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 identif ication 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 a ctive electrodes, in which each electrode was assigned to represent the out put of one filter in the normal SPEAK processor. In the present study sever al four-electrode processors were constructed by assigning the output of mo re than one filter to a single electrode. In all conditions speech sounds w ere still analyzed into 20 frequency bands and processed according to the u sual SPEAK processing strategy, but the location and spacing of the four st imulated electrode pairs were varied systematically. In Experiment I, the s pacing between stimulated electrodes was fixed at 3.75 mm and the cochlear location of the four electrode pairs was shifted from the most-apical posit ion up to 3.0 mm toward the base in 0.75 mm steps. In Experiment II, the sp atial separation between the four electrode pairs (each bipolar-plus-one) w as systematically changed from 1.5 mm to 4.5 mm while holding the most apic al active electrode fixed. In Experiment III, the spacing of active electro des was varied to represent equal tonotopic spacing to equal linear frequen cy intervals between pairs. Recognition of medial vowels and consonants was measured in three subjects with these custom four-electrode speech process ors. Results: In Experiment I, results showed that both vowel and consonant reco gnition 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 reco gnition scores decreased when the spacing of electrode pairs was changed fr om equal tonotopic spacing to equal linear frequency intervals. Overall, vo wel and consonant recognition were best at the most apical electrode locati ons and when the spacing of electrodes matched the frequency intervals of t he analysis filters. Consonant recognition was relatively robust to alterat ions in electrode location and spacing. The best vowel scores with four-ele ctrode speech processors were about 10 percentage lower than scores obtaine d with the full 20-electrode speech processors. However, the best consonant scores with four-electrode speech processors were similar to those obtaine d with the full 20-electrode speech processors. Information transmission an alysis revealed that temporal envelope cues (voicing and manner) were not s trongly affected by changes in electrode location and spacing, whereas spec tral cues, as represented by vowel recognition and consonantal place of art iculation, were strongly affected. Both spectral and temporal phoneme cues were strongly affected by the degree of tonotopic warping, created by alter ing both the location and spacing of the activated electrodes. Conclusion: The cochlear location and spacing of the activated electrodes h ad a clear effect on phoneme recognition. Temporal cues were less affected by tonotopic shifts or linear tonotopic stretching or shrinking, but were s usceptible to nonlinear tonotopic warping. Spectral cues were sensitive to all tonotopic manipulations: shifting, linear stretching, and nonlinear war ping. 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.