Binaural unmasking with multiple adjacent masking electrodes in bilateral cochlear implant users

Abstract

Bilateral cochlear implant (BiCI) users gain an advantage in noisy situations from a second implant, but their bilateral performance falls short of normal hearing listeners. Channel interactions due to overlapping electrical fields between electrodes can impair speech perception, but its role in limiting binaural hearing performance has not been well characterized. To address the issue, binaural masking level differences (BMLD) for a 125 Hz tone in narrowband noise were measured using a pair of pitch-matched electrodes while simultaneously presenting the same masking noise to adjacent electrodes, representing a more realistic stimulation condition compared to prior studies that used only a single electrode pair. For five subjects, BMLDs averaged 8.9 ± 1.0 dB (mean ± s.e.) in single electrode pairs but dropped to 2.1 ± 0.4 dB when presenting noise on adjacent masking electrodes, demonstrating a negative impact of the additional maskers. Removing the masking noise from only the pitch-matched electrode pair not only lowered thresholds but also resulted in smaller BMLDs. The degree of channel interaction estimated from auditory nerve evoked potentials in three subjects was significantly and negatively correlated with BMLD. The data suggest that if the amount of channel interactions can be reduced, BiCI users may experience some performance improvements related to binaural hearing.

Figure 1

(A) Block diagram of custom signal processing on-board the Spear3 research interface. Signal and noise were mixed, half-wave rectified, low-pass filtered, and compressed. The outputs were used to modulate a pulse train scaled to threshold and comfort levels for each electrode. The switches indicate specific stimulus conditions and masking electrodes that could be enabled and disabled. (B) Tested configurations of masking electrodes. For masking electrode configurations (MEC) #1–7, referred to as “full masking,” both signal and masking noise, N0S0 and N0Sπ were delivered to the target electrode pair. For each experimental condition, the same MEC was used bilaterally, with the pitch-matched electrode pair at the center. The target electrode pair, indicated in black, was the only pair to which the signal was delivered. Active masking electrodes, indicated in gray, always contained masking noise. Inactive electrodes are indicated in white. The second parallel set of conditions with noise removed from the target electrode (MEC #1n–7n) are referred to as “notched masking” and have the same masking electrode configurations as MEC #1–7.

Figure 2

Single electrode pair masking level thresholds for N0S0 and N0Sπ. Significantly different N0S0 (black) and N0Sπ (white) thresholds are indicated with *(unpaired t-test, p < 0.05) or **(p < 0.001). The mean group thresholds are labeled as “Mean,” and its associated error bars indicate standard error from five subjects. For group average the statistical test compared average paired N0S0 and N0Sπ thresholds from each subject. (paired t-test, p = 0.0009).

Figure 3

Masking electrode configurations (MEC). (A) N0S0 thresholds averaged across the five subjects, plotted according to electrode configuration [shown in Fig. 1B]. Error bars represent standard error for the five subjects. (B) N0Sπ thresholds. (C) BMLDs averaged across subjects for each electrode configuration. Significantly different N0S0 and N0Sπ thresholds, paired by subject, are indicated with *(paired t-test, p < 0.05). For reference, thresholds and BMLDs obtained from single electrode pairs are shown.

Figure 4

BMLD for multiple masking electrodes. Masking level thresholds for N0S0 and N0Sπ averaged over all electrode configurations. Format is similar to Fig. 2. *indicate statistically significant difference (paired t-test, p < 0.05) between N0S0 and N0Sπ conditions paired by electrode configuration. For the group mean, the thresholds were averaged over subject first, and then by MEC. The difference between average N0S0 and N0Sπ was significant when paired by subject (paired t-test, p = 0.010).

Figure 5

Full vs notched masking. (A) N0S0 thresholds are represented on the y-axis. Subjects and group average are listed on the x-axis. (B) BMLD. Data are grouped by subject and averaged over electrode configuration. Black bars represent conditions where noise was presented on the target electrode (MEC #1–7). White bars indicate conditions where masking noise was only presented on electrodes adjacent to the target electrode (MEC #1n–7n). Error bars are standard errors. Statistically significant differences between the two conditions are marked with *(paired t-test, p < 0.05). Mean and the associated error bars indicate average data and standard error. Data were averaged over subject first and then over MEC.

Figure 6

Spread of excitation measured through evoked potentials. (A) Subject CI1. (B) CI6. (C) CI7. In each panel, the upper and lower plots are data from the right and left ears, respectively. The y-axis indicates the magnitude of recorded neural activity. The x-axis indicates the masker active electrode. Symbol legend in (A) indicates the active electrode.

Figure 7

Channel interaction index (ChII). (A) Schematic of ChII calculation. Curves represent evoked potential data shown in Fig. 6. X,Y, and Z are the areas under each curve, XY and YZ are the intersected area. Y represents the target electrode. X and Z represent masker electrodes. (B) Right ChII plotted as a function of left ChII for masking electrode configurations #1–7 [see Fig. 1B]. Symbols represent different subjects. (C) ChII plotted against MEC #1–7. MEC #1n–7n have the same ChII values as their equivalent in #1–7 and are not shown. Left and right ears were averaged to produce a single ChII value for each MEC.

Figure 8

Behavioral correlations with average ChII. Each data point represents the mean threshold or BMLD from the 3 subjects shown in Figs. 67. (A) N0S0 thresholds plotted against ChII for full masking (MECs #1–7; filled circles) and notched masking (#1n–7n, open circles). (B) N0Sπ thresholds vs ChII. (C) BMLD plotted against ChII. Regression equations and statistics are listed in Table TABLE II..