For engineers, it may be easier to look at this “engineering schematic form” of the ear.
The function of the inner ear is to analyze the sound in a way that is quite consistent with the Fourier analysis, which we have been discussing earlier. Further, it also converts the mechanical movements that carry the sound to a code that would be understood by the rest of the perceptual system. Without going into too many details, it happens in the following way: Inside the cochlea (which is actually curled as shown in the left) is several membranes, the most important from our point of view being the basilar membrane. Movements of the cochlear fluid cause the basilar membrane to move with the frequency of the incoming sound.
The basilar membrane is much stiffer and narrower at one end ( at the entrance to the cochlea near the oval window) and much more pliable and wider at its other end. As it vibrates, it forms a traveling wave. Due to mechanical properties of the basilar membrane, this wave forms a maximum at a certain point along the length of the membrane and then it dies of. The position of this maximum depends on a frequency of the incoming sound. For low frequencies, whole basilar membrane is active, for high frequencies only the stiff part of the membrane is active.
It is possible to find out how selective if the basilar membrane (von Bekesy got a Nobel Prize for this). He observed by a microscope the amplitude of a displacement of a point on a basilar membrane (in dead ear) while changing a frequency of the incoming sound. Each point on the basilar membrane acts as a band-pass filter. The filters re asymmetric, the low frequency slopes are less steep than the high frequency slopes. This is because for low frequencies, whole basilar membrane is active, for high frequencies only the stiff part of the membrane is active.
Von Bekesy was not exactly correct. New methods of measurement of basilar membrane movements allow for measuring the basilar membrane action in live animal. Selectivity of the basilar membrane is much higher in the live animal.
Now we are ready to discuss how the ear tells the brain what it hears. The picture shows a cut through so called “organ of Corti”. This organ consists of two membranes, the basilar membrane (which is frequency selective as we have discussed earlier), and the tectorial membrane. Hairs are growing from the basilar membrane and into the tectorial membrane. As the basilar membrane moves, the hairs bend. The inner hair cells emit spikes of action potential whenever the hair bends of one direction. There is no spike when the hair moves in the other direction – the hair cells do one-way rectification of the acoustic signal. These spikes then go into higher levels of the hearing system and after further processing the information from the ear reaches the brain.
An interesting thing is that the brain also appear to have means to talk to the ear! The so called “outer hair cells” receive spikes from the brain. They appear to change mechanical properties of the organ of Corti in response to the information received from the brain. Thus, the hearing seems to provide for a feedback that could adapt the ear to a changing acoustic input. There is a strong evidence for this top-down flow of information in a form of so called “otoacoustic emissions”. When a sensitive microphone is placed in a vicinity of the eardrum, it can pick up a delayed echo (by several tens of ms) of the sound that entered the ear.
This a schematic summary of what is happening in the organ of Corti.
The Place Theory of Hearing says exactly that. The theory is strongly supported by the fact that the fibres connected to low frequency parts of the cochlea are in different parts of the auditory nerve than the fibres connected to high frequency part of the cochlea (so called “tonotopical” organization) and this tonotopical organization is preserved all the way to the brain – the low frequencies excite different parts of the brain than the high frequencies do.
However, the things are not all that easy.
So far, what we have heard indicates a relatively straightforward mechanism of the auditory analysis that is done in the ear. The ear could act as a bank of ban-pass filters (which can be approximately emulated by properly modified short-term Fourier analysis), firing rates on the individual fibres in the auditory nerve (which are connected to hair cells distributed along the length of the cochlea) indicate spectral energy of the signal (which would be an equivalent of the spectral energy computed by the engineering analysis).