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

Abstract

Current steering in partial tripolar (pTP) mode has been shown to improve pitch perception and spectral resolution with cochlear implants (CIs). In this mode, a fraction (σ) of the main electrode current is returned within the cochlea and steered between the basal and apical flanking electrodes (with a proportion of α and 1 − α, respectively). Pitch generally decreases when α increases from 0 to 1, although the salience of pitch change varies across CI users. This study aimed to identify the mechanism of pitch changes with pTP-mode current steering and the factors contributing to the intersubject variability in pitch-ranking sensitivity. The electrical fields were measured for steered pTP stimuli on the same main electrode with α = 0, 0.5, and 1 in five implanted ears using electrical field imaging (EFI). The related excitation patterns were also measured physiologically using evoked compound action potential (ECAP) and psychophysically using psychophysical forward masking (PFM). Consistent with the pitch-ranking results in this study, the EFI, ECAP, and PFM centroids shifted apically with increasing α. An apical shift was also observed for the PFM peak but not for the EFI or ECAP peak. The pattern width was similar with different α values within a given measure (e.g., EFI, ECAP, or PFM), but the ECAP patterns were broader than the EFI and PFM patterns, possibly because ECAP was measured with smaller σ values than EFI and PFM. The amount of pattern shift with α depended on σ (i.e., the total amount of current used for steering) but was not correlated with the pitchranking sensitivity across subjects. The results revealed that the pitch changes elicited by pTP-mode current steering were not only driven by the shifts of excitation centroid.

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

“…For each MP stimulation, such as the one on EL8, the electrical potential on the stimulated electrode was estimated by fitting the potentials on the apical nonstimulated electrodes with an exponential curve (e.g., Vanpoucke et al 2004a;Berenstein et al 2010), fitting those on the basal non-stimulated electrodes with another exponential curve and averaging the extrapolated potentials of the two curves on the stimulated electrode. Using this method, good agreement has been found between the recorded and estimated potentials on the non-stimulated electrodes of pTP stimuli (Berenstein et al 2010;Wu and Luo 2016).…”

“…As in Wu and Luo (2016), the peak (i.e., the electrode location with the highest amplitude) and the centroid (i.e., the arithmetic mean location of all electrodes weighted by the amplitudes) were calculated for each pattern. Wu and Luo (2016) calculated the width of each pattern at 75 % of the peak amplitude to quantify the spread of excitation only around the main electrode.…”

“…The EFI, ECAP, and PFM patterns of standard pTP (7,8,9) and symmetrically spanned pTP (6,8,10) were measured using the same methods as in Wu and Luo (2016). The stimuli and procedure for each measurement are briefly described here, and more details can be found in Wu and Luo (2016).…”

“…The stimuli and procedure for each measurement are briefly described here, and more details can be found in Wu and Luo (2016).…”

“…As in Wu and Luo (2016), the intracochlear electrode voltages as well as physiological and psychophysical excitation patterns of pTP (7,8,9) and pTP (6,8,10) were measured and compared in this study. In specific, electrical field imaging (EFI) records intrascalar potentials and relies on the assumption of uniform neural distribution along the cochlea.…”