Binaural sensitivity as a function of interaural electrode position with a bilateral cochlear implant user

Abstract

Experiments were conducted with a single, bilateral cochlear implant user to examine interaural level and time-delay cues that putatively underlie the design and efficacy of bilateral implant systems. The subject's two implants were of different types but custom equipment allowed presentation of controlled bilateral stimuli, particularly those with specified interaural time difference (ITD) and interaural level difference (ILD) cues. A lateralization task was used to measure the effect of these cues on the perceived location of the sensations elicited. For trains of fixed-amplitude, biphasic current pulses at 100 pps, the subject demonstrated sensitivity to an ITD of 300 micros, providing evidence of access to binaural information. The choice of bilateral electrode pair greatly influenced ITD sensitivity, suggesting that electrode pairings are likely to be an important consideration in the effort to provide binaural advantages. The selection of bilateral electrode pairs showing sensitivity to ITD was partially aided by comparisons of the pitch elicited by individual electrodes in each ear (when stimulated alone with fixed-amplitude current pulses at 813 pps): specifically, interaural electrodes with similar pitches were more likely (but not certain) to show ITD sensitivity. Significant changes in lateral position occurred with specific electrode pairs. With five bilateral electrode pairs of 14 tested, ITDs of 300 and 600 micros moved an auditory image significantly from right to left. With these same pairs, ILD changes of approximately 11% of the dynamic range (in microApp) moved an auditory image from the far left to the far right-significantly farther than the nine pairs not showing significant ITD sensitivity. However, even these nine pairs did show response changes as a function of the interaural (or confounding monaural) level cue. Overall, insofar as the access to bilateral cues demonstrated herein generalizes to other subjects, it provides hope that the normal binaural advantages for speech recognition and sound localization can be made available to bilateral implant users.

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.…”

Lateralization of Interimplant Timing and Level Differences in Children Who Use Bilateral Cochlear Implants

“…When pulse presentation rates were increased to 200 pps, ITD thresholds were higher (~200µs) (van Hoesel and Tyler 2003). Others have likewise reported that ITD thresholds increase with increases in stimulus rates (Laback et al 2004;Long et al 2003;Van Hoesel et al 2002;van Hoesel 2004;van Hoesel and Clark 1997). Our group has shown that auditory brain stem response amplitudes in children using cochlear implants deteriorate as the rate of electrical pulse rates increases (Davids et al 2008).…”

“…Despite decreased sensitivities to ITD cues, some studies (Long et al 2003;Van Hoesel et al 2002;van Hoesel 2004;van Hoesel and Tyler 2003) have nonetheless indicated that adults with bilateral cochlear implants can perceive differences in time and especially intensity between devices. This suggests that some degree of binaural processing is available to adult bilateral cochlear implant users.…”

“…Indeed, lateralization of interaural timing differences suffers when different frequencies of sound are delivered to each ear in normally hearing listeners (Henning 1974;Nuetzel and Hafter 1976). On the other hand, matching the place of stimulation in the two cochleae may not be as challenging to do with electrical stimuli compared to bilaterally presented acoustic input perhaps due to the wide spread of electrical current (Long et al 2003;Van Deun et al 2009a;van Hoesel 2004). In adult bilateral implant users, mismatches in place of stimulation of 7-8 electrode bands between the two cochleae diminishes ITD sensitivity but does not eliminate it entirely (van Hoesel 2004).…”

Sensitivity to interaural envelope correlation changes in bilateral cochlear-implant users

“…Pitch-matched pairs of electrodes were tested in these experiments, which often maximize sensitivity to binaural cues in CI listeners (Long et al, 2003;Poon et al, 2009;Kan et al, 2013). Thus interaural place-of-stimulation mismatch may have also reduced binaural sensitivity in the CI listeners if the pitch matches did not fully compensate for interaural frequency mismatch.…”