Threshold-distance functions predict speech recognition with cochlear implants

Cochlear implantation shows unexplained outcome variability. Among the key factors affecting outcomes measured by speech understanding is the functional state of the auditory nerve and the amount of its degeneration due to hearing loss. In the present study we actively varied the distance of the stimulating electrodes from the modiolus (the spiral ganglion), quantified it using fluoroscopy and cone-beam computed tomography and related it to electrically-evoked compound action potentials (eCAPs) in human subjects. Stimulation was monopolar. The distance from modiolus could explain up to 91% of the variability in the thresholds of eCAPs. The threshold varied between participants. For individual electrodes of the implant in the given participant, a linear relationship between threshold (in current level) and distance from modiolar axis was found, with different slopes in different participants. The slopes of the eCAP thresholds to modiolus distance of the electrode across ears could explain up to 67% of the variability of speech understanding in these participants. We suggest that the slope of the threshold-distance function might serve as a marker for the functional state of the target neurons (spiral ganglion cells) and can be used to assess this in individual subjects.