Sound Localization Bias and Error in Bimodal Listeners Improve Instantaneously When the Device Delay Mismatch Is Reduced

Abstract

Users of a cochlear implant (CI) in one ear, who are provided with a hearing aid (HA) in the contralateral ear, so-called bimodal listeners, are typically affected by a constant and relatively large interaural time delay offset due to differences in signal processing and differences in stimulation. For HA stimulation, the cochlear travelling wave delay is added to the processing delay, while for CI stimulation, the auditory nerve fibers are stimulated directly. In case of MED-EL CI systems in combination with different HA types, the CI stimulation precedes the acoustic HA stimulation by 3 to 10 ms. A self-designed, battery-powered, portable, and programmable delay line was applied to the CI to reduce the device delay mismatch in nine bimodal listeners. We used an A-B-B-A test design and determined if sound source localization improves when the device delay mismatch is reduced by delaying the CI stimulation by the HA processing delay (τHA). Results revealed that every subject in our group of nine bimodal listeners benefited from the approach. The root-mean-square error of sound localization improved significantly from 52.6° to 37.9°. The signed bias also improved significantly from 25.2° to 10.5°, with positive values indicating a bias toward the CI. Furthermore, two other delay values (τHA –1 ms and τHA +1 ms) were applied, and with the latter value, the signed bias was further reduced in some test subjects. We conclude that sound source localization accuracy in bimodal listeners improves instantaneously and sustainably when the device delay mismatch is reduced.

Frequency Fitting Optimization Using Evolutionary Algorithm in Cochlear Implant Users with Bimodal Binaural Hearing

“…Wess et al proposed several possible explanations possible for this variability: (1) intrisinc characteristics of the individual listeners (neural survival, current spread, duration of deafness, lack of cortical plasticity) that cannot be addressed through signal processing; (2) extrinsic distorsions rectifiable via signal processing and/or mapping procedures including different processing times between CI and contralateral HA or NH ear and frequency mismatch between the CI and the contralateral HA or NH ear [ 10 ]. For example Zirn et al [ 56 ] and Angermeier et al [ 17 ] showed that sound Localization in Bimodal Listeners could be improved instantaneously when the device delay mismatch (between CI and contralateral HA) was reduced.…”

“…Alternatively, binaural interactions could be improved through frequency band adjustments without electrode deactivation [ 10 ], or through adjustment of the temporal processing between the CI and the contralateral HA or ear [ 7 , 17 ].…”

Considerations for Fitting Cochlear Implants Bimodally and to the Single-Sided Deaf

“…With the CI side and the unaided acoustic side producing similar latencies, Zirn et al (2019) solely compensated for the HA processing latency. Angermeier et al (2021) where able to show an even better outcome using a delay compensation of HA processing latency plus an additional 1 ms for the difference between MED-EL CI and NH latency ( Seebacher et al, 2019 ).…”

“…In bimodal CI listeners, an acute compensation of the latency mismatch by adding a constant delay to the CI side was performed by Zirn et al (2019) and Angermeier et al (2021) . After one hour of acclimatization to the added delay, the subjects showed a significantly decreased rms-error, resulting in an improved sound localization accuracy in the test situation of more than 11% and on average a reduced lateralization bias by 15° compared to no latency correction.…”

“…If prerequisites on the input to these neurons are not met, especially in individuals with electric stimulation and/or very asymmetric hearing, they may miss those benefits, even under aided conditions ( Dieudonné & Francart, 2020 ), with respect to spatial release from masking). With respect to sound source localization, unnatural or asymmetric stimulation with broadband stimuli results in a root-mean-square (rms) localization error of 50 to 70° in bimodal and 30° in SSD-CI listeners, where 75° corresponds to chance performance ( Angermeier et al, 2021 ; Dorman et al, 2016 ). The better SSD-CI performance is likely caused by the monaural localization abilities of the NH ear, because even SSD patients without an implant perform similarly (e.g., Agterberg et al, 2014 ).…”