BINAURAL HEARING IN COMPLEX ENVIRONMENTS WITH COCHLEAR IMPLANTS B.U. Seeber1,2 and E.R. Hafter2 1 MRC Institute of Hearing Research, Nottingham, NG7 2RD, UK, 2 Auditory Perception Lab, Dept. of Psychology, UC Berkeley, Berkeley, CA, USA Abstract: The analysis of auditory scenes poses a severe problem to many listeners with cochlear implants (CIs): Single sounds can be identified and localized, but the presence of multiple sounds impairs this ability. In normal hearing, the precedence effect allows for unimpaired localization at the leading sound despite the presence of a delayed sound copy from a different direction. It is predominantly based on the evaluation of interaural time differences (ITDs) in the sound's temporal fine structure and envelope (onsets). However, Seeber and Fastl (2008) showed that localization with bilateral CIs instead relies on evaluating interaural level differences (ILDs). The present study investigated if this affects patient's ability to cope with single and multiple sound reflections when localizing sounds. Localization ability was severely impaired by the presence of a single reflection of equal amplitude to the direct sound and the precedence effect failed in more than half of the twelve patients. Patients localized a single sound image in-between the lead and the lag loudspeaker, but not at the lead. This suggests that binaural information from the leading and the lagging sound "cancelled" each other. Remarkable is that some patients showed the precedence effect for a stimulus of longer duration for which the leading sound and the reflection temporally overlapped. This requires access to binaural cues of the lead despite a waveform interaction with the lag. Since the CIs did not encode sound information in the pulse timing, patients must have used information in the temporal envelope. Localization ability was further assessed in a simulated living room. Room reverberation was created by playing sound reflections from free-field loudspeakers of the Simulated Open Field Environment (Seeber et al., 2010). Localization ability appeared generally less impaired as in the precedence effect test, possibly because room reflections have a lower level than the direct sound whereas the reflection in the precedence effect test had the same level. However, despite reverberation being moderate and realistic, some subjects had considerable problems localizing sounds. In conclusion, patients with bilateral CIs may regain the ability to localize sounds in quiet, but reverberation can impair this ability and only few patients show the precedence effect, i.e. correct localization at the direct sound. Support was thankfully provided by the intramural programme of the MRC (UK), by NIH RO1 DCD 00087 (USA) and by NOHR grant 018750 (USA). References: Seeber, B., Fastl, H. (2008). Localization cues with bilateral cochlear implants. J. Acoust. Soc. Am. 123, 1030-1042. Seeber, B.U., Kerber, S., Hafter, E.R. (2010). A system to simulate and reproduce audio-visual environments for spatial hearing research. Hearing Res. 260:1-10.