Exploring the Use of Interleaved Stimuli to Measure Cochlear-Implant Excitation Patterns

• Published in: Journal of the Association for Research in Otolaryngology 2024-04, Vol.25 (2), p.201-213
• Main Authors: Guérit, François, Middlebrooks, John C., Gransier, Robin, Richardson, Matthew L., Wouters, Jan, Carlyon, Robert P.
• Format: Article
• Language: English
• Subjects: Cochlea; Medicine; Medicine & Public Health; Neurobiology; Neurosciences; Original; Original Article; Otorhinolaryngology; Psychophysics; Transplants & implants
• ISSN: 1438-7573; 1525-3961; 1438-7573
• DOI: 10.1007/s10162-024-00937-2

Purpose Attempts to use current-focussing strategies with cochlear implants (CI) to reduce neural spread-of-excitation have met with only mixed success in human studies, in contrast to promising results in animal studies. Although this discrepancy could stem from between-species anatomical and aetiological differences, the masking experiments used in human studies may be insufficiently sensitive to differences in excitation-pattern width. Methods We used an interleaved-masking method to measure psychophysical excitation patterns in seven participants with four masker stimulation configurations: monopolar (MP), partial tripolar (pTP), a wider partial tripolar (pTP + 2), and, importantly, a condition (RP + 2) designed to produce a broader excitation pattern than MP. The probe was always in partial-tripolar configuration. Results We found a significant effect of stimulation configuration on both the amount of on-site masking (mask and probe on same electrode; an indirect indicator of sharpness) and the difference between off-site and on-site masking. Differences were driven solely by RP + 2 producing a broader excitation pattern than the other configurations, whereas monopolar and the two current-focussing configurations did not statistically differ from each other. Conclusion A method that is sensitive enough to reveal a modest broadening in RP + 2 showed no evidence for sharpening with focussed stimulation. We also showed that although voltage recordings from the implant accurately predicted a broadening of the psychophysical excitation patterns with RP + 2, they wrongly predicted a strong sharpening with pTP + 2. We additionally argue, based on our recent research, that the interleaved-masking method can usefully be applied to non-human species and objective measures of CI excitation patterns.