Sensitivity to Interaural Level and Envelope Time Differences of Two Bilateral Cochlear Implant Listeners Using Clinical Sound Processors

In Brief Objectives: To assess the sensitivity of two bilateral cochlear implant users to interaural level and time differences (ILDs and ITDs) for various signals presented through the auxiliary inputs of clinical sound processors that discard fine timing information and only preserve envelope cues. Design: In a lateralization discrimination experiment, the just noticeable difference (JND) for ILDs and envelope ITDs was measured by means of an adaptive 2-AFC method. Different stimuli were used, including click trains at varying repetition rates, a speech fragment, and noise bursts. For one cochlear implant listener and one stimulus, the sensitivity to envelope ITDs was also determined with the method of constant stimuli. The dependency of ILD-JNDs on the interaural place difference was studied with stimulation at single electrode pairs by using sinusoidal input signals in combination with appropriate single-channel processor fittings. In a lateralization position experiment, subjects were required to use a visual pointer on a computer screen to indicate in-the-head positions for blocks of stimuli containing either ILD or ITD cues. All stimuli were loudness balanced (before applying ILD) and fed directly into the auxiliary inputs of the BTE processors (TEMPO+, Med-El Corp.). The automatic gain control and the processors’ microphones were deactivated. Results: Both cochlear implant listeners were highly sensitive to ILDs in all broadband stimuli used; JNDs approached those of normal-hearing listeners. Pitch-matched single electrode pairs showed significantly lower ILD-JNDs than pitch-mismatched electrode pairs. Envelope ITD-JNDs of cochlear implant listeners obtained with the adaptive method were substantially higher and showed a higher test-retest variability than waveform ITD-JNDs of normal-hearing control listeners and envelope ITD-JNDs of normal-hearing listeners reported in the literature for comparable signals. The envelope ITD-JNDs for the click trains were significantly lower than for the speech token or the noise bursts. The best envelope ITD-JND measured was ca. 250 μs for the click train at 100 cycles per sec. Direct measurement of the psychometric function for envelope ITD by the method of constant stimuli showed discrimination above chance level down to 150 μs. The lateralization position experiment showed that both ILDs and envelope ITDs can lead to monotonic changes in lateral percept. Conclusions: The two cochlear implant users tested showed strong effects of ILDs in various broadband stimuli with respect to JNDs as well as lateralization position. The high dependency of ILD-JNDs on the interaural pitch difference suggests the potential importance of pitch-matched assignment of electrodes in the two ears by the speech processors. Envelope ITDs appear to be more ambiguous cues than ILDs, as reflected by the higher and more variable JNDs compared with normal-hearing listeners. The envelope ITD-JNDs of cochlear implant listeners depended on the stimulus. Interaural level and time differences (ILDs and ITDs) are important cues for the localization of sound sources and for the understanding of speech in noise. This study investigated the ability of bilateral cochlear implant (CI) listeners to lateralize on ILD and envelope ITD using various stimuli presented through the auxiliary inputs of clinical sound processors. The processing done by these sound processors preserves envelope cues but removes fine timing information. The results of a lateralization discrimination experiment using broadband stimuli revealed generally high sensitivity of CI listeners to ILDs but low sensitivity and high variability with respect to envelope ITDs compared with normal hearing listeners. The effect of interaural place difference on ILD sensitivity was studied with stimulation at single electrode pairs. Sensitivity was found to be significantly higher for pitch-matched electrode pairs than for pitch-mismatched pairs, emphasizing the potential importance of pitch-matched assignment of electrodes in the two ears by the speech processors. A lateralization position experiment showed that both ILDs and envelope ITDs can lead to monotonic changes in lateral percept.