The Relationship Between Insertion Angles, Default Frequency Allocations, and Spiral Ganglion Place Pitch in Cochlear Implants

Abstract

Objectives Commercially available cochlear implant systems attempt to deliver frequency information going down to a few hundred Hz, but the electrode arrays are not designed to reach the most apical regions of the cochlea which correspond to these low frequencies. This may cause a mismatch between the frequencies presented by a cochlear implant electrode array and the frequencies represented at the corresponding location in a normal hearing cochlea. In the following study, the mismatch between the frequency presented at a given cochlear angle and the frequency expected by an acoustic hearing ear at the corresponding angle is examined for the cochlear implant systems that are most commonly used in the United States. Design The angular insertion of each of the electrodes on four different electrode arrays (MED-EL Standard, MED-EL Flex28, Advanced Bionics HiFocus 1J, and Cochlear Contour Advance) was estimated from x-rays. For the angular location of each electrode on each electrode array, the predicted spiral ganglion frequency was estimated. The predicted spiral ganglion frequency was compared with the center frequency provided by the corresponding electrode using the manufacturer’s default frequency-to-electrode allocation. Results Differences across devices were observed for the place of stimulation for frequencies below 650 Hz. Longer electrode arrays (i.e. the MED-EL Standard and Flex28) demonstrated smaller deviations from the spiral ganglion map than the other electrode arrays. For insertion angles up to approximately 270°, the frequencies presented at a given location were typically approximately an octave below what would be expected by a spiral ganglion frequency map, while the deviations were larger for angles deeper than 270°. For frequencies above 650 Hz, the frequency to angle relationship was consistent across all four electrode models. Conclusions A mismatch was observed between the predicted frequency and default frequency provided by every electrode on all electrode arrays. The mismatch can be reduced by changing the default frequency allocations, inserting electrodes deeper into the cochlea, or allowing cochlear implant users to adapt to the mismatch. Further studies are required to fully assess the clinical significance of the frequency mismatch.

Comparison of Place-versus-Pitch Mismatch between a Perimodiolar and Lateral Wall Cochlear Implant Electrode Array in Patients with Single-Sided Deafness and a Cochlear Implant

“…The large variation in insertion depth was also reported by Landsberger et al [2015]. In contrast, they found a median insertion angle of the most apical electrode con-Audiol Neurotol 2019;24:38-48 DOI: 10.1159/000499154 tact of the Contour Advance electrode array of 375°, which is deeper than the median insertion angle of 307.3° in our study.…”

“…As the electrical stimulus activates spiral ganglion cells and not the organ of Corti, the classical place-pitch relation [Greenwood, 1990] has been amended for CI stimulation [Dorman et al, 2007;Stakhovskaya et al, 2007;Landsberger et al, 2015]. In pitch matching experiments in SSD patients implanted with a CI, the perceived pitch is 1 to 2 octaves lower than the pitch predicted by the amended place-pitch relation [Boëx et al, 2006;Dorman et al, 2007;Baumann and Nobbe, 2006;Schatzer et al, 2014].…”

Evidence for a neural source of the precedence effect in sound localization

“…In the case of CI subjects, although the cochlear place of stimulation for "apical" electrode pairs (given in Table 1) is unknown, a typical cochlear array insertion depth of 20 mm would place the apical-most electrode at a roughly ϳ1,000-Hz cochlear location (Stakhovskaya et al 2007; see also Landsberger et al 2015). Basal electrodes stimulate significantly higher-frequency locations.…”

Comparison of the HiFocus Mid-Scala and HiFocus 1J Electrode Array: Angular Insertion Depths and Speech Perception Outcomes

“…It was shown in earlier studies that deeply inserted electrode arrays result in increased intracochlear trauma and the loss of residual hearing , limited stimulation of the basal frequencies [Finley and Skinner, 2008] and confusion of the apical pitch due to neural interaction of the more densely located neural fibers in the apical regions in the cochlea [Gani et al, 2007]. On the other hand, other studies show that stimulation of the lower frequencies in the apical region of the cochlea are related to better speech perception outcomes [Hochmair et al, 2003;Yukawa et al, 2004], increased range of place pitches by stimulating a larger number of neural fibers [Landsberger et al, 2014], or reduced mismatch between the predicted and default frequencies by a further approximation of the normal frequency to place function of the cochlea [Landsberger et al, 2015]. Another study also showed that deeper inserted electrode arrays are not associated with scalar translocations [Wanna et al, 2014].…”