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. 2007 Mar;8(1):134-51.
doi: 10.1007/s10162-006-0069-0. Epub 2007 Jan 17.

Using evoked potentials to match interaural electrode pairs with bilateral cochlear implants

Zachary M Smith  1 , Bertrand Delgutte
Affiliations

Using evoked potentials to match interaural electrode pairs with bilateral cochlear implants

Zachary M Smith et al. J Assoc Res Otolaryngol. 2007 Mar.

Abstract

Bilateral cochlear implantation seeks to restore the advantages of binaural hearing to the profoundly deaf by providing binaural cues normally important for accurate sound localization and speech reception in noise. Psychophysical observations suggest that a key issue for the implementation of a successful binaural prosthesis is the ability to match the cochlear positions of stimulation channels in each ear. We used a cat model of bilateral cochlear implants with eight-electrode arrays implanted in each cochlea to develop and test a noninvasive method based on evoked potentials for matching interaural electrodes. The arrays allowed the cochlear location of stimulation to be independently varied in each ear. The binaural interaction component (BIC) of the electrically evoked auditory brainstem response (EABR) was used as an assay of binaural processing. BIC amplitude peaked for interaural electrode pairs at the same relative cochlear position and dropped with increasing cochlear separation in either direction. To test the hypothesis that BIC amplitude peaks when electrodes from the two sides activate maximally overlapping neural populations, we measured multiunit neural activity along the tonotopic gradient of the inferior colliculus (IC) with 16-channel recording probes and determined the spatial pattern of IC activation for each stimulating electrode. We found that the interaural electrode pairings that produced the best aligned IC activation patterns were also those that yielded maximum BIC amplitude. These results suggest that EABR measurements may provide a method for assigning frequency-channel mappings in bilateral implant recipients, such as pediatric patients, for which psychophysical measures of pitch ranking or binaural fusion are unavailable.

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Figures

FIG. 1
FIG. 1
Measuring the binaural interaction component (BIC). (A–C) EABR waveforms for binaural stimulation and monaural stimulation of each ear with single biphasic current pulses. Waves 2, 3, and 4 are seen, but Wave 1 is obscured because the first 1 ms of the EABR waveforms contains residual stimulus artifact. Triangles show the times used to measure the peak–peak amplitudes of each response. (D) BIC waveform computed by subtracting the sum of the monaural evoked responses from the binaural response.
FIG. 2
FIG. 2
(A) Average response waveforms recorded in the IC site with a silicon probe for bipolar stimulation of the contralateral ear with a biphasic pulse at eight different levels. Analysis window used to compute RMS amplitude is 10 ms in duration and starts 2.5 ms after the stimulus pulse. (B) RMS response amplitude as a function of stimulus level.
FIG. 3
FIG. 3
Effects of stimulus intensity on evoked responses. (A) Mean EABR amplitude-level functions for bipolar stimulation of the cochlea across all animals and electrodes. Increasing stimulus intensity results in greater binaural EABR, monaural EABR, and BIC amplitudes. Intensity is given in decibel re monaural EABR thresholds. (B) Mean monaural EABR amplitude-level functions for bipolar and monopolar stimulations. Error bars show ±1 SD. Bipolar stimulation results in a more gradual growth of EABR amplitude than monopolar stimulation. (C) Mean normalized BIC amplitude across all animals as a function of interaural intensity difference (ILD). The 0-dB point for each individual curve is set to where monaural EABR amplitudes are equal. The mean binaural level was kept constant as ILD was varied. BIC amplitude peaks at an ILD of 0 dB.
FIG. 4
FIG. 4
Estimating the BIC amplitude for matching stimulus strength in interaural electrode pairs. (A) Monaural EABR and BIC amplitudes are measured for a fixed intensity in the right ear (5 dB re 1 mA) and a range of intensities in the left ear (points fit with linear regression). The left- and right-ear intensities are considered matched when they evoke the same monaural EABR amplitude (7.9 dB in left ear, dashed black line). The BIC amplitude for this interaural electrode pair is measured for the matched intensities (purple arrow). The noise floor of the measurements is indicated by gray shading. (B) BIC amplitude as a function of the active electrode in the left ear, with the right-ear stimulation fixed at BP 3. The BIC amplitude measured in panel A was for the interaural electrode pair right BP 3 and left BP 4 (purple point). The same-level matching procedure was repeated for five different bipolar electrode locations in the left ear.
FIG. 5
FIG. 5
Effect of interaural electrode offset on BIC amplitude. (A) Complete set of BIC–electrode functions measured in the seven animals (BIC amplitude normalized). Curves are sorted into panels according to the location of the active electrode in the fixed ear (red contact and red arrow). Thin black lines show individual BIC–electrode curves, and thick purple lines show the mean data for each fixed electrode location. All curves are normalized so that the peak BIC amplitude is 1. (B) All BIC–electrode functions superimposed and plotted as a function of interaural electrode offset (varied electrode–fixed electrode). (C) Histogram of the interaural electrode offsets for which BIC peaks in B.
FIG. 6
FIG. 6
Acoustic tonotopy of the inferior colliculus. (A) Diagram showing the angle of penetration of the silicon probe into the IC. The probe is in the coronal plane, 45° from vertical so that the trajectory is from dorsolateral to ventromedial, parallel to the tonotopic arrangement of CFs in the central nucleus of the IC. With this electrode trajectory, low-CF neurons are located at shallow depths from the surface of the IC, and high-CF neurons are deeper. (B) CF as a function of electrode depth for one probe penetration in a normal-hearing animal. Data from three different probe positions are combined to cover the entire depth of the IC.
FIG. 7
FIG. 7
Neural response patterns with electrical stimulation in the inferior colliculus. (A) Neural response amplitude as a function of position along the IC and stimulus intensity. Color map indicates response amplitude. Each panel shows the neural response pattern for a different bipolar electrode location in the contralateral ear. White horizontal lines indicate stimulus intensities that elicited equal-amplitude EABRs. (B) Neural response amplitude as a function of probe depth in IC for bipolar stimulation with seven electrode pair (see top). These spatial profiles from the neural response patterns shown in A by choosing the stimulus intensity for each electrode to produce the same EABR amplitude. Stimulus intensities are shown in A as white horizontal lines. Curves are smoothed and interpolated with a cubic spline and normalized to their peak. (C) Location of peak of spatial profile in the IC as a function of active intracochlear electrode
FIG. 8
FIG. 8
Matching interaural electrode pairs from IC neural responses patterns. (A) Each panel shows the spatial response profile in the right IC for monaural stimulation of each ear (bipolar configuration). Stimulation in the right/ipsilateral cochlea is fixed at BP 2, whereas stimulation in the left/contralateral ear differs for each panel. The neural distance is calculated between the peaks of the right and left responses for each interaural pairing. (B) Distance between response peaks as a function of the varied electrode in the left/contralateral ear (location of fixed electrode in right ear indicated by red arrow). The neural distance is minimal at BP 2 in the varied ear.
FIG. 9
FIG. 9
Comparison of interaural electrode matches based on evoked potentials and IC neural recordings. (A) BIC–electrode curve for fixed electrode BP 2 (indicated by arrow) in one animal. (B) Flipped neural distance function for same animal and electrodes as in panel A (flipped version of Fig. 8B). (C) Normalized BIC amplitude versus neural distance for all data points measured in the same animals and same configurations. Data from four animals and 16 fixed electrodes are included. Solid curve shows best-fitting decaying exponential (R2 = 0.58, space constant = 700 μm). (D) Histogram of differences in interaural electrode matches between the two measurement methods. The 11/16 matches are in agreement between the two methods.
FIG. 10
FIG. 10
Comparison of BIC-amplitude falloff with electrode offset for bipolar and monopolar stimulations. (A–B) Thin black lines are individual BIC–offset curves, and thick colored lines are mean values. Curves are shifted so that BIC peak is at zero. (A) Bipolar stimulation. (B) Monopolar stimulation. (C) Comparison of mean bipolar (solid line with closed circles) and monopolar (dashed line with open triangles) stimulations. Error bars indicate ±1 SD.
FIG. 11
FIG. 11
Comparison of neural response patterns for bipolar and monopolar electrode configurations. Thick lines show means, and thin lines indicate ±1 SD from the mean. (A) Peak amplitude of neural activation patterns versus stimulus intensity for bipolar (solid lines) and monopolar (dashed lines) stimulations. The mean is taken over all animals and stimulating electrodes. (B) Halfwidth of neural activation patterns versus stimulus intensity for both configurations. (C) Halfwidth versus peak amplitude of neural activation patterns.
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