Localization cues with bilateral cochlear implantsBernhard U. Seebera兲and Hugo FastlInstitute for Human-Machine-Communication, Technische Universität München Arcisstr. 21,80333 München, Germany共Received 10 April 2007; accepted 11 November 2007兲Selected subjects with bilateral cochlear implants 共CIs兲 showed excellent horizontal localization ofwide-band sounds in previous studies. The current study investigated localization cues used by twobilateral CI subjects with outstanding localization ability. The first experiment studied localizationfor sounds of different spectral and temporal composition in the free field. Localization ofwide-band noise was unaffected by envelope pulsation, suggesting that envelope-interaural timedifference 共ITD兲 cues contributed little. Low-pass noise was not localizable for one subject andlocalization depended on the cutoff frequency for the other which suggests that ITDs played only alimited role. High-pass noise with slow envelope changes could be localized, in line withcontribution of interaural level differences 共ILDs兲. In experiment 2, processors of one subject wereraised above the head to void the head shadow. If they were spaced at ear distance, ITDs alloweddiscrimination of left from right for a pulsed wide-band noise. Good localization was observed witha head-sized cardboard inserted between processors, showing the reliance on ILDs. Experiment 3investigated localization in virtual space with manipulated ILDs and ITDs. Localization shiftedpredominantly for offsets in ILDs, even for pulsed high-pass noise. This confirms that envelopeITDs contributed little and that localization with bilateral CIs was dominated by ILDs.© 2008 Acoustical Society of America. 关DOI: 10.1121/1.2821965兴PACS number共s兲: 43.66.Pn, 43.66.Qp, 43.66.Sr, 43.66.Ts 关RYL兴 Pages: 1030–1042I. INTRODUCTIONLocalization of sounds is one of the most importanttasks for the auditory system as it not only helps orientationin space but it is also crucial for the segregation of multiplesounds in the auditory scene. Normal hearing subjects showan outstanding ability to localize sounds and directionalchanges of 1 ° can be detected in the front 共Mills, 1958;Blauert, 1997兲. Several recent studies demonstrated that co-chlear implant 共CI兲 subjects can regain the ability to localizesounds after bilateral implantation 共Tyler et al., 2002; vanHoesel et al., 2002; Grantham et al., 2003; van Hoesel andTyler, 2003; Laszig et al., 2004; Nopp et al., 2004; Schoen etal., 2005兲. One subject in a study by van Hoesel 共2004兲 madeno errors discriminating between the two frontal loudspeak-ers at ⫾13° in an array spanning 180° using a clinical stimu-lation strategy and a pink noise stimulus. However, rootmean square-error increased for non-frontal directions to10– 35°. Seeber et al. 共2004兲 studied localization of bilateralCI subjects as well as of subjects with hearing aid and co-chlear implant using a high-resolution pointing technique.All four bilateral CI subjects were able to localize but onesubject showed excellent localization ability with quartiles ofonly 4.4° and a regression slope of 1.15. This localizationability is close to one of normal hearing subjects whoshowed quartiles of 1.7° and a slope of 0.95 in the same task共Seeber, 2002兲. The purpose of the present article is thus toexplain this surprisingly good localization ability by studyingthe localization cues used by this and another subject.Several recent studies investigated the sensitivity of CIsubjects to binaural cues. All studies correspondinglyshowed high sensitivity to interaural level differences共ILDs兲. Lawson et al. 共1998兲, 共2000兲, and van Hoesel andTyler 共2003兲 found sensitivities as small as one current step.Sensitivity rarely exceeded a few current steps across severalsubjects and electrodes. Despite the dynamic range compres-sion in CI processors this translates into high sensitivity toILDs at the acoustical input. Van Hoesel 共2004兲 measuredjust-noticeable differences 共JNDs兲 for ILDs at the input of aresearch processor. Both of his subjects showed ILD-JNDssmaller than 1 dB. Likewise, Laback et al. 共2004兲 measuredILD-JNDs of two subjects at the direct input of clinical pro-cessors with disabled automatic gain controls 共AGCs兲. Theyfound ILD-JNDs of 1.4–2.7 dB 共S1兲 and 1 –5.2 dB 共S2兲 forvarious stimuli which were about 1 dB larger than for nor-mal hearing subjects.A larger range of sensitivities was found for interauraltime differences 共ITDs兲 in several recent studies. Severalsubjects demonstrated JNDs beyond 1 ms for ITDs in thecarrier, which is outside the naturally occurring range 共vanHoesel et al., 1993; van Hoesel and Clark, 1997; Lawson etal., 2000; van Hoesel, 2004兲. Selected subjects achievedhigher sensitivities at certain electrode combinations. Law-son et al. 共1998兲 found ITD-JNDs of 150␮s in one subjectwith synchronized processors for pulses at a rate of 480pulses per second 共pps兲. Van Hoesel and Tayler 共2003兲showed JNDs of 90, 150, and 250␮s each for two subjectsfor carrier ITDs in pulses at 50 pps. The subjects studied byLawson et al. 共2000兲 showed mixed results, but several sub-jects demonstrated ITD-JNDs of 50– 150␮s on at least oneelectrode combination. The bilateral CI-subject BW, whoa兲Author to whom correspondence should be addressed. Present address:MRC Institute of Hearing Research, University Park, Nottingham, NG72RD, United Kingdom; electronic mail: [email protected] J. Acoust. Soc. Am. 123 共2兲, February 2008 © 2008 Acoustical Society of America0001-4966/2008/123共2兲/1030/13/$23.00demonstrated excellent localization ability in our previousstudy 共Seeber et al., 2004兲, participated in their study 共ME5兲and showed ITD-JNDs of 50␮s on one and 150␮s on twoelectrode combinations. One subject in the study by Lawsonet al. 共2000兲 共ME8兲 showed ITD-JNDs of 50␮s on 5 elec-trode combinations and JNDs of 150␮s on 16 other testedcombinations which were by far the best and most consistentresults of any subject in the test. Because of his high sensi-tivity to ITDs this subject was selected to participate in thecurrent study 共DF兲.While previous studies investigated localization with bi-lateral CIs, the sensitivity to binaural cues, or lateralizationwith isolated binaural cues the present article focuses on thecombination of binaural cues for