Perceptual adaptation to a binaurally mismatched frequency to place map by omahafunk

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									Perceptual adaptation to a binaurally mismatched frequency-to-place map
Catherine Siciliano, Andrew Faulkner, Stuart Rosen
Department of Phonetics and Linguistics and UCL Centre for Human Communication
UCL, London, UK

 Background                                                                                                 Can listeners adapt?                                                 Results                                                                                   Conclusions
 The cochlea analyzes incoming sound into its frequency                                                     Simulations of cochlear implants were used to test whether                                   IEEE sentence intelligibility                                     The findings suggest that listeners rely on the natural
 components and passes this information on to higher centres                                                normal-hearing listeners could learn to integrate mismatched                           100                                                                     frequency-to-place map when it is available, even at the
 in the brain. A cochlear implant is an implantable device that                                             frequency maps. Processing was designed to ensure that the                                                                                                     expense of improved spectral detail. The perceptual
 can restore functionality to faulty cochleae. The implant                                                  integration of information from both ears would be maximally                                                                                                   adaptation must involve a rapid attuning to just the matched
 performs the frequency analysis and passes this information                                                beneficial.                                                                            80                                                                      frequency bands - subjects are not simply listening to the
 on to the brain via direct electrical stimulation.                                                                                                                                                                                                                        better ear.
 Historically, cochlear implantees have been fitted unilaterally
 with a single implant. However, research has shown that
                                                                                                                                                                                                   60
                                                                                                                                                                                                                                                 condition
                                                                                                                                                                                                                                                                           Hypothesis 1:
 hearing with two ears is better than hearing with a single ear,                                                                                                                                                                                                           Subjects may need a period of training with the shifted map in




                                                                                                                                                                              % keywords correct
 especially in background noise. This is especially true for                                                                                                                                                                                         dichotic unshifted    the absence of the natural frequency-place map to realize any
                                                                                                                                                                                                   40
 cochlear implantees. Accordingly, implant patients are now                                                                                                                                                                                          dichotic part-shifted improvements with the mismatched map. Experiments testing
 sometimes fitted with a hearing aid in the opposite ear to make                                                                                                                                                                                                           this hypothesis are currently underway.
 use of what residual hearing may be left. Some patients have                                                                                                                                      20                                                diotic part-shifted
 been fitted with two cochlear implants. In both cases, benefits                                                                                                                                                                                     3 unshifted channels Hypothesis 2:
 to speech perception have been demonstrated. [1,2,3]                                                                                                                                                                                                                      Performance never reached asymptotic level, so the training
                                                                                                                                                                                                    0                                                3 shifted channels    period may not have been long enough for subjects to adapt to
 Spectral shifts                                                                                                                                                                                               0             4           8
                                                                                                                                                                                                                                                                           the processor. Listeners may outperform the 3 matched
 The snail-shaped anatomy of the cochlea can prevent the                                                                                                                                                 training sessions (40 minutes each)                               channels after very long-term training with the mismatched
 cochlear implant electrode array from being fully inserted.                                                                                                                                                                                                               processor.
 Consequently, the cochlear implant may deliver its frequency                                                                                                                    Repeated-measures ANOVA:
 analysis to the 'wrong' place along the basilar membrane.                                                                                                                          No significant interaction between talker and condition                                Implications
                                                                                                            Experimental processor: 6-channel dichotic sine-vocoder with
 We can liken this to an upward spectral shift of frequency                                                                                                                         Significant training effect (p < 0.05) for all conditions                              While listeners can learn to adapt to very distorted speech,
                                                                                                            a 6mm shift in one ear [6]
 information. Shallow implant insertions have been shown to                                                                                                                      Bonferroni-adjusted paired comparisons (post-training):                                   they are quicker and better at adapting when this speech
 have adverse effects on speech perception immediately                                                      Subjects                                                                                                                                                       retains the natural relative frequency order (cf. [4]). This
 following surgery. Over time, though, many listeners can learn                                                                                                                      No significant difference between dichotic part-shifted,
                                                                                                                                                                                                                                                                           should be taken into account when optimizing bilateral devices
                                                                                                              Six normally-hearing native speakers of British English                diotic part-shifted and 3 unshifted channels (p > 0.05)
 to accommodate this 'spectral shift.' [4]                                                                                                                                                                                                                                 for speech recognition.
 Previous research has examined spectral shifts that are both                                               Training                                                                                     Vowel identification
 unilateral and uniform. Bilateral devices pose new questions                                                                                                                                 100
                                                                                                              10 minute familiarization block with unshifted processor
 about spectral shifts as frequency maps are likely to be
 asymmetric.                                                                                                  Dichotic part-shifted processor                                                                                                                              References
                                                                                                              Connected Discourse Tracking (CDT)                                                   80                                                                      [1] Ching, Y. C. et al. (2001). Ear & Hear 22(5), 365-380.
                                                                                                              8 40-minute training sessions                                                                                                                                [2] Tyler, R. et al. (2002). Ear & Hear 23(1), 80S-89S.
                                                                                                              5 minutes per session with visual feedback                                                                                                                   [3] Dorman, M. & Dahlstrom, L. (2004) Ear & Hear 25, 191-194.
                                                                                                                                                                                                   60                                          condition                   [4] Rosen, S., Faulkner, A. & Wilkinson, L. (1999). JASA 106, 3629-
                                                                                                            Testing                                                                                                                                dichotic unshifted      3636.
                                                                                                              Pre-test, mid-test and post-test                                                     40
                                                                                                                                                                           % vowels correct
                                                                                                                                                                                                                                                                           [5] Images adapted from Geisler, C.D. (1998). From Sound to
                                                                                                                                                                                                                                                   dichotic part-shifted   Synapse: Physiology of the Mammalian Ear.
  apex
                                                                                base                          IEEE/Harvard sentences
                                                                                                                                                                                                                                                   diotic part-shifted     [6] Greenwood, D. D. (1990) JASA 87(6), 2592-2605.
 The apex is impenetrable by the implant. Low                                                                 Vowel identification: 'Say bVd again.'                                               20
 frequency information important for speech is
 presented to structures further towards the base -       cochlear implant                                    1 male, 2 female talkers                                                                                                             3 unshifted channels
 a 'basalward' shift.                                     electrode
                                                                                                                                                                                                    0                                              3 shifted channels      Acknowledgments
 An unwound cochlea fitted with an implant (spectral shift)
                                                                                                                                                                                                              0          4          8                                      Work supported by EU project HEARCOM [FP6-004171].
                                                                                                            condition               right ear          left ear
                                      The base of the cochlea is most responsive to high-frequency                                                                                                      training sessions (40 minutes each)
                                      sounds. Here the inner hair cells (nerve receptors) have died, and    dichotic unshifted      1, 3, 5            2, 4, 6
                                      the cochlea is unresponsive.
                                                                                                                                                                                 Repeated-measures ANOVA:
                                                                                                            dichotic part-shifted   1, 3, 5 à 6mm      2, 4, 6
                                                                                                                                                                                    No significant interaction between talker and condition
                                                                                                                                    1, 3, 5 à 6mm      1, 3, 5 à 6mm                Significant training effect (p < 0.05) for all conditions
                                                                                                            diotic part-shifted
                  base                                                                          apex
                                                                                                                                    2, 4, 6            2, 4, 6                   Bonferroni-adjusted paired comparisons (post-training):
                                                                              Surviving hair cells at the
                                                                              apex respond to low
                                                                                                            3 unshifted channels                       2, 4, 6                       Performance with 3 matched channels better than with
                                                                              frequencies with the help                                                                              dichotic and diotic part-shifted conditions (p < 0.01)
                                                                              of a hearing aid.             3 shifted channels      1, 3, 5 à 6mm
  An unwound cochlea in the opposite ear (no spectral shift) [5]

								
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