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Fig. 6. These are tuning curves for an auditory system with normal OHCs (solid line) and damaged OHCs (dashed line). The upward displacement of the tip of the tuning curve in the impaired system represents a loss of the frequency-selective gain normally provided by OHCs.
Compression in hearing aids
In closing, we consider one form of aural rehabilitation for individuals with damaged OHCs, namely amplification via a hearing aid. The intent of hearing aids is to amplify sounds so that the individual with the hearing loss can hear them. The reduced dynamic range, however, provides a con- siderable challenge. This is illustrated in Fig. 7. In the top panel the growth of loudness is shown schematically for a normal (solid line) and an impaired (dashed line) ear. The goal of a hearing aid might be to amplify low-level sounds a great deal, but high-level sounds only a little, if at all. This would shift the response of the impaired ear to be more in line with the response of the normal ear (as indicated by the arrows). As noted previously, OHCs normally provide this type of level-dependent amplification: the amount by which they amplify the vibration of the basilar membrane decreases with increasing input level. The goal of level-dependent amplification will not be accomplished by simple linear amplification where all sounds are amplified by the same amount. Instead, compression amplification has become an increasingly more popular type of amplification for individ- uals with sensorineural hearing loss in order to deal success- fully with their reduced dynamic range. An illustration of one type of compression amplification is shown as an I-O func- tion in the bottom panel of Fig. 7. The output increases with increasing input, but the gain of the hearing aid (the differ- ence between the output and the input) decreases with increasing input level. This type of amplification has accom-
associated with the level-dependent gain resulting from nor- mally functioning OHCs in the cochlea. Damage to those cells results in elevated detection thresholds, a reduced dynamic range, and broader tuning. As discussed in several recent reviews5–8, it is likely that cochlear compression plays an even more pervasive role in hearing than outlined here. It may, for example, play an important role in auditory tempo- ral processing, thereby improving our ability to process the time-varying or dynamic aspects of sound.
Fig. 7. The top panel shows the growth of loudness for a normal (solid line) and an impaired (dashed line) ear. The length of the arrows indicates the amount by which the sound would need to be amplified to shift the impaired ear to be equal to the nor- mal ear. The bottom panel shows the I-O function for a hearing aid that employs com- pression. The amount of gain provided by the hearing aid is simply the difference between the output and the input.
Auditory Compression and Hearing Loss 33