Symmetric Electrode Spanning Narrows the Excitation Patterns of Partial Tripolar Stimuli in Cochlear Implants

Abstract

In cochlear implants (CIs), standard partial tripolar (pTP) mode reduces current spread by returning a fraction of the current to two adjacent flanking electrodes within the cochlea. Symmetric electrode spanning (i.e., separating both the apical and basal return electrodes from the main electrode by one electrode) has been shown to increase the pitch of pTP stimuli, when the ratio of intracochlear return current was fixed. To explain the pitch increase caused by symmetric spanning in pTP mode, this study measured the electrical potentials of both standard and symmetrically spanned pTP stimuli on a main electrode EL8 in five CI ears using electrical field imaging (EFI). In addition, the spatial profiles of evoked compound action potentials (ECAP) and the psychophysical forward masking (PFM) patterns were also measured for both stimuli. The EFI, ECAP, and PFM patterns of a given stimulus differed in shape details, reflecting the different levels of auditory processing and different ratios of intracochlear return current across the measurement methods. Compared to the standard pTP stimuli, the symmetrically spanned pTP stimuli significantly reduced the areas under the curves of the normalized EFI and PFM patterns, without shifting the pattern peaks and centroids (both around EL8). The more focused excitation patterns with symmetric spanning may have caused the previously reported pitch increase, due to an interaction between pitch and timbre perception. Being able to reduce the spread of excitation, pTP mode symmetric spanning is a promising stimulation strategy that may further increase spectral resolution and frequency selectivity with CIs.

Exploring the Use of Interleaved Stimuli to Measure Cochlear-Implant Excitation Patterns

“…Our data are broadly consistent with the results of a study by Luo and Wu [ 15 ]. They compared pTP to pTP + 1 masking patterns, both with psychophysical forward masking and a physiological measure (electrically-evoked compound action potentials), showing a small but significant sharpening with pTP + 1.…”

“…1 ) may not be ideal for minimising spread of excitation. For example, Luo and Wu [ 15 ] have reported that returning current from the adjacent-but-one electrodes can reduce both current spread and the width of forward-masked excitation patterns compared to the standard pTP method. Our intra-cochlear voltage recordings, described below, indicated that leaving two unused electrodes between the central and flanking electrodes can produce reduced current spread compared to ‘standard’ pTP.…”

Dynamic Current Focusing Compared to Monopolar Stimulation in a Take-Home Trial of Cochlear Implant Users

“…For many years, cochlear implant research has made an effort to narrow the electrical field of speech encoding strategies through multipolar stimulation of the cochlea, also known as current focusing. As a proposed solution to poor spectral discriminatory abilities and a way to improve speech recognition in quiet as well as in noise, current focusing can lead to narrower excitation patterns (van den Honert & Stypulkowski 1987; Kral et al 1998; Bonham & Litvak 2008; Landsberger et al 2012; Kalkman et al 2014; Wu & Luo 2016) and improved spectral resolution (Berenstein et al 2008; Landsberger et al 2012; Smith et al 2013); the ability to discriminate percepts in the frequency domain. As spectral discrimination correlates with speech recognition (Henry & Turner 2003; Won et al 2007; Henry et al 2005; Holden et al 2016; Lawler et al 2017; Luo et al 2020), current focusing can potentially lead to improved speech recognition in noise compared to monopolar stimulation (MP) (Srinivasan et al 2013; Arenberg et al 2018).…”

The intraoperative relationship between intracochlear electrical field and excitability of the auditory nerve

“…More recently, there has been growing interest in current focusing by using the intracochlear electrodes to ground the stimulating current. With tripolar stimulation, where the adjacent electrodes of the stimulating electrode are used as grounds and the current loop is entirely intracochlear, the spread of the EF can be reduced potentially achieving narrower patterns of stimulated SGNs and improved speech recognition in complex listening situations ( Snyder et al., 2004 ; Srinivasan et al., 2013 ; Padilla and Landsberger, 2016 ; Luo and Wu, 2016 ). However, to reach similar loudness levels as with monopolar stimulation with an extracochlear ground, the stimulating current in tripolar stimulation must be significantly increased ( Landsberger et al., 2012 ), potentially debilitating the beneficial effects of current focusing ( Morris and Pfingst, 2000 ).…”