Hearing loss is typically experienced as reduced sensitivity to sounds, possibly more so at some frequencies than others, or more in one ear than the other. There are many different causes of hearing loss, such as conductive hearing loss (sound does not properly reach the ear) and sensorineural hearing loss (damaged hair cells of the inner ear do not send a strong signal to the brain), but the key issue is that the audibility of sounds is reduced. In many cases, hearing aids that amplify the affected frequencies can partially restore audibility.
Spatial hearing can be affected by these types of hearing losses, even when the sounds are mostly audible. For example, hearing loss at high frequencies can limit the use of spectral cues for front/back and up/down localization. Unilateral hearing loss (affecting one ear more than the other) can disrupt the use of binaural cues. In both cases, we might expect hearing-impaired listeners to have problems localizing sounds, segregating competing sounds, and understanding speech when lots of different sounds compete. We may also expect hearing aids to help restore these functions by making the cues audible again.
There is another type of impairment that doesn't involve reduced audibility. A listener may be able to detect sound at a normal level, but still have trouble understanding, localizing, or segregating sounds. The term "Central Auditory Processing Disorder" is often used to describe these types of listening difficulties when hearing loss is not present. The term "Central" is used to suggest the involvement of the brain (part of the central nervous system), rather than the ear (part of the peripheral nervous system). However, the true nature of these impairments is poorly understood, and could involve distorted signals from the ear, disordered processing in the brain, or both.
One potential cause of central auditory processing disorders is damage to the brain itself, either due to disease or to traumatic brain injury. Exposure to intense explosive blasts, for example, can stretch and damage fibers that brain cells use to communicate. Some scientists think concussion might have similar effects. Because these fibers are essential for the brain to compare signals from the two ears, such damage could reduce the processing of binaural differences for localizing and segregating sound.
Many people experience difficulty listening in complex environments with multiple talkers, echoes, and reverberation. Aging and hearing impaired listeners seem to be particularly affected, even many whose hearing aids provide good audibility and understanding in quiet. Many scientists believe that disruptions to spatial hearing may lie at the root of those problems.
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