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Hearing Loss Hearing loss comes mainly in two forms, conductive loss, and sensorineural loss.  Conductive losses are caused by problems with the outer or
middle ear (see Fig. 1 for the structures of outer, middle, and inner ear). The
reasons can be buildup of wax or fluid, puncture of eardrum, middle-ear
infection, and diseases that immobilize the bones of the middle ear, which lead
to attenuation in the transmission of the signals. This type of hearing loss
typically reduces sensitivity but not the dynamic range. With sufficient
amplification that give equal boosts to soft as well as intense sounds, the
patient's perception of the acoustic world can be restored close to normal. For
these patients, hearing aids are an effective remedy. It should be noted that,
in many cases, these conditions can be corrected medically without resorting to
any hearing instruments. Sensorineural hearing losses can be caused by problems with the cochlea or the auditory nerve, as a result of exposure to noise and toxins, diseases, or simply aging. Hearing losses with cochlear origin exhibit a number of distinct characteristics. First, they typically occur at high frequencies, with low-frequency hearing affected to a less degree, or even being normal. Second, in the frequency region of significant loss (such as at high frequencies), the loss of sensitivity is mostly for soft sounds; loudness perception of intense sounds remains largely normal. As a result, the range of sounds that are perceptible, the dynamic range, is reduced. Finally, in a noisy environment, patients with such losses have difficulty understanding speech, even when the speech signal is clearly audible. This effect on speech perception is probably the most damaging, and it is also a defect that is most difficult to correct, with hearing aids, or any other means. To understand the symptoms of sensorineural hearing loss, it would be helpful to look at the functions of the key sensory cells of the auditory system, and, in particular, what happens to these sensory cells in a damaged cochlea. After the acoustic signal is converted into mechanic vibration by the middle ear, the middle-ear bone structure drives the energy into cochlea via the oval window, to create a localized vibration on the basilar membrane (BM). Along the length of the BM in the cochlea sit three rows of outer hair cells and one row of inner hair cells. In responding to an incoming tone, the basilar membrane will vibrate, with the location of the peak vibration depending on the frequency. In this mapping from signal frequency to BM place, the higher the signal frequency, the closer the peak of vibration will be to the entrance to the cochlea, or the oval window. At the peak location, the outer hair cells, through their motility, will amplify the vibration by pumping additional energy into the BM system. The inner hair cells, which are the auditory sensory receptors, are driven by the amplified vibration, and transmit the information to auditory nerves connected to them. The Outer hair cells serve two main functions. First, they provide amplification to weak sound but not to intense ones. This function is effectively an automatic gain control (AGC) system, compressing a wide dynamic range of input into something that can be managed by the auditory nerve system. Second, they sharpen the contour of the BM frequency response to tones, thus enhancing the frequency selectivity of the auditory system. Along the basilar membrane, sensory cells at high-frequency locations near the cochlear entrance are more vulnerable to damage than those at deeper, lower-frequency io locations. This explains why hearing loss usually begins at high frequencies. At a given BM location, the outer hair cells are more vulnerable than inner hair cells to insults to the cochlea. As a result, even at its early stage, sensorineural hearing loss can be identified with symptoms of losing the outer-hair-cell functions, i.e., a loss of sensitivity to soft sounds, a reduction of dynamic range, and a reduction of frequency selectivity. When the damage at a region on the BM is so severe that both outer and inner hair cells are destroyed, some researchers call this a “dead” region. Acoustic information normally transmitted via this region must now be transmitted via sensory cells at neighboring regions. Along with the reduction of dynamic range, this loss in the effective number of frequency channels hampers the capacity of the auditory system to transmit information of signals in the auditory world, particularly the speech signals. Speech signals are particularly affected by the decreased capacity of information transmission with sensorineural hearing loss because speech signals spread over a wide range in both intensity and frequency. Of the two major elements of speech sounds, the vowels are less affected, because they are relatively long and intense, and have energy concentrated at low frequencies. Consonants, on the other hand, are more affected in their perception, because they are short and soft, and typically have energy spread over a wide frequency range. This is unfortunate, because consonant recognition is key to speech understanding. Speech perception is made even more difficult because of impairment to the frequency resolution, and perhaps also to the temporal resolution. With compromised resolutions, speech segments that can be processed separately in normal ears will now interfere with other segments in time and frequency, making important speech features indistinct or even masked. The complex mechanisms of sensorineural hearing loss present a challenge for hearing aid industry. In many ways, a hearing aid does not have the same degree of magic effect that a pair of eyeglasses can produce, because the deficits of cochlear damage usually cannot be fully corrected. With that in mind, there is still much that can be done to improve the listening comfort, the sound quality, and, most of all, speech intelligibility. For so many who suffer the consequences of hearing losses, even a partial recovery of their auditory ability can go a long way in enlightening their life. For this, any effort for making a good hearing aid is a worthy effort. |