More on the perception of sound

Before the sounds we perceive are processed and interpreted by the brain, the first anatomical organ they encounter is the ear. The ear has a complex structure and its basic auditory functions include the perception of auditory stimuli, their analysis and their transmission further on to the brain. We can identify three components: the outer, the middle and the inner ear. The outer ear is mainly represented by the auricle or the pinna and the auditory meatus or the outer auditory canal. The auricle is the only visible part of the ear, constituting its outermost part, the segment of the organ projecting outside the skull. It does not play an essential role in audition, which is proved by the fact that the removing of the pinna does not substantially damage our auditory capacity. The auricle rather plays a protective role for the rest of the ear and it also helps us localize sounds.

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The meatus, or the outer auditory canal is a tubular structure playing a double role: it also protects the next segments of the ear – particularly the middle ear – and it functions as a resonator for the sound waves that enter our auditory system as well.

The middle ear is a cavity within the skull including a number of little anatomical structures that have an important role in audition. One of them is the eardrum or tympanic membrane. This is a diaphragm or membrane to which sound waves are directed from outside and which vibrates, acting as both a filter and a transmitter of the incoming sounds. The middle ear also contains a few tiny bones: the mallet or malleus, the anvil or incus and the stirrup or stapes. The pressure of the air entering our auditory system is converted by the vibration of the tympanic membrane and the elaborate movement of the little bones that act as some sort of lever system into mechanical movement which is further conveyed to the oval window, a structure placed at the interface of the middle and inner ear.

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As pointed out above, the middle ear has an important protective function. The muscles associated with the three little bones mentioned above contract in a reflex movement when sounds having a too high intensity reach the ear. Thus the impact of the too loud sounds is reduced and the mechanism diminishes the force with which the movement is transmitted to the structures of the inner ear. It is in the middle ear too, that a narrow duct or tube opens. Known as the Eustachian tube it connects the middle ear to the pharynx. Its main role is to act as an outlet permitting the air to circulate between the pharynx and the ear, thus helping preserve the required amount of air pressure inside the middle ear.

The next segment is the inner ear, the main element of which is the cochlea, a cavity filled with liquid. The inner ear also includes the vestibule of the ear and the semicircular canals. The vestibule represents the central part of the labyrinth of the ear and it gives access to the cochlea. The cochlea is a coil-like organ, looking like the shell of a snail. At each of the two ends of the cochlea there is an oval window, while the organ itself contains a liquid. Inside the cochlea there are two membranes: the vestibular membrane and the basilar membrane. It is the latter that plays a central role in the act of audition.

Look at an Animation of the chochlea here...

Also essential in the process of hearing is the so-called organ of Corti, inside the cochlea, a structure that is the real auditory receptor.

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Simplifying a lot, we can describe the physiology of audition inside the inner ear as follows: the mechanical movement of the little bony structures of the middle ear (the mallet, the anvil and the stirrup) is transmitted through the oval window to the liquid inside the snail-like structure of the cochlea; this causes the basilar membrane to vibrate: the membrane is stiffer at one end than at the other, which makes it vibrate differently, depending on the pitch of the sounds that are received. Thus, low-frequency (grave) sounds will make vibrate the membrane at the less stiff (upper) end, while high-frequency (acute) sounds will cause the lower and stiffer end of the membrane to vibrate. The cells of the organ of Corti, a highly sensitive structure because it includes many ciliate cells that detect the slightest vibrating movement, convert these vibrations into neural signals that are transmitted via the auditory nerves to the central receptor and controller of the entire process, the brain.

The way in which the human brain processes auditory information and, in general, the mental processes linked to speech perception and production are still largely unknown. What is clear, however, regarding the perception of sounds by man’s auditory system, is that the human ear can only hear sounds having certain amplitudes and frequencies. If the amplitudes and frequencies of the respective sound waves are lower than the range perceptible by the ear, they are simply not heard. If, on the contrary, they are higher, the sensation they give is one of pain, the pressure exerted on the eardrums being too great. As to the psychological processes involved by the interpretation of the sounds we hear, our knowledge is even more limited. It is obvious that hearing proper goes hand in hand with the understanding of the sounds we perceive in the sense of organizing them according to patterns already existing in our mind and distributing them into acoustic images. It is at this level that audition proper intermingles with psychological processes because our brain decodes, interprets, classifies and arranges the respective sounds according to the linguistic (phonological) patterns already existing in our mind. It is intuitively obvious that if we listen to someone speaking an unknown language it will be very difficult for us not only to understand what they say but we will have great, often insurmountable difficulties in identifying the actual sounds the person produced. The immediate, reflex reaction of our brain will be to assimilate the respective sounds to the ones whose mental images already exists in our brain, according to a very common cognitive reaction of humans that always have the tendency to relate, compare and contrast new information to already known information.