gms | German Medical Science

Introduction: For ABI stimulation there is no clearly defined tonotopic ordering of electrodes as occurs with the cochlear implant. There is only a trend from low pitches at the base of the implant array up to high pitches at the tip of the array.

Method: Between May 1996 and Feb 2010, 36 patients underwent implantation of an ABI in Hannover. Up to 2005, pitch ordering was performed simply through subjective description by the patients, which provided variable responses across testing sessions. We therefore switched to statistical based methods: pitch-scaling and pitch-ranking with repetitions. Due to the complex tonotopic organization of the cochlear nucleus, the pitch percept of a single electrode could change with current level. Thus pitch ranking was performed for two different loudness levels (comfortable and soft).

Results: A subset of significantly different electrodes and the percentage of significantly different pairs of electrodes can be calculated by the results of repeated pitch-scaling. These measures show good correlations to speech test results. For pitch-ranking a distance to the ideal performer was calculated. This measure shows good correlation to speech test results and to the measures derived from the pitch-scaling. Not in all cases electrodes could be ordered by the results of pitch-ranking – some patients preferred their “wrong” pitch ordering for understanding of speech.

The discrepancies in pitch ordering achieved through pitch-ranking with comfortable versus reduced loudnesses can be one of the reasons for the insufficient speech understanding in ABI patients. This should be considered by future speech coding strategies for ABI patients.