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Tolerable processing delay in electro-acoustic stimulation
Chalupper J. , Geissler G. , Büchner A. , Battmer R.
Advanced Bionics, European Research Center, Hannover, Germany, 2Medizinische Hochschule Hannover, Hannover, Germany,
Unfallkrankenhaus Berlin, Dept. of Otolaryngology, Berlin, Germany
Introduction: Recent studies using modern cochlear implant (CI) devices prove that the majority of subjects can
integrate electrical and acoustic stimulation either on the same or on different ears. This bimodal hearing offers
potentially complementary information that, when appropriately integrated in the brain, could allow for improved
speech perception in noise. In order to enable integration of ipsilateral electric and acoustic stimulation (EAS),
however, the delay between the electric and acoustic component may play a crucial role. While there are no
clear-cut research results available on maximum tolerable delay between electric and acoustic signals,
investigations by Stone & Moore [2003] indicate that delays up to about 10 ms are acceptable for acoustic
hearing. Depending on the devices, the difference in group delay between CIs and HAs can range from 0 ms up
to 15 ms. Another important parameter for subject acceptance and sound quality of the EAS fitting seems to be
the amount of electro-acoustic spectral overlap. In order to optimize sound processing and fitting for EAS, more
data on tolerable delays between electric and acoustic signal paths for different filterbank settings is needed.
Method: Eight CI-users with aidable residual hearing in the low frequencies on the implanted side participated in
three experiments. Experiment 1 investigated the just-noticeable delay between electric and acoustic stimulation
for five different stimuli. In Experiment 2, speech understanding in noise was measured for different delays below
and above the individual just-noticeable delay. Finally, subjects had to choose their preferred filterbank setting
(overlapping or non-overlapping) in Experiment 3. To be able to control the delay between acoustic and electric
stimulation, all stimuli were presented via direct audio input (DAI) to hearing aid (HA) and CI separately.
Results: The just noticeable delay varied substantially across individuals, but was in all cases clearly above the
values which are typical for state-of-the-art signal processing in HA and CI. Speech turned out to be the most
sensitive stimulus. Speech scores were not significantly affected by variations of the delay up to the just
noticeable threshold. For larger delays, however, speech recognition is degraded. While speech scores generally
were better with the non-overlapping filterbank, subjective preference varied highly across individuals.
Conclusion: The differences of processing delay in state-of-the-art acoustic and electric signal processing seem
to be tolerable for ipsilateral electro-acoustic hearing. However, patients with residual acoustic hearing in mid
and high frequencies may be more sensitive to delay differences than the group in this study.
Stone MA, Moore BC, 2003: Tolerable hearing aid delays. III. Effects on speech production and perception of
across-frequency variation in delay. Ear Hear 2003; 24: 175-183.