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      Auditory processing in individuals with auditory neuropathy

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          Abstract

          Background

          Auditory neuropathy is a disorder characterized by no or severely impaired auditory brainstem responses in presence of normal otoacoustic emissions and/or cochlear microphonics. Speech perception abilities in these individuals are disproportionate to their hearing sensitivity and reported to be dependent on cortical evoked potentials and temporal processing abilities. The disproportionate loss of auditory percept in presence of normal cochlear function is suggestive of impairment of auditory neural synchrony.

          Methods

          We studied the auditory evoked potentials and psychophysical abilities in 14 adults with auditory neuropathy to characterize their perceptual capabilities. Psychophysical tests included measurement of open set speech identification scores, just noticeable difference for transition duration of syllable /da/ and temporal modulation transfer function. Auditory evoked potentials measures were, recording of P 1/N 1, P 2/N 2 complex and mismatch negativity (MMN).

          Results

          Results revealed a significant correlation between temporal processing deficits and speech perception abilities. In majority of individuals with auditory neuropathy P 1/N 1, P 2/N 2 complex and mismatch negativity could be elicited with normal amplitude and latency. None of the measured evoked potential parameters correlated with the speech perception scores. Many of the subjects with auditory neuropathy showed normal MMN even though they could not discriminate the stimulus contrast behaviorally.

          Conclusion

          Conclusions drawn from the study are

          1. Individuals with auditory neuropathy have severely affected temporal processing.

          2. The presence of MMN may not be directly linked to presence of behavioral discrimination and to speech perception capabilities at least in adults with auditory neuropathy.

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          Most cited references 21

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          Effect of temporal envelope smearing on speech reception.

           R Plomp,  R Drullman,  J Festen (1994)
          The effect of smearing the temporal envelope on the speech-reception threshold (SRT) for sentences in noise and on phoneme identification was investigated for normal-hearing listeners. For this purpose, the speech signal was split up into a series of frequency bands (width of 1/4, 1/2, or 1 oct) and the amplitude envelope for each band was low-pass filtered at cutoff frequencies of 0, 1/2, 1, 2, 4, 8, 16, 32, or 64 Hz. Results for 36 subjects show (1) a severe reduction in sentence intelligibility for narrow processing bands at low cutoff frequencies (0-2 Hz); and (2) a marginal contribution of modulation frequencies above 16 Hz to the intelligibility of sentences (provided that lower modulation frequencies are completely present). For cutoff frequencies above 4 Hz, the SRT appears to be independent of the frequency bandwidth upon which envelope filtering takes place. Vowel and consonant identification with nonsense syllables were studied for cutoff frequencies of 0, 2, 4, 8, or 16 Hz in 1/4-oct bands. Results for 24 subjects indicate that consonants are more affected than vowels. Errors in vowel identification mainly consist of reduced recognition of diphthongs and of confusions between long and short vowels. In case of consonant recognition, stops appear to suffer most, with confusion patterns depending on the position in the syllable (initial, medial, or final).
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            PEST: Efficient Estimates on Probability Functions

             M Taylor (1967)
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              Human Discrimination of Auditory Duration

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                Author and article information

                Journal
                Behav Brain Funct
                Behavioral and brain functions : BBF
                BioMed Central (London )
                1744-9081
                2005
                1 December 2005
                : 1
                : 21
                Affiliations
                [1 ]Junior Research Fellow, Department of Audiology, All India Institute of Speech and Hearing, Manasagangothri, Mysore, Karnataka, 570006, India
                [2 ]Director, All India Institute of Speech and Hearing, Manasagangothri, Mysore, Karnataka, 570006, India
                Article
                1744-9081-1-21
                10.1186/1744-9081-1-21
                1322223
                16321163
                Copyright © 2005 Kumar and Jayaram; licensee BioMed Central Ltd.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                Categories
                Research

                Neurology

                llrs and mmn, temporal processing, auditory neuropathy, speech perception

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