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Keywords:

  • Sound localization;
  • cochlear implants;
  • monaural localization

Abstract

Objective: The main purpose of the study was to assess the ability of adults with unilateral cochlear implants to localize noise and speech signals in the horizontal plane.

Design: Six unilaterally implanted adults, all postlingually deafened and all fitted with MED-EL COMBI 40+ devices, were tested with a modified source identification task. Subjects were tested individually in an anechoic chamber, which contained an array of 43 numbered loudspeakers extending from –90° to +90° azimuth. On each trial, a 200 millisecond signal (either a noise burst or a speech sample) was presented from one of nine active loudspeakers, and the subject had to identify which source (from the 43 loudspeakers in the array) produced the signal.

Results: The relationship between source azimuth and response azimuth was characterized in terms of the adjusted constant error (Ĉ). Ĉ for three subjects was near chance (50.5°), whereas Ĉ for the remaining three subjects was significantly better than chance (35°–44°). By comparison, Ĉ for a group of normal-hearing listeners was 5.6°. For two of the three subjects who performed better than chance, monaural cues were determined to be the basis for their localization performance.

Conclusions: Some unilaterally implanted subjects can localize sounds at a better than chance level, apparently because they can learn to make use of subtle monaural cues based on frequency-dependent head-shadow effects. However, their performance is significantly poorer than that reported in previous studies of bilaterally implanted subjects, who are able to take advantage of binaural cues.