Effects of electrode location and spacing on phoneme recognition with the nucleus-22 cochlear implant

Objective: The objective of this paper was to determine how phoneme identification was affected by the cochlear location and spacing of the electrodes in cochlear implant listeners. Design: Subjects were initially programmed with the full complement of 20 active electrodes, in which each electrode was assigned to represent the output of one filter in the normal SPEAK processor. In the present study several four-electrode processors were constructed by assigning the output of more than one filter to a single electrode. In all conditions speech sounds were still analyzed into 20 frequency bands and processed according to the usual SPEAK processing strategy, but the location and spacing of the four stimulated electrode pairs were varied systematically. In Experiment I, the spacing between stimulated electrodes was fixed at 3.75 mm and the cochlear location of the four electrode pairs was shifted from the most-apical position up to 3.0 mm toward the base in 0.75 mm steps. In Experiment II, the spatial separation between the four electrode pairs (each bipolar-plus-one) was systematically changed from 1.5 mm to 4.5 mm while holding the most apical active electrode fixed. In Experiment III, the spacing of active electrodes was varied to represent equal tonotopic spacing to equal linear frequency intervals between pairs. Recognition of medial vowels and consonants was measured in three subjects with these custom four-electrode speech processors. Results: In Experiment I, results showed that both vowel and consonant recognition were best when the electrodes were in the most apical locations. In Experiment II, best speech recognition occurred when electrode pairs were separated by 3 to 3.75 mm. In Experiment III, both vowel and consonant recognition scores decreased when the spacing of electrode pairs was changed from equal tonotopic spacing to equal linear frequency intervals. Overall, vowel and consonant recognition were best at the most apical electrode locations and when the spacing of electrodes matched the frequency intervals of the analysis filters. Consonant recognition was relatively robust to alterations in electrode location and spacing. The best vowel scores with four-electrode speech processors were about 10 percentage lower than scores obtained with the full 20-electrode speech processors. However, the best consonant scores with four-electrode speech processors were similar to those obtained with the full 20-electrode speech processors. Information transmission analysis revealed that temporal envelope cues (voicing and manner) were not strongly affected by changes in electrode location and spacing, whereas spectral cues, as represented by vowel recognition and consonantal place of articulation, were strongly affected. Both spectral and temporal phoneme cues were strongly affected by the degree of tonotopic warping, created by altering both the location and spacing of the activated electrodes. Conclusion: The cochlear location and spacing of the activated electrodes had a clear effect on phoneme recognition. Temporal cues were less affected by tonotopic shifts or linear tonotopic stretching or shrinking, but were susceptible to nonlinear tonotopic warping. Spectral cues were sensitive to all tonotopic manipulations: shifting, linear stretching, and nonlinear warping. However, the present experiments could not differentiate whether the optimal mapping between analysis frequency bands and stimulation electrodes was determined by the normal acoustic tonotopic pattern or by the pattern learned from experience with the 20-electrode implant.