Effects of interaural time differences in fine structure and envelope on lateral discrimination in electric hearing

Abstract

Bilateral cochlear implant (CI) listeners currently use stimulation strategies which encode interaural time differences (ITD) in the temporal envelope but which do not transmit ITD in the fine structure, due to the constant phase in the electric pulse train. To determine the utility of encoding ITD in the fine structure, ITD-based lateralization was investigated with four CI listeners and four normal hearing (NH) subjects listening to a simulation of electric stimulation. Lateralization discrimination was tested at different pulse rates for various combinations of independently controlled fine structure ITD and envelope ITD. Results for electric hearing show that the fine structure ITD had the strongest impact on lateralization at lower pulse rates, with significant effects for pulse rates up to 800 pulses per second. At higher pulse rates, lateralization discrimination depended solely on the envelope ITD. The data suggest that bilateral CI listeners benefit from transmitting fine structure ITD at lower pulse rates. However, there were strong interindividual differences: the better performing CI listeners performed comparably to the NH listeners.

Modulation Enhancement in the Electrical Signal Improves Perception of Interaural Time Differences with Bimodal Stimulation

“…This is comparable to the best thresholds found in our previous studies with single and 3-channel stimuli (Francart et al 2009. The thresholds are also comparable to thresholds found for the best bilateral CI listeners with single-channel stimuli (Laback et al 2007(Laback et al , 2004Lawson et al 1998;Litovsky et al 2010Litovsky et al , 2012Long et al 2003;Majdak et al 2006;Senn et al 2005;van Hoesel 2004van Hoesel , 2007van Hoesel and Tyler 2003). That sensitivity with multiple-channel stimuli is comparable to performance with single-channel stimuli is unlike performance often reported for NH listeners, for whom sensitivity increases with increasing bandwidth.…”

Binaural jitter improves interaural time-difference sensitivity of cochlear implantees at high pulse rates

“…The improvements reach a maximum of 28% at 600 s for large jitter (P Ͻ 0.0001) and of 14% at 400 s for small jitter (P Ͻ 0.0001). Binaural jitter improves the performance even for ITDs that approach or exceed one-half of the IPI and thus contain ambiguous ongoing fine structure ITD cues (5). For example, the ITD of 400 s is ambiguous at all pulse rates from 800 to 1,515 pps (within one-quarter to three-quarters of the IPI), and still binaural jitter significantly improves the performance (both large and small jitter: P Ͻ 0.0001).…”

“…1a). An amplitude modulation of this type has been successfully used in a previous study (5). This signal attempts to roughly approximate the characteristics of real-world signals, in particular speech.…”

“…The respective electrode pairs for each subject are specified in the last column of Table 1, where electrodes are numbered from apex to base. For each pulse rate, current levels were determined to evoke a centralized auditory image at a comfortable level (5). The stimuli were created on a laboratory computer and directly transmitted to the CIs via an interaurally synchronized research interface (RIB, developed at the University of Innsbruck, Innsbruck, Austria) with an interaural timing accuracy of 2.5 s.…”

“…There is evidence that ITD in the fine structure of a sound is most important for sound localization (1,2) and for understanding speech in noise (3,4). Listeners bilaterally supplied with cochlear implants (CIs) have been shown to be sensitive to fine structure ITD, but their sensitivity disappears at a pulse rate of a few hundred pulses per second (5)(6)(7)(8). This is in contrast to normal hearing (NH) listeners, who are sensitive to ITD in the fine structure up to much higher frequencies (9,10).…”