title: Cortical encoding of timbre changes in cochlear implant users.
abstract: BACKGROUND
  Most cochlear implant (CI) users describe music as a noise-like and unpleasant sound. Using behavioral tests, most prior studies have shown that perception of pitch-based melody and timbre is poor in CI users.
  
  
  OBJECTIVE
  This article will focus on cortical encoding of timbre changes in CI users, which may allow us to find solutions to further improve CI benefits. Furthermore, the value of using objective measures to reveal neural encoding of timbre changes may be reflected in this study.
  
  
  METHODS
  A case-control study of the mismatch negativity (MMN) using electrophysiological technique was conducted. To derive MMNs, three randomly arranged oddball paradigms consisting of standard/deviant instrumental pairs: saxophone/piano, cello/trombone, and flute/French horn, respectively, were presented.
  
  
  METHODS
  Ten CI users and ten normal-hearing (NH) listeners participated in this study.
  
  
  METHODS
  After filtering, epoching, and baseline correction, independent component analysis (ICA) was performed to remove artifacts. The averaged waveforms in response to the standard stimuli (STANDARD waveform) and the deviant stimuli (DEVIANT waveform) in each condition were separately derived. The responses from nine electrodes in the fronto-central area were averaged to form one waveform. The STANDARD waveform was subtracted from the DEVIANT waveform to derive the difference waveform, for which the MMN was judged to be present or absent. The measures used to evaluate the MMN included the MMN peak latency and amplitude as well as MMN duration.
  
  
  RESULTS
  The MMN, which reflects the ability to automatically detect acoustic changes, was present in all NH listeners but only approximately half of CI users. In CI users with present MMNs, the MMN peak amplitude and duration were significantly smaller and shorter compared to those in NH listeners.
  
  
  CONCLUSIONS
  Our electrophysiological results were consistent with prior behavioral results that CI users' performance in timbre perception was significantly poorer than that in NH listeners. Our results may suggest that timbre information is poorly registered in the auditory cortex of CI users and the capability of automatic detection of timbre changes is degraded in CI users. Although there are some limitations of the MMN in CI users, along with other objective auditory evoked potential tools, the MMN may be a useful objective tool to indicate the extent of sound registration in auditory cortex in the future efforts of improving CI design and speech strategy.
authors.affiliation: Department of Communication Sciences and Disorders and.
authors.forename: Fawen
authors.initials: F
authors.lastname: Zhang
authors.forename: Chelsea
authors.initials: C
authors.lastname: Benson
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Cahn
mesh_terms: D000161:Acoustic Stimulation / Q000379:methods; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001303:Auditory Cortex / Q000502:physiology*; D016022:Case-Control Studies; D003054:Cochlear Implants; D004569:Electroencephalography; D005072:Evoked Potentials, Auditory / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009146:Music; D010898:Pitch Perception / Q000502:physiology*; D011571:Psychoacoustics; D013018:Sound Spectrography; D055815:Young Adult