An objective method to measure electrode independence in cochlear implant patients with a dual-masker forward masking...

This study introduced a dual-masker forward masking technique and evaluated whether this objective method could measure electrode independency in a cochlear implant; more particularly, whether the optimal locations and number of active electrodes could be determined. This method further enabled the investigation of the efficacy of current steering, because the proposed recording method could also be described as applying a sequentially current steered masker. The paradigm requires 5 frames involving 2 maskers and 1 probe and is referred to as the Apple Core method (MP5-AC). For each recording, both the masker and probe amplitude were varied independently, producing 3-D eCAP plots that showed the eCAP amplitude for independent variations of masker and probe amplitudes. A simple quantitative model was developed to aid interpretation of the results. Theory and model were clinically tested in 14 patients. On the basis of the model, the multi-variate, color-coded plots could be subdivided into seven distinct regions, each depicting a unique relationship between the probe and the maskers. The model's predictions supported interpretation of the results, and indicated independence for the probe electrode contacts only at lower current levels and/or at greater inter-electrode separations. The clinical results revealed a lack of selectivity in the electrode array for stimulus levels larger than 600 microA. This suggests that sequential current steering is only capable of producing a single excitation area at higher current levels, or smaller electrode distances, without additional loudness correction being applied. Thus, the MP5-AC paradigm provided insight concerning the independence of electrodes and the efficacy of current steering in clinical patients. However, its current clinical applicability is limited because measurements were adequate only in anesthetized patients.