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Question: Wasn't Helmholtz right?
Answer: Yes and no.
Dear Dr. Blumschein and List:
In my book (pp. 15-19), I discussed not only the traditional view
concerning the mechanics for stimulation within the inner ear, but also
some intriguing alternatives that appeared in several articles in the
recent literature. These articles, taken together, ascribe novel
complementary functions of inner and outer hair cells as tuned resonators
responsible for frequency analysis rather than the basilar membrane. The
primary dynamic function of the basilar membrane is considered to be that
of a tuned absorber of excessive vibrational energy that could otherwise
damage the hair cells. I attempted to summarize some of the supporting
evidence for these heretical views.
The view that hair cells are the tuned resonators rather than the basilar
membrane resembles Helmholtz's initial resonance theory presented in his
first edition of Sensations of Tone (1863). He then suggested that the
rods of Corti (located near the nerve fibers) as tuned resonators for
spectral components by virtue of their gradations in stiffness and
tension. He later abandoned this view in part because other investigators
found that these structures were absent in birds and amphibia, and in
subsequent editions of Sensations of Tone suggested that radial fibers
embedded in the basilar membrane were the resonant transducers.
Hence, if we substitute the stereocilia bundles of hair cells for the rods
of Corti, we are back to the first guess by Helmholtz. Interestingly,
while functional homology was involved in Helmholtz's dropping of resonant
rod transducers, the lack of travelling waves in the basilar papilla of
reptiles, together with the tuned resonance of their tonotopically
arranged hair cells, has led some investigators to a position close to
what Helmholtz initially suggested.
On Mon, 22 May 2000, Eckard Blumschein wrote:
> Date: Mon, 22 May 2000 13:54:04 +0200
> From: Eckard Blumschein <Eckard.Blumschein@E-TECHNIK.UNI-MAGDEBURG.DE>
> To: AUDITORY@LISTS.MCGILL.CA
> Subject: Wasn't v. Helmholtz right?
> Dear list,
> The recent edition of Auditory Perception by Richard M. Warren provides an
> excellent review of Mechanics for Stimulation within the Inner Ear.
> Unfortunately, the author preferred to leave some conclusions and possibly
> some notorious errors to the reader. He wrote correctly: The speed of sound
> in the cochlear liquids is very much faster ((than velocity of the
> traveling wave)), about 1,600 m/sec (this difference is of significance in
> determining whether the traveling wave or the sound pressure is the
> stimulus for receptor cell transduction...). He did not, however, mention
> the question whether or not the traveling wave is the result of energy
> transmission basilar from base to apex inside basilar membrane or it might
> rather be an epi-phenomenon, i.e. an attendant symptom of local resonance.
> Referring to Lewis, Leverence, and Bialek (1985), and also to de Boer and
> Nutall (1996), Dancer, Avan, and Magnan (1997) tried to belittle this
> discrepancy by calling the traveling wave a leitmotiv. Recio, Rich,
> Narayan, and Ruggero (1998) rejected this point of view. Can anybody point
> me to the final outcome of that discussion? Possibly, I am simply not yet
> aware of the latest news since I did neither attend a concerning conference
> in Japan last year nor the ARO meeting this year.
> It is my gut feeling that v. Helmholtz was pretty right with his idea of
> local resonance. Otherwise, I was wrong with my speculations on physiology
> of the inner ear of some animals, explanation of equivalence of net latency
> and 1/CF, problems with understanding of DPOAE, etc. I also realized
> evidence for the longitudinal coupling being fairly weak. Local resonance
> does neither exclude the appearance of a traveling wave nor the application
> of a modified transmission line model (with a nearly common upper potential
> along the whole length). I additionally imagine an additional oscillating
> motion back and forth in radial direction due to motility of the outer hair
> cells. Radial component of velocity was reported ten times larger than the
> longitudinal one.
> Once again, may I ask for hints to the ultimate elucidation? As Dancer et
> al. stated, the two positions should have implications in signal
> processing. Suggesting that psychoacoustics may be the touchstone for
> theories, I would be curious if it will really be possible to compensate
> for the traveling wave delay.
> Thank you very much,
> Dr. Eckard Blumschein
> Inst. of Electronics, Signal Processing, and Telecommunication
> Otto von Guericke Univ. Magdeburg, GERMANY