A Comparison of Alternating Polarity and Forward Masking Artifact-Reduction Methods to Resolve the Electrically Evoked Compound Action Potential

Abstract

Objective: Cochlear implant manufacturers utilize different artifact-reduction methods to measure electrically evoked compound action potentials (ECAPs) in the clinical software. Two commercially available artifact-reduction techniques include forward masking (FwdMsk) and alternating polarity (AltPol). AltPol assumes that responses to the opposing polarities are equal, which is likely problematic. On the other hand, FwdMsk can yield inaccurate waveforms if the masker does not effectively render all neurons into a refractory state. The goal of this study was to compare ECAP thresholds, amplitudes, and slopes of the amplitude growth functions (AGFs) using FwdMsk and AltPol to determine whether the two methods yield similar results.

Design: ECAP AGFs were obtained from three electrode regions (basal, middle, and apical) across 24 ears in 20 Cochlear Ltd. recipients using both FwdMsk and AltPol methods. AltPol waveforms could not be resolved for recipients of devices with the older-generation chip (CI24R(CS); N = 6).

Results: Results comparing FwdMsk and AltPol in the CI24RE- and CI512-generation devices showed significant differences in threshold, AGF slope, and amplitude between methods. FwdMsk resulted in lower visual-detection thresholds (p < 0.001), shallower slopes (p = 0.004), and larger amplitudes (p = 0.03) compared with AltPol.

Conclusions: Results from this study are consistent with recent findings showing differences in ECAP amplitude and latency between polarities for human CI recipients. When averaged, these differences likely result in a reduced ECAP response with AltPol. The next step will be to separate the effects of artifact-reduction method and stimulus polarity to determine the relative effects of each.

Fig. 1

Schematic illustrating the two primary methods of artifact reduction used in today’s commercial software. Left: Forward-masking subtraction paradigm. Four frames are presented (A—D) to obtain various templates of artifact and neural response. The formula A–B+C–D is applied to resolve the neural response (see text). MPI: masker-probe interval. Right: Alternating polarity paradigm. Responses and artifacts are obtained for opposing polarities and then averaged together (C+A)/2 (see text). Subtraction of the switching artifact is not applied with alternating polarity in the commercial software.

Fig. 2

Four screenshots from Cochlear’s Custom Sound EP software displaying AGFs obtained with FwdMsk (left panels, A and C) versus AltPol (right panels, B and D) from two participants with different internal devices. The top panels (A and B) show AGFs collected from a CI24RE internal device (participant F5, e20). The bottom panels (C and D) display AGFs obtained from a recipient with the older CI24R device (participant R6, e11).

Fig. 3

Individual AGFs from six participants (rows) displaying ECAP amplitude as a function of probe current level. Gray and white symbols represent FwdMsk and AltPol data, respectively. Data from the basal, middle, and apical electrodes are shown from left to right, respectively.

Fig. 4

ECAP threshold data for the two artifact-reduction methods (FwdMsk, gray; AltPol, white), separated by electrode region (basal, middle, apical). Box boundaries represent the 25^th and 75^th percentiles, whiskers represent the 10^th and 90^th percentiles, black circles represent outliers, and horizontal solid and dotted lines within the box represent medians and means, respectively. Mean threshold values are noted on each box.

Fig. 5

Mean slope data compared across artifact-reduction method and region. Data are plotted similar to Fig. 4.

Fig. 6

Example AGFs for both polarities from subject F7, electrode 5, illustrating how amplitude comparisons were made. The vertical dashed line represents the lowest current level that yielded non-zero amplitudes for both artifact-reduction methods. Amplitude values to the right of the dashed line (indicated with arrows) represents the current-level range at which amplitudes were compared between methods.

Fig. 7

Amplitude differences (FwdMsk minus AltPol) plotted across stimulus levels (as illustrated in Fig. 6) for all AGFs in all subjects (identified by color in the legend). Positive numbers indicate larger amplitudes with FwdMsk and negative numbers indicate larger amplitudes with AltPol. The horizontal dashed line represents no difference. Basal, middle, and apical electrode regions are differentiated by circles, triangles, and squares, respectively.

Fig. 8

Mean amplitude data compared across artifact-reduction method and regions. Data are plotted similar to Figs. 4 and 5.