New evidence: Also mammals hear without traveling wave (Martin Braun )

Subject: New evidence: Also mammals hear without traveling wave
From:    Martin Braun  <nombraun(at)POST.NETLINK.SE>
Date:    Mon, 27 Nov 2000 14:22:21 +0100

Subject: New evidence: Also mammals hear without traveling wave Findings: "Bone conduction" can occur without bones. It actually is tissue - and fl= uid conduction. Implications: In common with all other classes of vertebrates, also mammals hear by dir= ect 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 in= to the cochlea is passing through the vestibular aqueduct and/or the cochlea= r 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 traveli= ng 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 viscosit= y, 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 hai= r 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(at) [a]Department of Physiology, Hebrew University, Hadassah Medical School, P.O. Box 12272, Jerusalem 91120, Israel[b]Department of Otolaryngology/He= ad 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 addition= al 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 o= f skull bone did not alter the ABR to bone-conducted stimuli delivered to t= he 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 cavit= y 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 ma= jor pathway for cochlear excitation which is non-osseous: when a bone vibrato= r is applied to the skull, the bone vibrations may induce audio-frequency sound pressures in the skull contents (brain and cerebro-spinal fluid) wh= ich 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(at), 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, Hadas= sah 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 t= o bone conduction stimulation over the fontanelle and audiometric responses were obtained in neurosurgical patients with the bone vibrator on the ski= n over a craniotomy. There were no differences in threshold between these responses and those obtained to bone conduction stimulation over skull bo= ne in the same subjects. Audiometric thresholds in response to bone vibrator stimulation of the eye (a 'natural craniotomy') were no different from th= ose 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 vibratio= n (acceleration) measured at various sites around the head in response to b= one 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 ab= le to penetrate the skull, setting up audio-frequency pressures in the CSF which spread by fluid communications to the inner ear fluids, exciting th= e ear. Web-sites with data on vestibular and cochlear aqueducts: 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 Kl=E4ssbol Sweden nombraun(at)

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