gms | German Medical Science

Fourth International Symposium and Workshops: Objective Measures in Cochlear Implants

Medical University of Hannover

01.06. bis 04.06.2005, Hannover

The eCAP Exhibits Level-Dependent Non-Linearities

Meeting Abstract

  • corresponding author A. Westen - ENT department, Leiden University Medical Centre, Leiden, The Netherlands
  • R. van den Hooff - ENT department, Leiden University Medical Centre, Leiden, The Netherlands
  • J.J. Briaire - ENT department, Leiden University Medical Centre, Leiden, The Netherlands
  • J.H.M. Frijns - ENT department, Leiden University Medical Centre, Leiden, The Netherlands

Medical University of Hannover, Department of Otolaryngology. Fourth International Symposium and Workshops: Objective Measures in Cochlear Implants. Hannover, 01.-04.06.2005. Düsseldorf, Köln: German Medical Science; 2005. Doc05omci094

Die elektronische Version dieses Artikels ist vollständig und ist verfügbar unter: http://www.egms.de/de/meetings/omci2005/05omci094.shtml

Veröffentlicht: 31. Mai 2005

© 2005 Westen et al.
Dieser Artikel ist ein Open Access-Artikel und steht unter den Creative Commons Lizenzbedingungen (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.de). Er darf vervielf&aauml;ltigt, verbreitet und &oauml;ffentlich zug&aauml;nglich gemacht werden, vorausgesetzt dass Autor und Quelle genannt werden.


Gliederung

Text

Introduction

The eCAP amplitude is commonly interpreted as a measure of the number of excited nerve fibres. Decreasing amplitudes at high current strengths, usually ascribed to saturation effects and stimulus artefacts, are the subject of the present study.

Materials and Methods

eCAP responses were recorded in five guinea pigs implanted with the HiFocus electrode, using forward masking with independently varying masker and probe amplitudes. Experimental results were compared with our computational model of the implanted cochlea, calculating the eCAP from simulated single fibre contributions.

Results

A non-monotonous eCAP I/O-curve is found at high stimulus levels if the probe amplitude is either co-varied with the masker amplitude or varied alone. This is also found in the model. In line with the expectations, however, the model depicts a monotonously growing number of excited nerve fibres. Interestingly, at high levels the fibres located centrally in the excitation area yield an atypical response, without a clear negative peak in the single fibre action potential. The total number of excited fibres increases monotonously, but the number of fibres with atypical responses also increases (and often more rapidly) with stimulus level. Therefore, the overall N1P1 eCAP amplitude decreases above a certain stimulus level. Although our human cochlear model also predicts such an effect, it was not recognized consistently in actual human NRI recordings.

Conclusions

The observed level-dependent non-linearities of the eCAP I/O-curve are caused by atypical contributions of fibres close to the stimulating electrode. This has implications for the interpretation of such curves.