Reweighting of Binaural Localization Cues in Bilateral Cochlear-Implant Listeners

Abstract

Normal-hearing (NH) listeners rely on two binaural cues, the interaural time (ITD) and level difference (ILD), for azimuthal sound localization. Cochlear-implant (CI) listeners, however, rely almost entirely on ILDs. One reason is that present-day clinical CI stimulation strategies do not convey salient ITD cues. But even when presenting ITDs under optimal conditions using a research interface, ITD sensitivity is lower in CI compared to NH listeners. Since it has recently been shown that NH listeners change their ITD/ILD weighting when only one of the cues is consistent with visual information, such reweighting might add to CI listeners’ low perceptual contribution of ITDs, given their daily exposure to reliable ILDs but unreliable ITDs. Six bilateral CI listeners completed a multi-day lateralization training visually reinforcing ITDs, flanked by a pre- and post-measurement of ITD/ILD weights without visual reinforcement. Using direct electric stimulation, we presented 100- and 300-pps pulse trains at a single interaurally place-matched electrode pair, conveying ITDs and ILDs in various spatially consistent and inconsistent combinations. The listeners’ task was to lateralize the stimuli in a virtual environment. Additionally, ITD and ILD thresholds were measured before and after training. For 100-pps stimuli, the lateralization training increased the contribution of ITDs slightly, but significantly. Thresholds were neither affected by the training nor correlated with weights. For 300-pps stimuli, ITD weights were lower and ITD thresholds larger, but there was no effect of training. On average across test sessions, adding azimuth-dependent ITDs to stimuli containing ILDs increased the extent of lateralization for both 100- and 300-pps stimuli. The results suggest that low-rate ITD cues, robustly encoded with future CI systems, may be better exploitable for sound localization after increasing their perceptual weight via training.

Interaural time difference sensitivity under binaural cochlear implant stimulation persists at high pulse rates up to 900 pps

“…Furthermore, recent studies have demonstrated that lateralization training can lead to a reweighting of binaural cues in normal hearing listeners (Kumpik et al 2019; Klingel et al 2021) and biCI users (Klingel and Laback 2022), such that the listeners rely more or less heavily on ILDs or ITDs respectively depending on which cue is more reliable or informative. This could be relevant for CI listeners, as they could similarly down-weight ITD cues if the ongoing stimulation with clinical processors does not transmit reliable, informative ITDs which would thus encourage a reliance exclusively on ILDs to the detriment of ITDs.…”

“…The rats in these studies spent their entire lives before puberty without any auditory input, but, unlike human CI users, once they were implanted they were also never exposed to prolonged periods of stimulation with the uninformative and potentially misleading pulse timing ITDs that wearers of bilateral clinical speech processors will experience constantly. Note that recent studies have demonstrated that lateralization training can lead to a reweighting of binaural cues in normal hearing listeners [77; 78] and biCI users [79], such that the listeners rely more or less heavily on ILDs or ITDs, respectively, depending on which cue is more reliable or informative. The auditory pathway thus seems clearly capable of desensitizing to ITD cues if provided with prolonged stimulation that is bereft of informative ITDs.…”

“…Note that recent studies have demonstrated that lateralization training can lead to a reweighting of binaural cues in normal hearing listeners [77; 78] and biCI users [79], such that the listeners rely more or less heavily on ILDs or ITDs, respectively, depending on which cue is more reliable or informative. The auditory pathway thus seems clearly capable of desensitizing to ITD cues if provided with prolonged stimulation that is bereft of informative ITDs.…”

Binaural-cue Weighting and Training-Induced Reweighting Across Frequencies

“…Many CI stimulation strategies encode ITDs only via the envelope of the stimulus waveform and, even when encoding ITDs via the pulse timing, CI listeners’ sensitivity pattern resembles that for envelope ITDs in acoustic hearing ( Bernstein & Trahiotis, 2002 ; Laback et al, 2007 ). In fact, binaural-cue reweighting has recently been observed in CI listeners when ITDs were encoded via the pulse timing of low-rate pulse trains ( Klingel & Laback, 2021 ).…”

Interaural time difference sensitivity under binaural cochlear implant stimulation persists at high pulse rates up to 900 pps

“…The rats in these studies spent their entire lives before puberty without any auditory input, but, unlike human CI users, once they were implanted they were also never exposed to prolonged periods of stimulation with the uninformative and potentially misleading pulse timing ITDs that wearers of bilateral clinical speech processors will experience constantly. Note that recent studies have demonstrated that lateralization training can lead to a reweighting of binaural cues in normal hearing listeners 86,87 and biCI users 88 , such that the listeners rely more or less heavily on ILDs or ITDs, respectively, depending on which cue is more reliable or informative. The auditory pathway thus seems clearly capable of desensitizing to ITD cues if provided with prolonged stimulation that is bereft of informative ITDs.…”