It is concerned with speech perception, principally how the brain forms perceptual representations of the input it receives. Basicly, it focus on listener´s ear and listener´s brain.


The ear is divided into three different parts:

The only visible part of the ear is the pinna (the auricle) which - with its special helical shape - is the first part of the ear that reacts with sound. The pinna acts as a kind of funnel which assists in directing the sound further into the ear. Without this funnel the sound waves would take a more direct route into the auditory canal. This would be both difficult and wasteful as much of the sound would be lost making it harder to hear and understand the sounds.

The pinna is essential due to the difference in pressure inside and outside the ear. The resistance of the air is higher inside the ear than outside because the air inside the ear is compressed and thus under greater pressure.

In order for the sound waves to enter the ear in the best possible way the resistance must not be too high. This is where the pinna helps by overcoming the difference in pressure inside and outside the ear. The pinna functions as a kind of intermediate link which makes the transition smoother and less brutal allowing more sound to pass into the auditory canal (meatus).

Once the sound waves have passed the pinna, they move two to three centimetres into the auditory canal before hitting the eardrum, also known as the tympanic membrane.

The eardrum (tympanic membrane), which marks the beginning of the middle ear, is extremely sensitive. In order to protect the eardrum, the auditory canal is slightly curved making it more difficult for insects, for example, to reach the eardrum. At the same time, earwax (cerumen) in the auditory canal also helps to keep unwanted materials like dirt, dust and insects out of the ear.

In addition to protecting the eardrum, the auditory canal also functions as a natural hearing aid which automatically amplifies low and less penetrating sounds of the human voice. In this way the ear compensates for some of the weaknesses of the human voice, and makes it easier to hear and understand ordinary conversation.

2.- the middle ear:

Three bones
The eardrum is very thin, measures approximately 8-10 mm in diameter and is stretched by means of small muscles.

The pressure from sound waves makes the eardrum vibrate. The vibrations are transmitted further into the ear via three bones: the hammer (malleus), the anvil (incus) and the stirrup (stapes). These three bones form a kind of bridge, and the stirrup, which is the last bone that sounds reach, is connected to the oval window.

The oval window is a membrane covering the entrance to the cochlea in the inner ear. When the eardrum vibrates, the sound waves travel via the hammer and anvil to the stirrup and then on to the oval window.

When the sound waves are transmitted from the eardrum to the oval window, the middle ear is functioning as an acoustic transformer amplifying the sound waves before they move on into the inner ear. The pressure of the sound waves on the oval window is some 20 times higher than on the eardrum. The pressure is increased due to the difference in size between the relatively large surface of the eardrum and the smaller surface of the oval window. The same principle applies when a person wearing a shoe with a sharp stiletto heel steps on your foot: The small surface of the heel causes much more pain than a flat shoe with a larger surface would.

The Eustachian tube
The Eustachian tube is also found in the middle ear, and connects the ear with the rearmost part of the palate. The Eustachian tube equalises the air pressure on both sides of the eardrum, ensuring that pressure does not build up in the ear. The tube opens when you swallow, thus equalising the air pressure inside and outside the ear.

In most cases the pressure is equalised automatically, but if this does not occur, it can be brought about by making an energetic swallowing action. The swallowing action will force the tube connecting the palate with the ear to open, thus equalising the pressure.

Built-up pressure in the ear may occur in situations where the pressure on the inside of the eardrum is different from that on the outside of the eardrum. If the pressure is not equalised, a pressure will build up on the eardrum, preventing it from vibrating properly. The limited vibration results in a slight reduction in hearing ability. A large difference in pressure will cause discomfort and even slight pain. Built-up pressure in the ear will often occur in situations where the pressure keeps changing, for example when flying or driving in mountainous areas.
3.- The inner ear:

Once the vibrations of the eardrum have been transmitted to the oval window, the sound waves continue their journey into the inner ear.

The inner ear is a maze of tubes and passages, referred to as the labyrinth. In the labyrinth can be found the vestibular and the cochlea.

The cochlea
In the cochlea, sound waves are transformed into electrical impulses which are sent on to the brain. The brain then translates the impulses into sounds that we know and understand.

The cochlea resembles a snail shell or a wound-up hose. The cochlea is filled with a fluid called perilymph and contains two closely positioned membranes. These membranes form a type of partition wall in the cochlea. However, in order for the fluid to move freely in the cochlea from one side of the partition wall to the other, the wall has a little hole in it (the helicotrema). This hole is necessary, in ensuring that the vibrations from the oval window are transmitted to all the fluid in the cochlea.

When the fluid moves inside the cochlea, thousands of microscopic hair fibres inside the partition wall are put into motion. There are approximately 24,000 of these hair fibres, arranged in four long rows.

The hair fibres are all connected to the auditory nerve and, depending on the nature of the movements in the cochlear fluid, different hair fibres are put into motion.

When the hair fibres move they send electrical signals to the auditory nerve which is connected to the auditory centre of the brain. In the brain the electrical impulses are translated into sounds which we recognise and understand. As a consequence, these hair fibres are essential to our hearing ability. Should these hair fibres become damaged, then our hearing ability will deteriorate.

Another important part of the inner ear is the organ of equilibrium, the vestibular.

The vestibular
The vestibular registers the body's movements, thus ensuring that we can keep our balance.

The vestibular consists of three ring-shaped passages, oriented in three different planes. All three passages are filled with fluid that moves in accordance with the body's movements. In addition to the fluid, these passages also contain thousands of hair fibres which react to the movement of the fluid sending little impulses to the brain. The brain then decodes these impulses which are used to help the body keep its balance.


How the brain filters noise:
Our left side of the brain is more active when we discriminate relevant sounds from background noise, according to the findings of a study by an international team of scientists.

A night out is often a frustrating experience for hearing impaired people. They find the words of their conversation partners drowned out by the conversations of others, music or street noise. They lack the so-called cocktail party ability of people with normal hearing to separate relevant sounds from background noise.

 Left side of brain sorts out the sounds
Brain researchers have investigated what happens in the brain when discriminating between the sounds we listen for and other noise. The study was headed by Hideko Okamoto of the University of Münster, Germany. He and his team exposed a number of individuals to test sounds and background noise in one or both ears while monitoring their brain activity. The recorded brain activity indicated greater activity in the left half of the brain when discriminating sounds from noise. In other words, the cocktail party effect occurs in the left side of the brain.

As of yet, the researchers are unable to determine why hearing impaired people’s ability to discriminate sounds from noise is diminished. This is a matter for future research.

Knowledge about the brain functions will eventually benefit hearing impaired people in terms of the development of new treatment methods and assistive devices.
Source: BMC Biology