The Greenwood Function shows close alignment with Pitch Perceived by Cochlear Implant Patients with Long, Flexible Electrode Arrays and Fine-Structure Stimulation

Andreas Büchner^1,2Tobias Weller^1,2Richard Penninger³Luke Helpard³Hanif M Ladak^4,5,6Sumit Agrawal^4,5,6Thomas Lenarz^1,2Daniel Schurzig^1,3*

• ¹Department of Otolaryngology, Head and Neck Surgery, Hannover Medical School, Hannover, Germany
• ²German Hearing Center (DHZ), Hannover, Germany
• ³MED-EL (Austria), Innsbruck, Tyrol, Austria
• ⁴Department of Otolaryngology-Head and Neck Surgery, Western University, London, ON, Canada
• ⁵Department of Medical Biophysics, Western University, London, ON, Canada
• ⁶Department of Electrical and Computer Engineering, Western University, London, ON, Canada

Introduction: The natural, tonotopic frequency distribution of the inner ear is typically described by the Greenwood function. Recent developments in cochlear implant (CI) programming aim to improve sound quality and music perception through consideration of the Greenwood frequency distribution when assigning frequency bands to the individual electrode contacts. This approach is commonly referred to as anatomy-based fitting (ABF). However, empirical validation of the Greenwood function to accurately describe pitch as perceived by CI users is lacking. Methods: Twelve CI patients with single-sided deafness (SSD) participated in the study. The cochlear anatomy of each participant was reconstructed based on clinical imaging to derive the location of the stimulating contacts relative to the basilar membrane, allowing for the comparison of the pitch perceived by participants to the frequency suggested by the Greenwood function. A pitch matching task was conducted at four different appointments and with two different fitting maps (standard and ABF). Participants were asked to set the frequency of a pure tone presented through a loudspeaker to the pitch perceived when stimulated with the single contacts of the CI electrode array. The cochlear anatomy of the patients was reconstructed based on clinical imaging to derive the location of the stimulating contacts relative to the basilar membrane, allowing for the comparison of the pitch perceived by the patients to the frequency suggested by the Greenwood function for each stimulating contact. Results: In general, subjective pitch percepts were found to agree well with the frequency suggested by the Greenwood function independent of subject, contact, or applied fitting map. Differences between pitch matches and Greenwood were found to be not statistically significant. At least part of the outcomes of previous studies reporting a basal frequency shift can be explained by the tonotopic mapping functions applied within these studies. Discussion: The present results suggest that the Greenwood function is well-suited for representing the tonotopic frequency distribution not only for normal hearing subjects but for CI recipients as well. Further advances in frequency mapping should also take the neural health of the cochlea into account, allowing for additional individualization of frequency mapping in CIs.