Sensitivity to interaural time differences and localization accuracy in cochlear implant users with combined electric-acoustic stimulation

Abstract

In this study, localization accuracy and sensitivity to acoustic interaural time differences (ITDs) in subjects using cochlear implants with combined electric-acoustic stimulation (EAS) were assessed and compared with the results of a normal hearing control group. Methods Eight CI users with EAS (2 bilaterally implanted, 6 unilaterally implanted) and symmetric binaural acoustic hearing and 24 normal hearing subjects participated in the study. The first experiment determined mean localization error (MLE) for different angles of sound incidence between ± 60˚(frontal and dorsal presentation). The stimuli were either low-pass, high-pass or broadband noise bursts. In a second experiment, just noticeable differences (JND) of ITDs were measured for pure tones of 125 Hz, 250 Hz and 500 Hz (headphone presentation). Results Experiment 1: MLE of EAS subjects was 8.5˚, 14.3˚and 14.7˚, (low-, high-pass and broadband stimuli respectively). In the control group, MLE was 1.8˚(broadband stimuli). In the differentiation between sound incidence from front and back, EAS subjects performed on chance level. Experiment 2: The JND-ITDs were 88.7 μs for 125 Hz, 48.8 μs for 250 Hz and 52.9 μs for 500 Hz (EAS subjects). Compared to the control group, JND-ITD for 125 Hz was on the same level of performance. No statistically significant correlation was found between MLE and JND-ITD in the EAS cohort. Conclusions Near to normal ITD sensitivity in the lower frequency acoustic hearing was demonstrated in a cohort of EAS users. However, in an acoustic localization task, the majority of the subjects did not reached the level of accuracy of normal hearing. Presumably, signal processing time

Hearing Asymmetry Biases Spatial Hearing in Bimodal Cochlear-Implant Users Despite Bilateral Low-Frequency Hearing Preservation

“…We hypothesize that hearing asymmetries introduce localization biases to the better hearing ear ( Figure 1b to d). This would suggest that idiosyncratic localization abilities as demonstrated in earlier studies ( Dunn et al, 2010 ; Gifford Grantham et al, 2014 ; Dorman et al, 2016 ; Körtje et al, 2020 ) with errors ranging from near-normal (10 deg) to extremely poor (70 deg) may well depend on individual hearing asymmetries. Indeed, more symmetric hearing in bimodal EAS users correlates with the ability to process ITDs ( Gifford Grantham et al, 2014 ) and localize sounds ( Loiselle et al, 2015 ).…”

“…Criteria for cochlear implantation have been expanded to allow adults with good low-frequency hearing thresholds and severe-to-profound high-frequency hearing loss in both ears to receive a single cochlear implant. The combined electric and acoustic stimulation (EAS) in the implanted ear and acoustic stimulation in the other provides these cochlear implant recipients with binaural low-frequency acoustic hearing, which has been shown to improve horizontal-plane sound localization ( Dunn et al, 2010 ; Gifford et al., 2014 ; Plant & Babic; 2016; Kortje et al, 2020 ). However, these EAS listeners effectively have monaural (electrical) hearing for high frequencies and asymmetric (bimodal) hearing for mid frequencies.…”

“…The EAS listeners with low-frequency binaural hearing ( Figure 1a , low frequency region) can potentially localize sounds in the horizontal plane using interaural time differences (ITDs; Figure 1b ( Dorman et al, 2013 ; Gifford et al., 2013 ; Gifford Graham et. al., 2014 ; Dorman et al, 2016 ; Gifford & Stecker, 2020 ; Körtje et al, 2020 ). However, typical EAS listeners will have no high-frequency hearing in the non-implanted ear ( Figure 1a , high frequencies) and so they lack access to interaural level differences (ILDs) necessary for high-frequency horizontal ( Blauert, 1996 ) sound localization ( Figure 1d ).…”

“…Furthermore, we hypothesize that, as hearing asymmetry depends on frequency ( Figure 1a ), sound localization by EAS listeners depends on the sound spectrum as well. Specifically, since bimodal EAS users with binaurally symmetric low-frequency hearing can rely on low-frequency ITDs ( Gifford Grantham et al, 2014 ; Kortje et. al., 2020 ; Figure 1a ), they should be able to localize low-frequency sounds a­ccurately ( Figure 1b ).…”

A Computational Model for Evaluating Transient Auditory Storage of Acoustic Features in Normal Listeners

“…The preservation of the sensitivity to the binaural gap even when an interaural delay was introduced indicated that acoustic temporal fine structure features of noise were maintained for the duration of the interaural interval [11,25,26] and allowed similarity computation between the binaural sound inputs. Thus, measuring the impact of interaural delay when the binaural gap is detected [27][28][29] can provide a way of investigating the transient memory of acoustic features.…”