Electrode Position, Channel Interaction, and Speech Perception in Cochlear Implant Listeners

Abstract

Cochlear implants (CI) are surgically implanted devices that provide sound input to individuals with severe-to-profound hearing loss. While relatively successful, speech understanding abilities are highly variable among implant listeners in both quiet and noisy environments. One source of this variability is the electrode-neuron interface, which refers to how well each electrode activates target auditory neurons. Poor electrode-neuron interfaces may increase channel interaction, which can distort spectral information and result in decreased speech understanding. Electrode position, bone and tissue growth, and the integrity of the auditory neurons are important factors that affect the electrode-neuron interface. Currently, it is not possible to directly measure neural integrity in CI listeners; therefore, obtaining information about electrode position is an alternative approach to assessing the electrode-neuron interface. Information about electrode position is available through computerized tomography (CT) imaging. However, CT imaging is not readily available to audiologists, thus limiting its usefulness in a clinical setting. The purpose of this dissertation work was to examine the relationship between CT-estimated electrode position and two measures of channel interaction: an objective measure, the electrically evoked compound action potential (Experiment 1) and a behavioral measure, the psychophysical tuning curve (Experiment 2). In Experiment 3, we created novel listener strategies based on channels with poor electrode position and excessive channel interaction (based on measures from Experiment 2) in an effort to improve spectral resolution and speech understanding. Results from Experiment 1 and 2 demonstrated that the electrically evoked compound action potential and psychophysical tuning curves, two measures of channel interaction, are correlated with CT-estimates of electrode position. In other words, those electrodes farther from the inner wall of the cochlea tended to exhibit excessive channel interaction with these measures. Results from Experiment 3 suggest that CI listeners can gain improved speech understanding when manipulating channels to compensate for poor electrode position or excessive channel interaction. Importantly, manipulating the “wrong” channels can lead to a detriment in performance. This dissertation work lays the groundwork for assessment of an important aspect of the electrode-neuron interface and the practical application of these measures to CI listener strategies.