Coherent Coding of Enhanced Interaural Cues Improves Sound Localization in Noise With Bilateral Cochlear Implants

Abstract

Bilateral cochlear implant (BCI) users only have very limited spatial hearing abilities. Speech coding strategies transmit interaural level differences (ILDs) but in a distorted manner. Interaural time difference (ITD) information transmission is even more limited. With these cues, most BCI users can coarsely localize a single source in quiet, but performance quickly declines in the presence of other sound. This proof-of-concept study presents a novel signal processing algorithm specific for BCIs, with the aim to improve sound localization in noise. The core part of the BCI algorithm duplicates a monophonic electrode pulse pattern and applies quasistationary natural or artificial ITDs or ILDs based on the estimated direction of the dominant source. Three experiments were conducted to evaluate different algorithm variants: Experiment 1 tested if ITD transmission alone enables BCI subjects to lateralize speech. Results showed that six out of nine BCI subjects were able to lateralize intelligible speech in quiet solely based on ITDs. Experiments 2 and 3 assessed azimuthal angle discrimination in noise with natural or modified ILDs and ITDs. Angle discrimination for frontal locations was possible with all variants, including the pure ITD case, but for lateral reference angles, it was only possible with a linearized ILD mapping. Speech intelligibility in noise, limitations, and challenges of this interaural cue transmission approach are discussed alongside suggestions for modifying and further improving the BCI algorithm.

Interaural Pitch-Discrimination Range Effects for Bilateral and Single-Sided-Deafness Cochlear-Implant Users

“…Note, however, that tolerance to interaural mismatch may be limited to single-electrode stimulation. Multi-electrode stimulation (particularly when attempting to transmit low-rate ITD information) may be less tolerant to interaural mismatch (Williges et al 2018). Overall, the current results strongly support at least following the recommendations of Carlyon et al (2010), who advised that an experimenter should test multiple ranges to determine the veracity of the pitch match.…”

“…To achieve optimal binaural hearing with CIs, one should match interaural place of stimulation on individual electrodes (Kan et al 2013;Hu and Dietz 2015;Kan et al 2015;Williges et al 2018). Current clinical practice does not presently employ such a strategy.…”

Electro-Haptic Enhancement of Spatial Hearing in Cochlear Implant Users

“…Future work should investigate the benefits of electro-haptic stimulation in more complex acoustic environments, with multiple simultaneous sound sources. In such environments, it may be possible to improve performance through the use of algorithms that magnify spatial hearing cues, aid the segregation of multiple sounds, and reduce background noise [49][50][51] .…”

Binaural hearing measured with the temporal limits encoder using a vocoder simulation of cochlear implants

“…To enhance ITDs with electrical stimulation, most strategies lower the stimulation rate of the low frequency channels by timing the firing of the pulses at the time of zero-crossing [11] or peak [7] of the corresponding bands' signal, but only small binaural benefits have been observed [12]. Recently, some proof-of-concept studies have proposed novel approaches to improving ITD sensitivity in BiCI users: combining beamformer-based direction estimation with peak timing [13]; introducing some pulses with short interpulse intervals in high stimulation rates to promote ITD sensitivity [14]; interleaving high and low rate stimulation on different channels [15]. All studies showed significant improvement when measuring ITD sensitivity, but there are two main limitations: 1. no speech intelligibility tests were carried out; and 2. their applications might need a real-time ITD estimation algorithm which is a difficult engineering task for real scenarios.…”