Using Musical Pitch Interval Comparisons to Assess Cochlear Implant Frequency-to-Place Maps

Rebecca M Lewis, AuD, PhD ¹ Melanie L Gilbert ^1* Jordan A. Beim ² Andrew J Oxenham ² Charles J. Limb ¹

• ¹ University of California, San Francisco, San Francisco, United States
• ² University of Minnesota Health Sciences, University of Minnesota Medical Center, Minneapolis, Minnesota, United States

Introduction: Music perception remains challenging for many cochlear implant (CI) recipients, due in part to a frequency mismatch between the original tonotopic cochlear map and the allocation of frequencies along the electrode array during programming. Individual differences in ear anatomy, electrode array length, and surgical insertion can lead to variability in the positions of electrodes within the cochlea, but these differences are not typically accounted for by current CI programming techniques.Objectives: Flat panel computed tomography (FPCT) can be used to visualize electrode locations and calculate corresponding spiral ganglion characteristic frequencies. Such FPCT-based CI frequency mapping may improve pitch perception accuracy, and thus music appreciation and speech perception. The present study seeks to develop a behavioral assessment metric for how well place-based pitch is represented across the frequency spectrum by evaluating how accurately listeners perceive and compare pitch intervals across different frequency regions.Methods: The study included two groups: normal-hearing (NH) listeners and CI recipients. Listeners were asked to match the pitch interval created by two tones, played sequentially, across different frequency ranges to estimate how evenly pitch is distributed across the CI array. This test was initially evaluated with pure tones in NH listeners, using both unprocessed and vocoder-processed sounds to simulate matched and mismatched frequency-to-place maps. We hypothesized that vocoded stimuli would be more difficult to match than unprocessed stimuli, and that a warped map (as may occur with current clinical maps) would produce poorer matches than a veridical and well-aligned map (as may be achieved using FPCT-based frequency allocation).Results: Preliminary results suggest the task can reveal differences between veridical and warped maps in NH listeners under vocoded conditions. A small cohort of CI recipients were tested with the same pure tone stimuli (without vocoding). CI recipient performance was similar to that of NH listeners with default vocoded stimuli, and both groups showed less accurate interval matching compared to NH listeners tested under matched-frequency conditions.Discussion: The results suggest promise for this method when comparing perceptual effects on pitch interval perception of traditional clinical maps and FPCT-based frequency allocation.