Current Steering with Partial Tripolar Stimulation Mode in Cochlear Implants

Abstract

The large spread of excitation is a major cause of poor spectral resolution for cochlear implant (CI) users. Partial tripolar (pTP) mode has been proposed to reduce current spread by returning an equally distributed fraction (0.5×σ) of current to two flanking electrodes and the rest to an extra-cochlear ground. This study tested the efficacy of incorporating current steering into pTP mode to add spectral channels. Different proportions of current [α×σ and (1−α)×σ] were returned to the basal and apical flanking electrodes respectively to shape the electric field. Loudness and pitch perception with α from 0 to 1 in steps of 0.1 was simulated with a computational model of CI stimulation and tested on the apical, middle, and basal electrodes of six CI subjects. The highest σ allowing for full loudness growth within the implant compliance limit was chosen for each main electrode. Pitch ranking was measured between pairs of loudness-balanced steered pTP stimuli with an α interval of 0.1 at the most comfortable level. Results demonstrated that steered pTP stimuli with α around 0.5 required more current to achieve equal loudness than those with α around 0 or 1, maybe due to more focused excitation patterns. Subjects usually perceived decreasing pitches as α increased from 0 to 1, somewhat consistent with the apical shift of the center of gravity of excitation pattern in the model. Pitch discrimination was not better with α around 0.5 than with α around 0 or 1, except for some subjects and electrodes. For three subjects with better pitch discrimination, about half of the pitch ranges of two adjacent main electrodes overlapped with each other in steered pTP mode. These results suggest that current steering with focused pTP mode may improve spectral resolution and pitch perception with CIs.

Spatial Restriction of Neural Activation Using Focused Multipolar Stimulation With a Retinal Prosthesis

“…34 In this method, weights are calculated the same way; however, a multiplier is applied to the diagonal values of the transimpedance matrix, thereby affording a greater ratio of current to the PSE. 34 This method is similar to other focusing methods, such as partial-TP 42 and quasimonopolar stimulation. 32,56 Using this method, lower thresholds could be attained possibly without compromising on selectivity, allowing for titration of the two factors.…”

“…These weights have been referred to as the phased array. 42 Focused multipolar weights typically presented as concentric circles of alternating positive and negative weights progressively approaching zero with greater distance from the PSE (see Fig. 1).…”

“…31,40,41 However, this technique also results in a significant increase in stimulation threshold to elicit a cortical response, 31,32,38,41 which is consistent with studies showing increased perceptual thresholds for cochlear implant patients when receiving bipolar and TP stimulation. 27,33,42 While, at matched current levels, cortical activation has been shown to be reduced with HP stimulation compared to monopolar (MP) stimulation. 38 When responses have been compared at the same point along the dynamic range of each stimulation mode, no significant change in cortical spread has been found.…”

Speech perception with F0mod, a cochlear implant pitch coding strategy

“…There have also been attempts to improve spectral resolution by changing parameters of the fi lter bank (Geurts & Wouters, 2004;Kasturi & Loizou, 2007;Laneau et al, 2004), or using current steering (Donaldson et al, 2011;Luo et al, 2010;Wu & Luo, 2013), with varying degrees of success. Finally, there are several strategies that aim to improve periodicity pitch perception by increasing modulation depth due to F0 and modulating electrodes synchronously (Green et al, 2004(Green et al, , 2005Laneau et al, 2006;Milczynski et al, 2009Milczynski et al, , 2012Vandali et al, 2005;Vandali & van Hoesel, 2011, also with varying degrees of success in terms of effect on pitch perception and speech recognition.…”

Loudness and pitch perception using Dynamically Compensated Virtual Channels

“…Computational models have been successfully used to investigate multipolar stimulation in the implanted cochlea (e.g. Litvak et al, 2007; Frijns et al, 2011; Snel-Bongers et al, 2013; Wu and Luo, 2013; Kalkman et al, 2014). Modeling studies are less time-consuming than testing human CI users.…”