A Frequency-Position Function for the Human Cochlear Spiral Ganglion

TLDR

Greenwood’s frequency‑position function is widely used to map cochlear‑implant electrodes to cochlear place, but because modern implants target spiral‑ganglion cells near the modiolus and the ganglion’s extent differs from the organ of Corti, a mismatch may exist between the organ‑of‑Corti and spiral‑ganglion frequency maps. The authors examined seven human cadaveric cochleae, stained with osmium, measured organ‑of‑Corti and spiral‑ganglion lengths, and traced radial fiber trajectories to locate frequency‑matched points on each structure. They derived a cubic function that tightly correlates organ‑of‑Corti frequency to spiral‑ganglion position, offering a neural frequency map that could improve electrode design, channel‑filter mapping, music perception, and safe insertion distances for individual patients.

Abstract

Greenwood’s frequency-position function for the organ of Corti (OC) is commonly used to estimate represented frequencies for cochlear implant (CI) electrodes, both in temporal bone studies and in imaging studies of living CI recipients. However, many contemporary CIs position stimulating electrodes near the modiolus, directly targeting the spiral ganglion (SG) cells within Rosenthal’s canal. At the extreme base and apex, the SG does not extend as far as the OC, and the radial nerve fibers take a tangential course into the modiolus resulting in a potential offset between the frequency maps of the OC and SG. In this investigation, human cadaveric cochleae (n = 7) were studied in surface preparations after osmium staining. The OC and SG lengths were measured and radial fiber trajectories traced to identify frequency-matched points on each structure. These data allowed derivation of a mathematical function correlating represented frequency along the OC to position along the SG. A cubic function fit the data with a very high intersubject correlation. Better knowledge of the human SG ‘neural frequency map’ may help to refine electrode design, and to more accurately map CI channel filter bands to the appropriate cochlear place along the SG, which may be advantageous for more sophisticated CI outcomes, such as music appreciation. These data also could be valuable for electroacoustic stimulation, by defining the insertion distance of a CI electrode required to reach specific frequencies (based upon preoperative imaging) in an individual subject, thus helping to avoid trauma to cochlear regions with residual hearing.