Spatial channel interactions in cochlear implants

Abstract

The modern multi-channel cochlear implant is widely considered to be the most successful neural prosthesis for its ability to restore partial hearing to post-lingually deafened adults and to allow essentially normal language development in pre-lingually deafened children. However, the implant performance varies greatly in individuals and is still limited in background noise, tonal language understanding, and music perception. One main cause for the individual variability and the limited performance in cochlear implants is spatial channel interaction from the stimulating electrodes to the auditory nerve and brain. Here we systematically examined spatial channel interactions at the physical, physiological, and perceptual levels in the same 5 modern cochlear implant subjects. The physical interaction was examined using an electric field imaging technique, which measured voltage distribution as a function of electrode position in the cochlea in response to stimulation of a single electrode. The physiological interaction was examined by recording electrically evoked compound action potentials as a function of electrode position in response to stimulation of the same single electrode position. The perceptual interactions were characterized by changes in detection threshold as well as loudness summation in response to in-phase or out-of-phase dual-electrode stimulation. To minimize potentially confounding effects of temporal factors on spatial channel interactions, stimulus rates were limited to 100 Hz or lower in all measures. Several quantitative channel interaction indexes were developed to define and compare the width, slope, and symmetry of the spatial excitation patterns derived from these physical, physiological, and perceptual measures. The electric field imaging data revealed a broad but uniformly asymmetrical intracochlear electric field pattern, with the apical side producing wider half-width and shallower slope than the basal side. On the contrary, the evoked compound action potential and perceptual channel interaction data showed much greater individual variability. It is likely that actual reduction in neural and higher level interactions, instead of simple sharpening of electric current field, would be the key to predict and hopefully improve the variable cochlear implant performance. The present results are obtained with auditory prostheses but can be applied to other neural prostheses, in which independent spatial channels, rather than high stimulation rate, are critical to their performance.

The effect of the resistive properties of bone on neural excitation and electric fields in cochlear implant models

“…Predicted data is compared to the length constant (3.15 mm) calculated from EFI measurements by Tang et al (2011). The bone resistivity value required to match data from literature is around 6500 Ohm cm for all three head configurations indicating that potential decay inside the scala is largely unaffected by current paths to the return electrode.…”

“…The electric field data from literature came from a study by Tang et al (2011) where electrical field imaging (EFI) data of five implanted cochleae are presented. An EFI curve represents the voltages measured on all the electrodes of an implanted array when a stimulus is presented through a single electrode.…”

“…The averaged EFI profiles of these electrodes in all the ears were compared to the modelled data in the present study. All the ears in the Tang et al (2011) study were implanted with Clarion HiFocus electrode arrays; subsequently the volume conduction models in the present study were also implemented with Clarion HiFocus electrode arrays.…”

“…In Tang et al (2011), the EFI profiles (averaged over five ears) of three stimulating electrodes (numbers 1, 9 and 15) are presented. The same electrodes were stimulated in the present models to make data comparable.…”

Excitation Patterns of Standard and Steered Partial Tripolar Stimuli in Cochlear Implants

“…The EFI patterns in individual ears are dependent on the anatomy and conductivity of cochlear tissues as well as the placement and surface properties of electrode contacts. For simultaneous stimulation of multiple electrodes, the EFI pattern can be adequately modeled by linear summation of the potential distributions of individual stimulated electrodes (e.g., Berenstein et al 2010;Tang et al 2011;Snel-bongers et al 2012) and thus may change with the stimulation configuration (e.g., the α value in steered pTP mode).…”