The acoustic pathways to the cochlea

Ernest Glen Wever and Merle Lawrence

The Journal of the Acoustical Society of America 1950 July, 22: 460-467

Abstract:

Measurements are made of the relative effectiveness of the round window as a route of entrance of sounds to the cochlea. When the ear is normal this route is of no importance, but when the middle ear apparatus is absent and its advantage is lost to the oval window route a sound will have nearly equal access by both windows. When the inner ear is reached by both pathways at once the cochlear potentials represent the vector sum of what would result from the two separate waves. As phase and intensity relations between the two pathways are altered the potentials pass through maximum and minimum values, which for equal intensities of the two waves vary from a 6 db gain to a complete loss. Evidence is produced to show that each pathway of stimulation excites the same sensory cells and in the same intensity pattern. Over the major portion of the frequency range a minimum of response results when the waves are in phase as they enter the oval and round windows. Departures from this relation, which are most prominent at the highest frequencies, are explained as the result of reactance differences between the two windows. These results apply to the explanation of forms of deafness caused by interruption of the ossicular chain, and also to the explanation of the treatment of otosclerosis by means of the fenestration operation. (Bold text emphasis by Martin Braun)

Comment:

Wever and Lawrence fed sound into the cat cochlea via two separate routes simultaneously, first via the normal pathway, and second via a tube sealed over the round window. Great care was taken that the two sound sources could not interact before entering the cochlea. Two pure tones of the same frequency and adapted levels causing the same amplitude of cochlear microphonics (CM), when applied to their respective routes alone, were then applied simultaneously while varying their phase relation. At a certain phase setting the combined CM response approached zero, and at a phase setting of 180 degrees away from this minimum it reached a maximum that was about twice as large as a single route response. Interestingly, this vector summation of CM failed at high sound levels. Here the combined response was well below the vector sum of the single responses. At a phase setting for the maximum it could even be less than a single-route response.

The authors did not discuss a possible cause of the high-level effect, and apparently this issue has never been discussed in the literature of the past 52 years. It is an important detail, however, when considering the question WHERE in the cochlea the vector summation of sound waves actually took place. Wever and Lawrence assumed that it occurred at the basilar membrane (BM), whose excursions would be nulled at a certain phase setting and amplified 180 degrees away from it. This view was in line with models of hair cell excitation that had been published 50 years earlier. Today we know that in OHC-off conditions the BM remains unmoved at all sound levels up to about 60 dB SPL. Therefore the place of vector summation for sound input up to this level must have been the hair cells (OHC cilia) themselves. Interestingly, 28 year later, Wever himself reported compelling evidence that the place of vector summation most probably were the hair cells. In a salamander, which has no BM excursions but a fluid coupling of both inner ears via the brain cavity, he found again the same phase dependent amplification and cancellation effects when stimulating the two ears by independent sound sources (Wever, 1978, Sound transmission in the salamander ear, Proc. Natl. Acad. Sci. USA 75, 529-530). Clearly, in this experiment there was no other possibility for the place of vector summation than the hair cells themselves.
In mammals, however, typically at levels above ca 60 dB the BM starts to be driven directly by the applied sound pressure and increasingly absorbs, and takes out, energy from the intracochlear mechanical system. This is obviously reflected in the results of Wever and Lawrence (1950), which showed for high sound levels a continued negative summation (cancellation), but a nonlinearly decreasing positive summation (amplification), at the respective phase settings.

In conclusion, more than 50 years ago, Wever and Lawrence were possibly the first who documented the mechanism of overload protection by high-level sound absorption in the mammalian cochlea. (Comment Martin Braun)

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