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Comparative Study
. 2015 Dec 1:19:2331216515617143.
doi: 10.1177/2331216515617143.

Comparison of Interaural Electrode Pairing Methods for Bilateral Cochlear Implants

Hongmei Hu  1 , Mathias Dietz  2
Affiliations
Comparative Study

Comparison of Interaural Electrode Pairing Methods for Bilateral Cochlear Implants

Hongmei Hu et al. Trends Hear. .

Abstract

In patients with bilateral cochlear implants (CIs), pairing matched interaural electrodes and stimulating them with the same frequency band is expected to facilitate binaural functions such as binaural fusion, localization, and spatial release from masking. Because clinical procedures typically do not include patient-specific interaural electrode pairing, it remains the case that each electrode is allocated to a generic frequency range, based simply on the electrode number. Two psychoacoustic techniques for determining interaurally paired electrodes have been demonstrated in several studies: interaural pitch comparison and interaural time difference (ITD) sensitivity. However, these two methods are rarely, if ever, compared directly. A third, more objective method is to assess the amplitude of the binaural interaction component (BIC) derived from electrically evoked auditory brainstem responses for different electrode pairings; a method has been demonstrated to be a potential candidate for bilateral CI users. Here, we tested all three measures in the same eight CI users. We found good correspondence between the electrode pair producing the largest BIC and the electrode pair producing the maximum ITD sensitivity. The correspondence between the pairs producing the largest BIC and the pitch-matched electrode pairs was considerably weaker, supporting the previously proposed hypothesis that whilst place pitch might adapt over time to accommodate mismatched inputs, sensitivity to ITDs does not adapt to the same degree.

Keywords: bilateral cochlear implant; binaural interaction component; electrically evoked auditory brainstem response; interaural electrode pairing; interaural time difference; pitch comparison.

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Figures

Figure 1.
Figure 1.
CI stimulus and trigger for EEG recording. Because the trigger was generated from the RIB II placed within the booth, the trigger was sent out 25 ms before the CI stimulation to avoid any trigger-off-artifact to coincide with the eABR response. CI = cochlear implant; EEG = electroencephalography; eABR = electrically evoked auditory brainstem response.
Figure 2.
Figure 2.
EABRs and the BICs of Subject S6: (a) results of the eABR at 12 probe electrodes and (b) EABRs of left CI stimulated only, right CI stimulated only, both CI stimulated simultaneously, L + R, and the BIC. Reference electrode is L4, and probe electrode is R3. The ordinate is their amplitude values in µV. The CI stimulation electric artifact was removed by linear interpolation as described in Hu et al. (2015). The wave eV and BIC are visible at approximately 3.6 and 4.1 ms, respectively. BIC = binaural interaction component; eABR = electrically evoked auditory brainstem response; CI = cochlear implant.
Figure 3.
Figure 3.
Results of S1 with reference electrode L4: (a) wave eV BIC across the probe electrode pairs. The different probe electrodes were offset along the ordinate, and the abscissa is the time in ms. The peak and the trough of each BIC are marked with diamonds and squares, respectively. The error bar shows the standard deviation of the BIC, which was estimated as the square root of the summed variance of the three measurements; (b) scatter plot of the wave eV and BIC amplitudes with the number referring to the probe electrode number. The ordinate and the abscissa are the amplitudes of eV and BIC in µV. The numbers followed by asterisks indicate electrodes where the eABR peak-to-peak amplitude is smaller than the eABR root mean square (RMS) amplitude within the analysis time window of 3 ms to 4.5 ms. The r2 is the coefficient of determination between the eV amplitude and the BIC amplitude; (c) shows the results of interaural pairwise pitch comparison in percentage (%). The blue circles on the dash-dotted line indicate the percentage of how often the probe electrode resulted in a higher pitch percept than the reference electrode in the pitch matching experiment; the black squares in panel (d) are the correct rates in percentage (%) of lateralization judgment in the IPTD experiment. The line 50% indicates chance level for both the pitch and the IPTD task. Lines at 36% and 64% mark the respective 95% confidence intervals. Panel (e) plots the BIC amplitude values in µV and the red pentagrams in panel (f) are the normalized BIC amplitude values. The abscissain panel (c) to (f) indicates the probe electrode number. BIC = binaural interaction component; IPTD = interaural pulse time difference; eABR = electrically evoked auditory brainstem response.
Figure 4.
Figure 4.
Results of S2 with reference electrode L5 (same format as Figure 3). (d) The squares on the dashed line are the correct rates of lateralization judgment in the IPTD experiment with IPTD = 700 µs. The diamonds on the dashed line are the correct rates of lateralization judgment in the IPTD experiment with IPTD = 400 µs. BIC = binaural interaction component; IPTD = interaural pulse time difference.
Figure 5.
Figure 5.
Results of S3 with reference electrode L4 (same format as Figure 3): no normalization for S3. BIC = binaural interaction component; IPTD = interaural pulse time difference.
Figure 6.
Figure 6.
Results of S4 with reference electrode L4 (same format as Figure 3): no normalization for S4. BIC = binaural interaction component; IPTD = interaural pulse time difference.
Figure 7.
Figure 7.
Results of S5 with reference electrode L4 (same format as Figure 3). BIC = binaural interaction component; IPTD = interaural pulse time difference.
Figure 8.
Figure 8.
Results of S6 with reference electrode L4 (same format as Figure 3): no normalization for S6. BIC = binaural interaction component; IPTD = interaural pulse time difference.
Figure 9.
Figure 9.
The results of S7 with reference electrode L4 (same format as Figure 3): no normalization for S7. BIC = binaural interaction component; IPTD = interaural pulse time difference.
Figure 10.
Figure 10.
The results of S8 with reference electrode L4 (same format as Figure 3): no normalization for S8. BIC = binaural interaction component; IPTD = interaural pulse time difference.
Figure 11.
Figure 11.
Visualization of the three possible pairwise method comparisons: (a) pitch versus IPTD, (b) BIC versus IPTD, and (c) BIC versus pitch. The abscissa and ordinate are the electrode offsets ΔE, that is, the difference between processor pair and method-specific pair: offset for best IPTD performance (ΔEIPTD), pitch comparison (ΔEpitch), and largest BIC amplitude (ΔEBIC). The root mean square error (RMSE) is indicated in each panel as a measure of pairwise similarity, derived from subjects S1 to S3 and S5 to S7. BIC = binaural interaction component; IPTD = interaural pulse time difference.
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