Auditory System
Another very interesting human sensory capability is the hearing system. The human ear consists of several parts, the ear canal, the middle ear, and the inner ear. The functioning of all of these elements is not fully understood. In many cases, one or more parts of the ear can undergo structural damage, and some functionality is preserved.
Fig 2. Human Auditory System
Some partial description is possible though. Sound waves travel down the ear canal in the form of compression pressure waves. These pressure waves induce deflection of the eardrum, and the various structures attached to it. Part of this structure is coupled to the inner ear in a way which seems to produce an outward deflection in reaction to an inward pressure force. The inner ear is configured as a spiral with two chambers. One of the chambers is connected to the eardrum through the mechanism described above, while the other is in acoustic contact with the eardrum. The apparent result of all of this complexity is the development of an oscillating force between the chambers of the inner ear.
The inner ear is a seashell-like spiral structure, in which the two chambers are separated by a thin diaphragm impregnated with rapid adapting mechanoreceptors. Acoustic signals produce oscillations in this diaphragm. One effect of the shape of the spiral diaphragm is that the resonant frequency is a function of position along the spiral. As a result, any particular acoustic signal frequency will produce a mechanical oscillation in the inner ear at a particular physical location. In this way, the inner ear acts as an acoustic spectrum analyzer, with individual mechanoreceptors configured for detection of particular audio frequencies. The central nervous system receives all of these signals and processes them into recognizable patterns.
The semicircular canals are three fluid filled ring-like structures with hairs that are sensitive to motion of the fluid. A rotational acceleration will be detected by the canals, making them useful for maintaining balance. The three canals are oriented orthogonally to one another, providing information about all three axes. It is rather impressive to realize that humans are walking around with a couple of 3-axis rotational accelerometers in their heads whose output is automatically processed by the brain to provide information about any rotational accelerations that are experienced.
This mechanical system is capable of reasonable resolution for audio signals. Experiments have shown that particular mechanoreceptors are sensitive to audio frequencies in a 10-20% band around their primary peak. Clearly, the nervous system has acquired substantial post-processing because humans are capable of resolving tones to a much higher degree of accuracy (<1%).
One interesting experiment to perform is to study the capability for spatial location of signal sources. With your eyes closed in a quiet location, concentrate on the location of particular signals. It is best if you can have some new signal produced in random locations. In class, I ask everyone to close their eyes, and I sneak to some new location, and then say something. Without opening their eyes, I ask the class to point in my direction. Remarkably, the class is accurate to within a couple of degrees.
The class is encouraged to spend some time in the evening sitting quietly with eyes closed and think about how they are able to determine the location of objects and to see under which circumstances the results are most likely to be incorrect.
