title: Interaural level differences and sound source localization for bilateral cochlear implant patients.
abstract: OBJECTIVE
  The aims of this study were (i) to determine the magnitude of the interaural level differences (ILDs) that remain after cochlear implant (CI) signal processing and (ii) to relate the ILDs to the pattern of errors for sound source localization on the horizontal plane.
  
  
  METHODS
  The listeners were 16 bilateral CI patients fitted with MED-EL CIs and 34 normal-hearing listeners. The stimuli were wideband, high-pass, and low-pass noise signals. ILDs were calculated by passing signals, filtered by head-related transfer functions (HRTFs) to a Matlab simulation of MED-EL signal processing.
  
  
  RESULTS
  For the wideband signal and high-pass signals, maximum ILDs of 15 to 17 dB in the input signal were reduced to 3 to 4 dB after CI signal processing. For the low-pass signal, ILDs were reduced to 1 to 2 dB. For wideband and high-pass signals, the largest ILDs for ±15 degree speaker locations were between 0.4 and 0.7 dB; for the ±30 degree speaker locations between 0.9 and 1.3 dB; for the 45 degree speaker locations between 2.4 and 2.9 dB; for the ±60 degree speaker locations, between 3.2 and 4.1 dB; and for the ±75 degree speaker locations between 2.7 and 3.4 dB. All of the CI patients in all the stimulus conditions showed poorer localization than the normal-hearing listeners. Localization accuracy for the CI patients was best for the wideband and high-pass signals and was poorest for the low-pass signal.
  
  
  CONCLUSIONS
  Localization accuracy was related to the magnitude of the ILD cues available to the normal-hearing listeners and CI patients. The pattern of localization errors for the CI patients was related to the magnitude of the ILD differences among loudspeaker locations. The error patterns for the wideband and high-pass signals, suggest that, for the conditions of this experiment, patients, on an average, sorted signals on the horizontal plane into four sectors-on each side of the midline, one sector including 0, 15, and possibly 30 degree speaker locations, and a sector from 45 degree speaker locations to 75 degree speaker locations. The resolution within a sector was relatively poor.
authors.affiliation: 1Department of Speech and Hearing Science, Arizona State University, Tempe, Arizona, USA; 2MED-EL Corporation, Durham, North Carolina, USA; and 3Department of Communication Science and Disorders, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Dorman
authors.forename: Louise
authors.initials: L
authors.lastname: Loiselle
authors.forename: Josh
authors.initials: J
authors.lastname: Stohl
authors.forename: William A
authors.initials: WA
authors.lastname: Yost
authors.forename: Anthony
authors.initials: A
authors.lastname: Spahr
authors.forename: Chris
authors.initials: C
authors.lastname: Brown
authors.forename: Sarah
authors.initials: S
authors.lastname: Cook
mesh_terms: D000328:Adult; D000368:Aged; D016022:Case-Control Studies; D019929:Cochlear Implantation / Q000379:methods*; D003054:Cochlear Implants; D054062:Deaf-Blind Disorders / Q000503:physiopathology / Q000534:rehabilitation*; D005260:Female; D006801:Humans; D008297:Male; D012815:Signal Processing, Computer-Assisted; D013017:Sound Localization; D013067:Speech Perception; D055815:Young Adult