INTERNATIONAL CONGRESS ON ACOUSTICS MADRID , 2-7 SEPTEMBER 2007 AUDITORY SCENE ANALYSIS AND THE PRECEDENCE-EFFECT WITH COCHLEAR IMPLANTS – PREDICTIONS FROM SIMULATIONS

The analysis of auditory scenes is based on the evaluation of monaural spectral and temporal cues as well as on binaural cues. Using current devices cochlear implant (CI) subjects have very limited access to interaural time differences (ITDs) and spectral information while interaural level cues (ILDs) and the temporal envelope are relatively well reproduced. Localization in quiet is often possible but the presence of other sounds disturbs it. We previously showed that the precedence-effect was disturbed in a noise-band vocoder CI-simulation for ongoing sounds and lead and lag images were heard in separation. The present study shows that temporal quantization of the envelope in the vocoder has only minor effect on precedence. Further study varied the interaural match of the carrier frequencies in a sinusoidal vocoder. Precedence broke down if carriers were not matched in frequency. However, for zero or small frequency offsets some subjects showed precedence and fusion into one image. Interestingly, the interaural phase of the carrier played only a limited role which shows that precedence of ongoing sounds can solely be based on ILDs and envelope-ITDs provided that spectral cues are matched across the ears. The results suggest that proper place matching of CI-electrodes would help the analysis of concurrent sounds. INTRODUCTION The auditory system possesses the amazing ability to analyze the auditory scene into separate objects that are present simultaneously, often at different locations. The information for this is taken from the spectrum, from temporal relationships of components, and from binaural cues in time and intensity. In impaired hearing and with CIs in particular, some of this information is missing. Current generations of CIs do not encode ITDs in the carrier pulses and access to spectral information (pitch) is very limited. For coping with situations of hearing in noise or reverberation, however, previous studies found that carrier-ITDs are essential, at least for the localization of longer duration sounds [1-5]. An explicit way to study the auditory system’s ability to deal with reverberation is to investigate precedence or summing localization: Subjects localize a sound that is accompanied by its echo. Localization will be at the leading sound for short delay times and the echo will not be audible by itself. When we previously simulated CIlistening with a noise-band vocoder we found that precedence breaks down, i.e. that the echo is not suppressed, but separately audible [6]. The following two mechanisms which we will address in the present paper might contribute to the breakdown of precedence: a) The information present in the envelope might decorrelate signals on both ears so far that two images appear in both ears even if a correlated carrier is used. We investigated the influence of envelope quantization on precedence. The quantization should better simulate the pulsatile stimulation of CIs compared to low-pass filtering of the envelope which we previously used. Similar quantization on both ears should also reduce the availability of envelope-ITDs and increase the interaural correlation of the envelope. b) We wonder if the breakdown of precedence is caused by an absence of auditory object grouping in and across the ears. Auditory scene analysis is predominantly influenced by spectral grouping. The occurrence of separate images in both ears might thus be based on the lack of spectral grouping across the ears. We studied this hypothesis by varying carrier frequencies in a sinusoidal vocoder.