Use of "phantom electrode" technique to extend the range of pitches available through a cochlear implant.

OBJECTIVE: The range of pitch sensations available in cochlear implants (CIs) is conventionally thought to be limited by the location of the most apical and basal electrodes. However, partial bipolar stimulation, in which current is distributed to two intracochlear electrodes and one extracochlear electrode, can produce "phantom electrode" (PE) pitch percepts that extend beyond the pitch range available with physical electrodes. The goals of this study were (1) to determine the PE configuration that generated the lowest pitch relative to monopolar (MP) stimulation of the most apical electrode and (2) to determine the amount of pitch shift produced by different PE configurations.
DESIGN: Ten Advanced Bionics CI users (9 unilateral and 1 bilateral), implanted with the CII or HiRes 90k implant and the HiFocus 1, HiFocus 1j, or Helix electrode arrays participated in this study. PEs were created by simultaneously stimulating the primary and compensating electrodes in opposite phase. To test different PE configurations, the proportion of current delivered to the compensating electrode (sigma) and the electrode separation between the primary and compensatory electrode (D) were varied. To estimate the relative pitch of PEs, the lowest pitched PEs with primary electrodes 4 and 8 were compared with subsets of MP electrodes (1, 2, 3, 4, 5 and 5, 6, 7, 8, 9, respectively).
RESULTS: In all subjects, it was possible to identify sigma and D values that produced a PE that was lower in pitch than the MP stimulation of the primary electrode. In some subjects, increasing sigma and/or D produced progressively lower pitch percepts, whereas in others, PE pitch changed nonmonotonically with sigma and/or D. The amount of PE pitch shift could be estimated only for 14 cases; in seven cases, the pitch shift was <1 MP electrode, and in seven other cases, the pitch shift was between 1 and 2 MP electrodes.
CONCLUSIONS: PE stimulation can elicit pitch percepts lower than that of the most apical MP electrode; the PE pitch is lower by the equivalent of 0.5 to 2 MP electrodes.