Objective: To quantify binaural advantage for auditory localization in the horizontal plane by bilateral cochlear implant (CI) recipients. Also, to determine whether the use of dual microphones with one implant improves localization.<br/>Methods: Twenty subjects from the UK multicenter trial of bilateral cochlear implantation with Nucleus 24 K/M device were recruited. Sound localization was assessed in an anechoic room with an 11-loudspeaker array under four test conditions: right CI, left CI, binaural CI, and dual microphone. Two runs were undertaken for each of five stimuli (speech, tones, noise, transients, and reverberant speech). Order of conditions was counterbalanced across subjects.<br/>Results: Mean localization error with bilateral implants was 24[degrees] compared with 67[degrees] for monaural implant and dual microphone conditions (chance performance is 65[degrees]). Normal controls average 2 to 3[degrees] in similar conditions. Binaural performance was significantly better than monaural performance for all subjects, for all stimulus types, and for different sound sources. Only small differences in performance with different stimuli were observed.<br/>Conclusions: Bilateral cochlear implantation with the Nucleus 24 device provides marked improvement in horizontal plane localization abilities compared with unilateral CI use for a range of stimuli having different spectral and temporal characteristics. Benefit was obtained by all subjects, for all stimulus types, and for all sound directions. However, binaural performance was still worse than that obtained by normal hearing listeners and hearing aid users with the same methodology. Monaural localization performance was at chance. There is no benefit for localization with dual microphones
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