Implications of Deep Electrode Insertion on Cochlear Implant Fitting

Abstract

Using long Med-El Combi40+ electrode arrays, it is now possible to cover the whole range of the cochlea, up to about two turns. Such insertion depths have received little attention. To evaluate the contribution of deeply inserted electrodes, five Med-El cochlear implant users were tested on vowel and consonant identification tests with fittings with first one, two, and up to five apical electrodes being deactivated. In addition, subjects performed pitch-ranking experiments, using loudness-balanced stimuli, to identify electrodes creating pitch confusions. Radiographs were taken to measure each electrode insertion depth. All subjects used each modified fitting for two periods of about 3 weeks. During the experiment, the same stimulation rate and frequency range were maintained across all the fittings used for each individual subject. After each trial period the subject had to perform three consonant and three vowel identification tests. All subjects showed deep electrode insertions ranging from 605-to 720-. The two subjects with the deepest electrode insertions showed significantly increased vowel-and consonant-identification performances with fittings with the two or three most apical electrodes deactivated compared to their standard fitting with all available electrodes activated. The other three subjects did not show significant improvements in performance when one or two of their most apical electrodes were deactivated. Four out of five subjects preferred to continue use of a fitting with one or more apical electrodes deactivated. The two subjects with the deepest insertions also showed pitch confusions between their most apical electrodes. Two possible reasons for these results are discussed. One is to reduce neural interactions related to electrodes producing pitch confusions. Another is to improve the alignment of the frequency components of sounds coded by the electrical signals delivered to each electrode to the overall pitch of the auditory perception produced by the electrical stimulation of auditory nerve fibers.

Forward-masked spatial tuning curves in cochlear implant users

“…Some of the factors that limit spatial resolution by cochlear implant users, notably those associated with the speech processor and electrode array, can be mitigated by improvements in technology. For example, deeper insertion of electrode arrays allows for less compression of the basic frequency-to-place map Shannon, 2003, 2005;Gani et al, 2007). Similarly, the implementation of current steering in speech processing strategies can potentially reduce limitations related to fixed electrode locations by allowing current peaks to be "steered" to varying locations between pairs of adjacent electrodes (McDermott and McKay, 1994;Busby and Plant, 2005;Donaldson et al, 2005;Kwon and van den Honert, 2006;Firszt et al, 2007;Koch et al, 2007).…”

Considerations for design of future cochlear implant electrode arrays: Electrode array stiffness, size,

“…This compression may make achieving spatial selectivity between apical stimulation channels increasingly difficult. Thus, increased channel interaction may explain, at least in part, the results of psychophysical studies that indicate that the performance increase achieved with very deeply inserted electrodes (up to 720°) is minimal for many subjects [40][41]. In addition, temporal bone studies using deeply inserted electrodes of the same design resulted in significantly increased trauma compared with moderate depth insertions [25].…”

Psychophysical assessment of stimulation sites in auditory prosthesis electrode arrays

“…Seven of the nine subjects tested showed improvements on at least one of four speech-recognition tests when switched to the reduced-site maps. Several recent studies have also shown that removing some sites from the processor map can result in improved speech recognition, although the effects depend on the subjects and on what other conditions are changed (Arnoldner et al, 2007;Finley et al, 2007;Gani et al, 2007). In the remainder of this section, we will discuss additional data and considerations relevant to a more general development of the site-selection strategy.…”