ANALYSES OF SPEECH PROCESSING STRATEGIES FOR COCHLEAR IMPLANTS AND THE EFFECTS OF ELECTRODE INTERACTION

Author: Stickney, Ginger
Advisor: Philip Loizou
URL: http://www.utdallas.edu/~loizou/thesis/ginger_phd_thesis.pdf

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http://www.utdallas.edu/~loizou/thesis/ginger_phd_thesis.pdf
Completion Date: May 2001
Degree: Ph.D.
Institution: University of Texas at Dallas
Abstract: Multichannel cochlear implants electrically stimulate the auditory nerve to restore partial hearing to the profoundly deaf patient. The multichannel implant was designed to selectively stimulate discrete populations of spiral ganglion cells along the length of the cochlea. However, selective stimulation is not often, or at least imperfectly, achieved even with the most modern cochlear implant designs and speech processing strategies. When multiple electrodes are stimulated simultaneously, electrical fields generated around each electrode can interact with the electrical fields of neighboring electrodes, thereby reducing selectivity. Several studies have suggested that electrical-field interactions can disrupt the acoustic properties of the signal and severely degrade speech intelligibility, however this relationship has not been directly tested. Electrical-field interactions can be reduced by decreasing the current levels delivered to each electrode through improved electrode positioning and design, or by using speech processing strategies that maximize the separation between simultaneously stimulated electrodes or stimulate the electrodes sequentially. The proximity of the cochlear implant electrode array to the modiolus has been shown to reduce the amount of current required to reach threshold (Rebscher et al., 1994). When less current is required, current spread and electrical field overlap is reduced. Recently, cochlear implant manufacturers have taken interest in designing “positioners” which place the electrode array in close proximity to the spiral ganglion cells and new electrode arrays which attempt to direct their current toward spiral ganglion cell bodies. The following experiments examine electrical-field interactions and speech recognition performance for three electrode designs: patients implanted with the Enhanced Bipolar Clarion electrode array without a “positioner”, patients implanted with the Clarion Electrode Positioning SystemTM (EPS) and the Enhanced Bipolar electrode array, and patients with the EPS and the Clarion Hi-FocusTM electrode array. A simultaneous masking task was used to measure electrical-field interactions as a function of electrode separation for monopolar and bipolar configurations. The relationship between electrical-field interaction and speech recognition was also examined for several speech strategies varying in the number of electrodes stimulated simultaneously. Subjects identified consonants, vowels, and sentences with each of the following speech strategies, listed in order from sequential stimulation to fully simultaneous stimulation: Continuous Interleaved Sampler (CIS), Paired Pulsatile Sampler (PPS), Quadruple Pulsatile Sampler (QPS), Hybrid Analog Pulsatile (HAPs), and Simultaneous Analog Stimulation (SAS). Based on previous research, susceptibility to electrical-field interactions is expected to vary as a function of electrode design, the speech processing strategy used in the device, and factors specific to each patient. The contribution from each of these variables was investigated. The results showed a moderate to strong negative correlation between electrical-field interaction and speech recognition performance, which indicates that patients with lower levels of electrical-field interaction have higher speech recognition scores than patients with high levels of electrical-field interaction. In addition, patients with strong susceptibility to electrical-field interactions produced higher speech recognition scores for sequential than simultaneous speech strategies. An information analysis revealed that vowel recognition and consonant place-of-articulation were most affected by electrical-field interactions, demonstrating that electrode interactions severely disrupt spectral cues. The pattern of results also suggests that, with acute listening trials, patients achieve the highest speech recognition scores with the speech processing strategy most similar to their own. Future studies are needed to determine if patients with minimal levels of electrical-field interaction can benefit from the partially-simultaneous QPS or HAPs strategies with more listening exposure.