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

Monika Körtje,Tobias Weissgerber,Uwe Baumann,Timo Stöver

doi:10.1371/journal.pone.0241015

Monika Körtje, Tobias Weissgerber + Show 2 more

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https://doi.org/10.1371/journal.pone.0241015

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Abstract

ObjectivesIn 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.MethodsEight 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).ResultsExperiment 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.ConclusionsNear 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 delay differences between devices used on both sides are deteriorating the transfer of precise binaural timing cues.

Highlights

Accurate localization of sound incidence is of utmost importance to prevent potential dangerous situations in everyday life
No statistically significant correlation was found between mean localization error (MLE) and just noticeable differences (JND)-Interaural time differences (ITDs) in the electric-acoustic stimulation (EAS) cohort
Near to normal ITD sensitivity in the lower frequency acoustic hearing was demonstrated in a cohort of EAS users

Summary

Introduction

Accurate localization of sound incidence is of utmost importance to prevent potential dangerous situations in everyday life (e.g. traffic situations). Acoustic localization supports auditory scene analysis and improves speech perception e.g. in noisy environments. Interaural time differences (ITDs) and interaural level differences (ILDs) are the most relevant cues for the localization of sound sources. Normal hearing persons (NH) are most sensitive for ILD cues in higher frequencies above 1.5 kHz, whereas ITDs are relevant for sound localization of signals with lower frequencies, explored by e.g. Blauert in 1997 [1]. ITD cues derived by analysis of the envelope of high frequency sounds are involved in localization, but with less relevance [2, 3]. Sensitivities of ITD and ILD are highly correlated and a complex interaction between both neuronal identification networks can be assumed, even when hearing sensitivity is deteriorated as in persons with hearing loss [4,5,6]

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