Similar performance in sound localisation with unsynchronised and synchronised automatic gain controls in bilateral cochlear implant recipients

Abstract

Objective

One proposed method to improve sound localisation for bilateral cochlear implant (BiCI) users is to synchronise the automatic gain control (AGC) of both audio processors. In this study we tested whether AGC synchronisation in a dual-loop front-end processing scheme with a 3:1 compression ratio improves sound localisation acuity.

Design

Source identification in the frontal hemifield was tested in in an anechoic chamber as a function of (roving) presentation level. Three different methods of AGC synchronisation were compared to the standard unsynchronised approach. Both root mean square error (RMSE) and signed bias were calculated to evaluate sound localisation in the horizontal plane.

Study sample

Six BiCI users.

Results

None of the three AGC synchronisation methods yielded significant improvements in either localisation error or bias, neither across presentation levels nor for individual presentation levels. For synchronised AGC, the pooled mean (standard deviation) localisation error of the three synchronisation methods was 24.7 (5.8) degrees RMSE, for unsynchronised AGC it was 27.4 (7.5) degrees. The localisation bias was 5.1 (5.5) degrees for synchronised AGC and 5.0 (3.8) for unsynchronised.

Conclusions

These findings do not support the hypothesis that the tested AGC synchronisation configurations improves localisation acuity in bilateral users of MED-EL cochlear implants.

Keywords:

• Bilateral cochlear implantation
• automatic gain control
• sound localisation
• signal processing
• spatial hearing
• interaural level control

Acknowledgements

We express our gratitude to all participants in this study. Manuscript writing assistance was provided by P. Connolly (MED-EL).

Ethical approval

At the time when these studies were conducted (mid 2002), the 5^th Revision of the Declaration of Helsinki was in effect (WMA (World Medical Association) 2000). In that revision, submission of a study protocol to an ethical review committee was not a requirement. All aspects of the study were conducted in accord with the principles of the 5^th revision, and informed consent was obtained from all study participants prior to enrolment in the study.

Disclosure statement

All authors are employees of MED-EL Medical Electronics, Innsbruck, Austria.