Georg von Békésy's Mechanical Model of the Inner Ear
This page is a more detailed exploration of Dr. Békésy's mechanical model of the inner ear. The text is excerpted from Dr. Békésy's Nobel Lecture*. The photos were taken at the Laboratory of Sensory Sciences, University of Hawaii.
... After some modification, the final version of the model consists of a
plastic tube filled with water, and a membrane 30 cm in length; when
it is stimulated with a vibration it shows travelling waves of the
same type as those seen in the normal human ear. The usable
frequency range is two octaves.
The model ear is on the left. Connected to it on the right is a large vibrator for producing frequency-controlled stimulation. Photo by Hinano Akaka.
Another view of the model ear showing the cochlear cylinder and vibrator. Photo by Hinano Akaka.
I decided to go one step further and make a model of the inner ear
with a nerve supply. An attempt to use a frog skin as a nerve supply
had at an earlier time proved to be impractical, and so I simply
placed my arm against the model.
Dr. Békésy's technician, Walter Karplus, demonstrates how the "nerve supply" was implemented on the model ear. Photo by Hinano Akaka.
Another view of Walter Karplus demonstrating the model ear with nerve supply. Photo by Hinano Akaka.
To my surprise, although the
travelling waves ran along the whole length of the membrane with
almost the same amplitude, and only a quite flat maximum at one spot,
the sensations along my arm were completely different. I had the
impression that only a section of the membrane, 2 to 3 cm long, was
vibrating. When the frequency of the vibratory stimulus was
increased, the section of sensed vibrations travelled toward the
piston (at the right of the figure), which represents the stapes
footplate of the ear; and when the frequency was lowered, the area of
sensation moved in the opposite direction. The model had all the
properties of a neuromechanical frequency-analyzing system, in
support of our earlier view of the frequency analysis of the ear. My
surprise was even greater when it turned out that two cycles of
sinusoidal vibration are enough to produce a sharply localized
sensation on the skin, just as sharp as for continuous stimulation.
This was in complete agreement with the observations of Savart, who
found that two cycles of tone provide enough cue to determine the
pitch of the tone. Thus the century-old problem of how the ear
performs a frequency analysis -- whether mechanically or neurally --
could be solved; from these experiments it was evident that the ear
contains a neuromechanical frequency analyzer, which combines a
preliminary mechanical frequency analysis with a subsequent
sharpening of the sensation area.
I think the happiest period of my research was when I started to
repeat all the great experiments that have been done on the ear in
the past -- but now on the model ear with nerve supply. All the
small details could be duplicated on the skin. Nothing has been more
rewarding than to concentrate on the little discrepancies that I love
to investigate and see them slowly disappear. This always give me
the feeling of being on the right track, a new track.
The simple fact that on the model the whole arm vibrates (as can be
seen under stroboscopic illumination), but only a very small section
is recognized as vibrating, proves that nervous inhibition must play
an important role. Further investigation has shown that every local
stimulus applied on the skin produces strong inhibition around the
place of stimulation. This seems to be true, not only for the skin,
but also for the ear and the retina. Involuntarily, this had led me
to begin an investigation of the analogies between the ear and the
skin and the eye. Maybe the time is not too far off when these three
sense organs -- ear, skin, and eye -- so sharply separated in the
textbooks of physiology, will have some chapters in common. This
would lead toward a simplification of our descriptions of the sense
*Text excerpted from: Concerning the pleasures of observing, and the mechanics
of the inner ear. Nobel Lecture, December 11, 1961, Georg von
Békésy (in Nobel Lectures Physiology or Medicine 1942 -
1962, Elsevier, 1964, pp. 722-746).