I must admit that I don’t really know how the inner ear works. And it’s not just me. The ear is a dynamic system with feedback loops that only function well at optimal fluid pressures and biochemistries. We have many theories, but unfortunately (or fortunately for the listener) these are based on dead people, lower mammals, or other species. In addition to these petri dish, or scanning microscopy structures and results, we usually only have glimpses at static snap shot images of how the inner ear works. Some dynamic analyses have been achieved and some of these have been derived from mathematical and computer models.
So, we really have a pretty good idea – at least we think that we have a pretty good idea. I like to compare what we know about the ear as being like the Newtonian model of the universe. There are well-defined equations and processes which appear to account for much of the data. Then Max Planck and Albert Einstein came along and showed that Newton’s model of the universe broke down for some of the more massive, and quickly moving elements of the universe. In the auditory science field, our Newton would be von Bekesy and perhaps H.F. Schuknecht.
We haven’t quite yet had a Planck/Einstein breakthrough of the function of the inner ear, and part of this relates to the fact that as soon as we kill the volunteer donators of their cochleae, much of the dynamic activity ceases.
Having said this, we do have some ingenious pieces of data for the live dynamic cochlea and this is related to the study and assessment of both transient and distortion product otoacoustic emissions as well as a few other (capacitive) probing techniques performed on non-human mammals.
Between the late 1970s (when I was a lowly graduate student) and now, the model and our understanding of the inner ear has changed dramatically. For example, we now know about the feedback features in the ear that serve to enhance low levels sounds (a motor effect); we now know more about the biochemistry of the various structures of the inner ear; we now know more about the various proteins that are genetically controlled; and we know more about the nature of temporary and permanent hearing loss from prolonged and also sudden high sound levels.
We are getting there but we don’t yet have the full picture. And we won’t really know that we are “there yet” for many years to come, just as we thought first that the Newtonian view, and later the Relativity and Quantum Mechanics view, was the final answer in physics in the 1920s and 1930s, we may not know that any new model is the “final” one.
Recently a new optical scanning technique used in the visual domain has been used for cochlear mechanics. The technique is called optical coherence tomography and uses sound a bit like a sonic depth charge, to measure the resultant vibration inside a hard walled structure such as the cochlea. In a recent study by Rebecca Warren and her colleagues, published in the Proceedings of the National Academy of Sciences, this technique was used on a guinea pig. The guinea pig did not say whether he found the probing noise to be too great or whether the technique causes subsequent hearing loss. (I did contact the various Worker’s Compensation Boards around the world and to date there are no hearing loss claims by guinea pigs).
The researchers specifically examined the part of the cochlea responsible for low frequency hearing and found, among other things, that the function of the inner ear for low frequency sounds was both qualitatively and quantitatively different than what occurs in the mid to higher frequency region. In the lower frequency region, they found much less basilar membrane motion than expected from models based on the function for the higher frequency region. This may provide some insight into why some hard of hearing patients, musicians or not, desire more or less hearing aid amplification (or hearing protection) than a theoretical target, especially in the lower frequency region.
I suspect that such a probing analysis will be restricted to the laboratory (and to non-human mammals) but the results would be applicable to our study of hearing in people. After all, most of what we do know about the cochlear hearing mechanism of people is based on experiments performed on our less fortunate mammal cousin.
