gms | German Medical Science

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.

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.

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.

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.