There are a number of ways in which the hearing mechanism can break down, starting with the eardrum and working inward. Damage to the eardrum itself can occur in many ways, the most common being the effect of external pressure. such as (hat caused by a blow to the ears, a nearby explosion, or extreme water pressure during a scuba dive. Ordinarily the pressure within the middle ear is matched to the external air pressure by means of the air entering the Eustachian tube, a small channel that runs from the rear of the mouth to the middle ear. However, a difference in external air pressure may puncture the eardrum if changes in pressure occur too rapidly or if the Eustachian tube is blocked because of a heavy cold or a failure to swallow frequently enough during changes in pressure-as in a scuba dive or when an airplane takes off or lands. This damage can usually be repaired surgically. but the scar tissue In the expired eardrum may produce permanent changes in the ear’s response to various frequencies.
In a common form of deafness the tiny bones in the middle ear fuse because of calcium deposits, and this fusion effectively blocks the mechanical transmission of sound to the cochlea. Delicate surgical techniques can break the calcium deposits and restore the flexibility of the bones. This surgery produces a dramatic and almost complete return of normal hearing. The basilar membrane is susceptible to damage in a number of ways. First of all, there is the progressive deterioration due to age, which seems to affect the high-frequency regions of the membrane first. This produces heightened thresholds for the higher frequencies. A hearing aid designed to amplify the higher frequencies can restore normal hearing to many elderly people.
A very similar pattern of hearing loss occurs if a patient is given extensive doses of the drug mycine, which seems to cause progressive damage to the hair cells in the basilar membrane similar to the degeneration caused by age. Again, a hearing aid may restore normal hearing if the damage is not too extensive, (Extensive damage to the hair cells results in profound hearing loss. See the application, ”The Bionic Ear,” for a report on a new type of hearing aid specially developed to compensate for this damage.)
Exposure to very intense sounds can also cause damage to the basilar membrane. If the sound is primarily of one frequency, there may be damage to only one region, producing a tonal gap; that is, a loss of sensitivity to a narrow range of tones. If the damage is not too great, a special hearing aid that amplifies only those frequencies to which sensitivity is reduced may restore normal hearing. Fortunately, there is tremendous redundancy in most sounds which means that the ability to hear only a small part of the total frequency range may be all that is necessary for comprehension. People with tonal gaps are often unaware of any problem, because their brain automatically compensates for the lost frequencies.
There are also various forms of nerve damage in the acoustic pathways or in the auditory projection areas in the cortex. These may be caused by a variety of factors, including birth defects, disease, blows to the head, and tumors. Nerve damage may result in profound deafness. It is hard to make up for total loss of hearing. One way is to use another sensory system that provides the same information in a different form. For example, deaf people become quite adept at interpreting speech from lip movement, While normal people also rely on lip movements to some extent, it is only when hearing is lost that most people fully use the redundant visual information to interpret speech.
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