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It has been previously shown that patients with tinnitus and normal audiometry presented with worse pure-tone detection thresholds in the presence of competitive noise. While the data obtained in this study alone do not allow one to state what actually makes performances different, a recent hypothesis for this to occur would be the reduction in the number of afferent auditory fibers (deafferentations), culminating in a reduction in feedback from the medial olivocochlear system and contributing to the increase of tone detection thresholds in noise. Thus, normal hearing thresholds can also be accompanied by an impaired function of the efferent fibers, which protrude from the brainstem into the cochlea.
In individuals with tinnitus and normal hearing, it is possible that the deafferentation of the auditory nerve fibers is present, as evidenced by the intensity discrimination thresholds, which are significantly higher in the tinnitus group with. Thus, obtaining higher thresholds in the presence of noise could be the psychoacoustic result of a possible auditory information processing deficit.
The hypothesis of deafferentation has been discussed by several authors, but it has only been proven in animal models. Although it presents as a subclinical alteration, auditory deafferentation could cause a reduction in the probability and synchrony of nerve fiber firings and, consequently, impair speech coding, especially in unfavorable listening conditions. The reduction of auditory information from the injured area would decrease the inhibition that the medial olivocochlear system normally exerts on the hair cells. One of the actions resulting from this mechanism would be the reduction of the masking effect, produced by noise or other sounds.
Considering the functions of the medial olivocochlear system, if deafferentation modifies auditory information input and, consequently, interferes with the firing and the functioning of this system, the amplification of the previously reduced noise, favoring the perception of the target signal in the latter’s presence, will no longer take place. Accordingly, the recognition of stimuli in the presence of noise may be compromised or, at least, different from that in an intact system. Bearing this in mind, this behavior could be observed in the tinnitus group’s worse performance in the LSP test, with both noises. In other words, the signal/noise ratio required for 50% target sentence recognition was statistically higher in the tinnitus group, indicating that this noise, which should be attenuated by the medial olivocochlear system, was making the conditions for the main sound to be heard less favorable.