generally, as not yet pointed out by the authors
themselves.

4) They showed that hearing occurs, when the only possible sound input into
the cochlea is passing through the vestibular aqueduct and/or the cochlear
aqueduct.

5) These canals (see web-sites below) can transport sound-waves from
cerebral tissue to cochlear hair cells, but they can not transport fluid
shifts (bulk flow), which is needed to trigger a basilar membrane traveling
wave.

6) Note that both aqueducts are much longer and also narrower than the
helicotrema, which is regarded as a "plug" for bulk flow, due to viscosity,
at acoustic frequencies, except at the very low ones (Patuzzi, 1996; ref.
below).

Conclusion:
A basilar membrane traveling wave can not be a necessary condition of hair
cell stimulation.


Abstracts:

Hearing Research
Volume 146, Issue 1-2
August 2000
Pages 72-80

Bone conduction experiments in animals - evidence for a non-osseous
mechanism

Sharon Freeman a, Jean-Yves Sichel b and Haim Sohmer aA
sohmer@md2.huji.ac.il
[a]Department of Physiology, Hebrew University, Hadassah Medical School,
P.O. Box 12272, Jerusalem 91120, Israel[b]Department of Otolaryngology/Head
and Neck Surgery, Hadassah University Hospital, Jerusalem, Israel
A Corresponding author. Tel.: +972 (2) 6758385; Fax: +972 (2) 6439736
Manuscript received 3 December 1999 Accepted 28 April 2000;

Abstract
Bone conducted stimuli are used to differentiate between conductive and
sensori-neural hearing loss. It has been thought that the main route for the
transfer of vibratory energy from the point of application of the bone
vibrator on the skull to the inner ear is completely osseous. An additional
mechanism may play a prominent role. In rats, a bone vibrator was applied to
the skull and also directly on the brain, after removing bone (a
craniotomy), exposing the brain. Auditory nerve-brainstem evoked response
(ABR) could be elicited not only with the vibrator on bone, but also with
the vibrator directly on the brain. Similar results were obtained in
guinea-pigs and fat sand rats. Noise masked this ABR. Extensive removal of
skull bone did not alter the ABR to bone-conducted stimuli delivered to the
exposed brain. Experimental elimination of the ossicular chain inertial
mechanism and of the occlusion effect did not greatly alter the bone
conduction response. A reduction in the fluid volume of the cranial cavity
induced threshold elevations of the bone conducted ABR but not of the air
conducted ABR. These findings can be interpreted as evidence that the
'classical' bone conduction mechanisms should be modified to include a major
pathway for cochlear excitation which is non-osseous: when a bone vibrator
is applied to the skull, the bone vibrations may induce audio-frequency
sound pressures in the skull contents (brain and cerebro-spinal fluid) which
are then communicated by fluid channels to the fluids of the inner ear.


Hearing Research
Volume 146, Issue 1-2
August 2000
Pages 81-88

Bone conduction experiments in humans - a fluid pathway from bone to ear

Haim Sohmer aA sohmer@md2.huji.ac.il, Sharon Freeman a, Miriam Geal-Dor b,
Cahtia Adelman b and Igal Savion c
[a]Department of Physiology, Hebrew University - Hadassah Medical School,
P.O.B. #12272, Jerusalem 91120, Israel[b]Speech and Hearing Center, Hadassah
University Hospital, Jerusalem, Israel[c]Department of Maxillofacial
Prosthesis, Hadassah University Hospital, Jerusalem, Israel
A Corresponding author. Tel.: +972 (2) 6758385; Fax: +972 (2) 6439736
Manuscript received 8 December 1999 Accepted 26 April 2000;

Abstract
Animal experiments in this laboratory have led to the suggestion that a
major pathway in bone conduction stimulation to the inner ear is via the
skull contents (brain and CSF). This hypothesis was now tested in humans.
Auditory nerve brainstem evoked responses could be recorded in neonates to
bone conduction stimulation over the fontanelle and audiometric responses
were obtained in neurosurgical patients with the bone vibrator on the skin
over a craniotomy. There were no differences in threshold between these
responses and those obtained to bone conduction stimulation over skull bone
in the same subjects. Audiometric thresholds in response to bone vibrator
stimulation of the eye (a 'natural craniotomy') were no different from those
to bone stimulation delivered to several sites on the head. Thus there is no
need to vibrate bone in order to obtain 'bone conduction' responses. Bone
vibrator thresholds to stimulation at the head region with thinnest bone
(temporal) were better than those to stimulation at the forehead region
which has much thicker bone, implying that the vibrations penetrate the
skull at the site of the vibrator. In addition, the magnitude of vibration
(acceleration) measured at various sites around the head in response to bone
vibrator stimulation at a fixed point on the forehead generally decreased
with distance from the point of vibration. Therefore it seems that the
vibrations produced by a bone vibrator at a point on the head are also able
to penetrate the skull, setting up audio-frequency pressures in the CSF
which spread by fluid communications to the inner ear fluids, exciting the
ear.


Web-sites with data on vestibular and cochlear aqueducts:

http://oto.wustl.edu/cochlea/intro1.htm
http://oto.wustl.edu/cochlea/model/cochdim.htm
http://206.39.77.2/temporalbone/chapter/Tchapter.html


Reference:

Patuzzi, R. (1996), Cochlear micromechanics and macromechanics. In: P.
Dallos et al. (Eds.), The Cochlea, Springer-Verlag New York, pp.186-257.


Martin Braun
Neuroscience of Music
Gansbyn 14
S-671 95 Klässbol
Sweden
nombraun@post.netlink.se