Location Matters in the Cochlea – The Ear Hears Differently Than Expected
Whether you are using a cochlear implant to replace the lost sensation of sound, or recreating binaural sound within a virtual environment, a precise understanding of how the ear works is always helpful.
However, it seems that our current understanding of how the tiny hairs in the cochlea process sound, is not entirely accurate. If we wish to artificially recreate sounds accurately regardless of whether it is for a VR or a self-augmentation angle then the actual process needs to be understood. It may well be that our artificial ears' basic design can be significantly improved by such an understanding.
The tiny hairs in the cochlea -the cilia, vibrate according to the pitch of noise heard. Each hair responds to a specific pitch, and each hair is not replaced if it is damaged or destroyed. This is information about the inner ear's function that has long been known.
However, what has only recently been discovered is that these cells vibrate strongly at different sound frequencies not depending on individual differences within the hairs, but depending on their actual location within the spiral of the cochlea. In other words the shape of the cochlear itself is essential to the frequency detection process, and the different signals picked up by the nerve endings that form the ends of the auditory nerve? Those are also solely dependent on the location they terminate at. There is no frequency differentiation between the cilia themselves, and they are just a standardised sound detection system, not unique as was previously thought.
Dr. Elizabeth Olson and Dr. Wei Dong, both of Columbia University, designed tiny sensors that could simultaneously measure small pressure fluctuations and cell-generated voltages at specific locations within the ears of live gerbils, specifically to measure this process.
Dr. Dong and Dr. Olson discovered that a shift in the timing of this feedback voltage activates amplification at the right frequencies. With the shift, hair cell forces pump energy into cochlear motion, much like a child increases a swing's motion by pumping his legs at the right time. In addition to detecting the amplification trigger, the researchers' sensors verified the amplification that results.
It makes recreating artificial sound systems a much more straightforward process, and means that at least some of the distortion current implants face, are because they are based on incorrect assumptions about the design of the ear. By understanding which part of the cochlea deals with which frequency, we can tailor implants far more efficiently, and potentially even bypass the cochlea entirely now that we know the location of the cilia the nerves are attached to and thus the location of the nerve endings is what is important,l as opposed to studying each cilia individually.
Detection of Cochlear Amplification and Its Activation (Paper) (Paywalled)