Hearing and hearing loss

How the ear functions and possible causes of hearing loss

To "survive and thrive", humans (and all mamals) are equipped with two ears as an efficient physiological system, to gather information about the surrounding sound environment.  Two ears provide the physiological link between the physical world of sound events, and our brain which interprets and enjoys the world of sound.

With two ears, we distinguish the direction of sound.  The closer ear receives the sound a few microseconds earlier and at a slightly higher intensity than the opposite ear.  This is how we know the direction of a truck's approach, a calling voice, or a warning whistle.  This is nature's way of protecting us.

The integration of information from two ears also helps us understand the speech signal we want to hear when there is background noise.  Furthermore, the quality of speech is better when it is heard stereophonically, i.e. with two ears.

The ear is a very complex organ consisting of three parts: the outer ear, the middle ear and the inner ear.  From the inner ear, the auditory nerve transmits information to the brain, where the final auditory processing takes place.  Hearing loss can result from an obstruction or damage to any of these parts.  A hearing loss resulting from a problem located in the outer or middle ear is called a conductive hearing loss, while a loss resulting from damage to the inner ear is called sensorineural.  It is also possible to have a hearing loss which results from a combination of both; it is then called a mixed hearing loss.  In order to gain a better understanding of hearing loss, it is important to know how the ear functions.

The outer ear includes the auricle, the auditory canal and the eardrum (tympanic membrane).  It serves as the channel by which sounds are first introduced to the hearing system. 

The auricle helps gather the sound waves from the environment.  The auditory canal then directs them further to the eardrum which vibrates in response to the sound waves.

The middle ear is an air-filled cavity which contains the smallest bones in the human body (the malleus, incus and stapes).  They are connected to the eardrum on one side, and on the other side, to a thin membrane-covered opening on the wall of the inner ear.  It is the movement of these bones, activated by the eardrum's vibrations, which transmits the sound further to the inner ear.  The middle ear is also connected to the throat via the eustachian tube whose main function is to keep the air pressure in the middle ear equal to that in the surrounding environment.

In the inner ear, auditory input is processed by the cochlea, while information regarding balance is processed by the semicircular canals. 

Along the entire length of the cochlea, which is fluid filled, there are tiny hair cells.  When the fluid in the cochlea is displaced by sound waves that have been passed on through the action of the middle ear bones, the hair cells bend.  This triggers a chemical response which activates the corresponding nerve endings.  These, in turn, transmit the message further to the area of the brain in charge of processing and interpreting auditory input.

 

The possible causes of hearing loss in...

Typical problems include excessive accumulation of ear wax and infection of the auditory canal (otitus externa). 

Typical problems include perforation of the eardrum, infection (otitus media) and otosclerosis (a calcification around the stapes, limiting its ability to move).  Many outer and middle ear problems can be treated medically or surgically.  When this is not possible, a remaining conductive hearing loss can be assisted by using hearing instruments - usually very successfully.

The vast majority of hearing losses are a result of damage to the inner ear structures.  Typical problems include the natural aging process (the most frequent cause of hearing loss), excessive exposure to noise (sudden impact noise and/or longer term exposure to high levels of noise), intake of medication which is toxic to the auditory system, head trauma and hereditary factors.  In such cases, the tiny hair cells in the cochlea are damaged; usually permanently.  As a rule, this damage cannot be corrected medically but the majority of sensorineural hearing losses can be assisted by the use of hearing intruments.

Between the two extremes of hearing well and hearing nothing at all, there is a wide range in the degree of hearing loss a person may experience.  The terms used to indicate the degree of hearing loss are : mild, moderate, severe and profound.  Most hearing losses are "mild to moderate" severity.

A conductive hearing loss usually results in a hearing deficiency in the low frequency range.  That is, the low pitched sounds (deep sounds such as "o", "u") which provide a lot of the "volume" of speech are affected the most.  This lowers the loudness of the incoming speech signal but does not affect its clarity.

A sensorineural hearing loss, on the other hand, mostly affects hearing acuity in the high frequency range.  The high pitched speech sounds (such as "s", "f", "sh", "t") play a crucial role in our ability to understand speech clearly.  This is why a person with a sensorineural loss often say "I can hear but often I don't understand what I said".