Electrically Evoked Compound Action Potential Measures for Virtual Channels Versus Physical Electrodes

Abstract

Objectives-The number of distinct pitch percepts for cochlear implant (CI) listeners is somewhat limited by the number of physical electrodes in the array. Newer-generation CIs have the capability to potentially increase this number by stimulating areas of the cochlea between the physical electrodes. Currently this is achieved by electrically coupling adjacent electrodes or by simultaneously activating two electrodes with independent current sources (i.e., "current steering"). Presumably, either type of dual-electrode stimulation will generate neural excitation patterns that are intermediate to those generated by either physical electrode alone (henceforth termed virtual channel). However, it is not clear whether virtual-channel stimulation yields neural recruitment patterns with similar shapes and rates of growth as compared with each physical electrode alone. The purpose of this study was to compare basic electrically evoked compound action potential (ECAP) measures for physical electrodes and virtual channels to determine whether properties of the respective excitation patterns were similar.Design-Data were collected for 12 adult CI recipients (N = 6 Nucleus Freedom CI24RE, N = 2 Advanced Bionics HiResolution 90K, N = 4 Advanced Bionics CII). ECAP responses were measured for a set of three adjacent physical electrodes and two corresponding intermediate virtual channels (e.g., physical electrodes 4, 5, and 6 and virtual channels 4+5 and 5+6) at three positions along the electrode array (basal, middle, and apical). Virtual channels for Nucleus subjects were produced via electrical coupling of adjacent electrode pairs ("dual-electrode mode"). For AB subjects, virtual channels were produced via simultaneous, in-phase stimulation of adjacent electrode pairs with 50 % of the total current delivered to each electrode in the pair. Specific ECAP measures were: (1) threshold and slope of the input/output (I/O) functions, (2) amplitude for a masker-probe interval of 1500 μsec (measure of refractory recovery), and (3) the relative location of spread-of-excitation (SOE) functions among virtual channels and adjacent physical electrodes. Measures for virtual channels were compared with those for the flanking physical electrodes using a multivariate analysis of variance.Results-There were no statistically significant differences between physical electrodes and virtual channels for ECAP thresholds, slope of the I/O function, or refractory recovery. On average, SOE functions for the virtual channels were spatially located approximately halfway between SOE functions for the adjacent physical electrodes.Conclusions-Results from this study suggest that virtual channels produce neural recruitment patterns with properties similar to those elicited by the adjacent physical electrodes.

Pitch ranking, electrode discrimination, and physiological spread of excitation using current steering in cochlear implants

“…ECAP amplitudes were then normalized for each subject according to the highest amplitude across the two physical-electrode probe functions within a set and separately for each region. This was done to preserve the spatial relationships among the probe SOEs being compared and to allow for SOE comparisons across electrode sets and subjects (Hughes, 2008;Hughes and Goulson, 2011;Hughes et al, 2013). Following methodology from Hughes (2008), the amount of spatial separation (R) among each of the pairs of SOE functions was quantified.…”

“…The ECAP SOE function has been examined for virtual-channel stimulation (Busby et al, 2008;Saoji et al, 2009;Hughes and Goulson, 2011;SnelBongers et al, 2012;Hughes et al, 2013). These studies have shown that virtual channels produce ECAP SOE patterns similar to those of physical electrodes.…”

“…These studies have shown that virtual channels produce ECAP SOE patterns similar to those of physical electrodes. More recently, Hughes and Goulson (2011) determined that virtual-channel SOE amplitudes fall approximately half-way between the amplitudes of adjacent physical electrodes for both dualelectrode mode and current steering. Hughes et al (2013) showed that spatial separation of SOE patterns was greater between adjacent physical electrodes than between the intermediate virtual channels and the flanking physical electrodes, indicating measurable separation between neural populations.…”

Correlations Between Pitch and Phoneme Perception in Cochlear Implant Users and Their Normal Hearing Peers

“…Studies (Pretorius andHanekom 2008, Goldsworthy et al 2013) have found CI discrimination thresholds between 5 and 30 %; furthermore, Vandali et al (2014) demonstrated that training can improve CI discrimination thresholds by more than a factor of 2. While the use of synthetic tones presented through the CI processor is relatively unexplored, there is a rich literature investigating CI pitch perception based on electrode psychophysics (Tong et al 1982;McDermott and McKay 1997;Hanekom and Shannon 1998;Zeng 2002;Henry and Turner 2003;Carlyon et al 2010;Hughes and Goulson 2011;Goldsworthy and Shannon 2014) as well as using more natural stimuli (Fujita and Ito 1999;Gfeller et al 2002;McDermott 2004;Looi et al 2008). In the present article, the suggestion put forth by Pretorius and Hanekom (2008) that pure and complex tones can be used to probe different aspects of perception is extended toward understanding correlations with phoneme identification in noise.…”

Threshold Levels of Dual Electrode Stimulation in Cochlear Implants

“…However, implanted adults and children are still experiencing problems in challenging listening situations, such as background noise or listening to music. One reason for this might be reduced spectral resolution caused by the limited number of physical electrodes in the array (Brendel et al 2009;Hughes and Goulson 2011). Dual electrode stimulation (DES), either simultaneous (Donaldson et al 2005;Townshend et al 1987) or sequential (Kwon and van den Honert 2006;McDermott and McKay 1994), has been proposed as a method to increase the number of perceivable pitches with CIs.…”