From the aforementioned studies one can hypothesize that impaired ability to follow amplitude variations in speech signal may in part be the root cause of the extreme difficulty experienced by listeners with AN in understanding speech in the presence of noise. This has been proposed as one of the major reasons for reduced speech intelligibility in the presence of competing signals,. Background noise reduces the modulation depth of temporal envelopes and introduces spurious modulations, which obscures the relevant speech modulations. The exact mechanisms underlying these extreme perceptual difficulties in noise are not clear.Ī number of previous investigators have examined reasons for the reduction in speech identification scores in the presence of background noise for listeners with normal hearing –. Similar results have been demonstrated for children by Rance et al. Also, identification scores in noise were much lower for those who had poor identification scores (50%). A greater reduction in identification scores in noise was observed for listeners with AN than those with normal hearing. Narne and Vanaja have evaluated speech identification scores in quiet and in the presence of noise (signal to noise ratio of 10, 5 and 0 dB). Further, it has been hypothesized that this impaired ability to follow amplitude variations may reduce the consonant-vowel distinction, resulting in extreme difficulty in understanding speech. These investigators have suggested that perceptual difficulties experienced by listeners with AN may be due to an impaired ability to follow amplitude variations in speech signals (temporal envelope cues). Attempts have been made through psychoacoustic and simulation studies to explain factors responsible for perceptual difficulties experienced by listeners with AN in quiet –. Studies have shown that these perceptual difficulties cannot be explained based on the degree of hearing loss or audibility. In listeners with AN, speech perception difficulties in quiet range from minimal to severe and these difficulties are exaggerated in presence of competing background noise. Listeners with cochlear hearing loss also have difficulty in understanding speech in quiet, more so in the presence of competing signals. Speech perception difficulties (in quiet and background noise) are not unique to listeners with AN. Psychoacoustic experiments conducted on listeners with AN indicate a significant impairment in temporal processing and this leads to extreme difficulty in understanding speech. One main characteristic of AN is disrupted auditory nerve activity, evidenced by absent or severely abnormal auditory brainstem response, with normal or near normal cochlear outer hair cell function, as observed by the presence of oto-acoustic emissions and/or cochlear microphonics. Auditory Neuropathy (AN) is a term used to describe auditory disorders with dysfunction of the auditory nerve in the presence of preserved cochlear outer hair-cell function.
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