Effects of electrode configuration and frequency allocation on vowel recognition with the Nucleus-22 cochlear implant

This study was conducted to understand vowel recognition in cochlear implants as a function of the cochlear location and separation of the stimulated electrode pairs and as a function of the matching between speech spectral information and the location of the stimulated electrodes.

Four-electrode speech processors with a continuous interleaved sampling speech processing strategy were implemented through a custom interface in five subjects implanted with the Nucleus-22 cochlear implant. The temporal envelopes from four broad frequency bands were used to modulate 500 pps, 100 microsec/phase interleaved pulse trains delivered to four electrode pairs. Ten different frequency allocations and five sets of four-electrode configurations were tested. Each frequency allocation represented the same cochlear extent but different cochlear locations based on Greenwood's frequency-to-place formula. Recognition of multi-talker medial vowels was measured for each combination of parameters with no period of practice or adjustment.

Results showed that recognition of multi-talker vowels was highly dependent on frequency allocation for all electrode configurations. For a given electrode configuration maximum vowel recognition was observed with a specific frequency allocation. When the stimulated electrodes were shifted basally by 3 mm, the frequency allocation that produced the best performance also shifted basally by 3 mm. A similar pattern of vowel recognition was observed as a function of frequency allocation for electrode configurations that had the same apical-most electrode in each pair, regardless of location of the basal-most electrode in the pair. Subjects with different electrode insertion depths had similar trends in vowel recognition for each frequency allocation.

For a given electrode configuration, the best performance was obtained with processors with a specific frequency allocation. In addition, the apical-most member of each electrode pair had a much stronger influence on vowel recognition in electric hearing. Finally, results from this study also suggest that over time, patients with implants can partially adapt to a basal shift in place of stimulation.