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

Abstract

Interaural timing cues are important for sound source localization and for binaural unmasking of speech that is spatially separated from interfering sounds. Users of a cochlear implant (CI) with residual hearing in the non-implanted ear (bimodal listeners) can only make very limited use of interaural timing cues with their clinical devices. Previous studies showed that bimodal listeners can be sensitive to interaural time differences (ITDs) for simple single-and three-channel stimuli. The modulation enhancement strategy (MEnS) was developed to improve the ITD perception of bimodal listeners. It enhances temporal modulations on all stimulated electrodes, synchronously with modulations in the acoustic signal presented to the non-implanted ear, based on measurement of the amplitude peaks occurring at the rate of the fundamental frequency in voiced phonemes. In the first experiment, ITD detection thresholds were measured using the method of constant stimuli for five bimodal listeners for an artificial vowel, processed with either the advanced combination encoder (ACE) strategy or with MEnS. With MEnS, detection thresholds were significantly lower, and for four subjects well within the physically relevant range. In the second experiment, the extent of lateralization was measured in three subjects with both strategies, and ITD sensitivity was determined using an adaptive procedure. All subjects could lateralize sounds based on ITD and sensitivity was significantly better with MEnS than with ACE. The current results indicate that ITD cues can be provided to bimodal listeners with modified sound processing.

Interaural Time Difference Perception with a Cochlear Implant and a Normal Ear

“…In particular Francart et al (2009) and Francart et al (2011) used the 50%-correct point from a constant stimuli procedure, which may lead to lower JNDs than estimated with the typically used target point of the adaptive procedure of around 70%. However, Francart et al (2014) compared the results of their constant stimuli procedure with an 1up/2down, 70% correct, -ITD +ITD procedure, and found similar results when the thresholds obtained from the adaptive procedure were multiplied by 2.…”

“…With current fitting and sound coding strategies, it is unlikely that CI users with contralateral normal-hearing (or impaired hearing) would be able to use ITD cues in practice because (1) the required temporal information is poorly coded by most CI sound processing strategies (Francart et al, 2014), (2) many CI sound processors have a processing delay longer than the required delay of electrical stimulation for synchronisation with the acoustic signal, and for broad-band signals the delay of the sound processor does not vary in the same way with frequency as the delay of the traveling wave in the acoustically stimulated ear, (3) with the standard frequency-to-electrode allocation of the sound processor, the place of stimulation in the two cochleae is not optimally matched for ITD perception, for most subjects (4) for the human head size, the maximal physically plausible ITD is around 690 us (Feddersen, 1957). If the smallest ITD that can be detected is in the best case 200 μs, and in many cases much more, it is unlikely to yield much practical use.…”

“…If a certain subject is sufficiently sensitive to ITD in controlled laboratory conditions, the following changes to sound coding and fitting would be required to allow the use of ITDs for sound source localisation: on the CI side faithfully encode the timing information in the temporal envelope of the incoming acoustical signal, in a way compatible with the temporal envelopes available in the acoustically stimulated ear (Francart et al, 2014). While we did not explicitly investigate sensitivity to temporal fine-structure ITDs in SSD CI listeners, we previously found no sensitivity to temporal fine-structure ITDs in bimodal listeners (Lenssen et al, 2011).…”

Effect of Channel Envelope Synchrony on Interaural Time Difference Sensitivity in Bilateral Cochlear Implant Listeners

“…In practice, this could be achieved in a sound processor by using either a single modulation waveform to modulate all channel envelopes, such as in the Modulation Enhancement Strategy (Francart et al 2014), or using overlapping filters in the filter bank from which the envelopes are extracted, which may be optimized as a good compromise between providing sufficiently comodulated signals and providing separate information channels. Delays in this order of magnitude were tested in the present study.…”

Bimodal Cochlear Implant Listeners’ Ability to Perceive Minimal Audible Angle Differences

“…Despite the significant degree of acoustic ear hearing loss, it may be possible that even limited overlapping frequency regions could support binaural computation of ITDs and ILDs, which may improve sound localization performance (Francart et al, 2009b;Veugen et al, 2016b). Sensitivity to ITDs has been demonstrated in bimodal listeners in laboratory conditions using highly controlled stimuli (e.g., pulse trains or noise stimuli with experimental devices under direct computer control of electrode stimulation patterns; Francart and Wouters, 2007;Francart et al, 2009b;2011b;Francart et al, 2014). However, another study found that even for individuals trained on an ITD task (n 5 3), they demonstrated no or little sensitivity to transposed sinusoids or pulse trains (Lenssen et al, 2011).…”