Identifying cochlear implant channels with poor electrode-neuron interfaces: electrically evoked auditory brain stem responses measured with the partial tripolar configuration

Abstract

Objectives: The goal of this study was to compare cochlear implant behavioral measures and electrically evoked auditory brain stem responses (EABRs) obtained with a spatially focused electrode configuration. It has been shown previously that channels with high thresholds, when measured with the tripolar configuration, exhibit relatively broad psychophysical tuning curves. The elevated threshold and degraded spatial/spectral selectivity of such channels are consistent with a poor electrode-neuron interface, defined as suboptimal electrode placement or reduced nerve survival. However, the psychophysical methods required to obtain these data are time intensive and may not be practical during a clinical mapping session, especially for young children. Here, we have extended the previous investigation to determine whether a physiological approach could provide a similar assessment of channel functionality. We hypothesized that, in accordance with the perceptual measures, higher EABR thresholds would correlate with steeper EABR amplitude growth functions, reflecting a degraded electrode-neuron interface.

Design: Data were collected from six cochlear implant listeners implanted with the HiRes 90k cochlear implant (Advanced Bionics). Single-channel thresholds and most comfortable listening levels were obtained for stimuli that varied in presumed electrical field size by using the partial tripolar configuration, for which a fraction of current (σ) from a center active electrode returns through two neighboring electrodes and the remainder through a distant indifferent electrode. EABRs were obtained in each subject for the two channels having the highest and lowest tripolar (σ = 1 or 0.9) behavioral threshold. Evoked potentials were measured with both the monopolar (σ = 0) and a more focused partial tripolar (σ ≥ 0.50) configuration.

Results: Consistent with previous studies, EABR thresholds were highly and positively correlated with behavioral thresholds obtained with both the monopolar and partial tripolar configurations. The Wave V amplitude growth functions with increasing stimulus level showed the predicted effect of shallower growth for the partial tripolar than for the monopolar configuration, but this was observed only for the low-threshold channels. In contrast, high-threshold channels showed the opposite effect; steeper growth functions were seen for the partial tripolar configuration.

Conclusions: These results suggest that behavioral thresholds or EABRs measured with a restricted stimulus can be used to identify potentially impaired cochlear implant channels. Channels having high thresholds and steep growth functions would likely not activate the appropriate spatially restricted region of the cochlea, leading to suboptimal perception. As a clinical tool, quick identification of impaired channels could lead to patient-specific mapping strategies and result in improved speech and music perception.

Figure 1

Single-channel behavioral thresholds across subjects and configurations. Each panel plots the single-channel detection thresholds for a given subject (indicated in the top right corner). The abscissa represents cochlear implant channel from apical to basal and the ordinate represents detection threshold in decibels relative to 1 mA. Electrode configuration is indicated by symbols and for the triangles is either 0.9 or 1. The vertical dashed lines indicate the lowest and highest threshold channels obtained with the largest pTP fraction for each subject (0.9 for S9, S22, and S26 and 1.0 for S24, S29, and S30).

Figure 2

Electrically-evoked auditory brainstem response (EABR) waveforms for the lowest (A, left two panels) and highest (B, right two panels) pTP threshold channels. The ordinate is response amplitude in μV and the abscissa is post-stimulus time in ms. Within each panel the EABR waveform is plotted with increasing stimulus levels from 0 current (bottom) to the most comfortable level (top) indicated at the left of each waveform. There are two waveforms plotted for each stimulus representing the replication of each waveform shown in black and grey lines. Within A and B, the left panels show responses to the MP configuration while the right panels show responses to pTP stimuli with a fraction of σ = 0.6. Data are from S26.

Figure 3

Wave V amplitude growth functions for each subject and configuration. Amplitude was measured for Wave V of the EABR from the peak to the following trough in μV (ordinate) and is plotted as a function of stimulus level in dB (abscissa). The left and right columns are for the channels with low and high TP thresholds, respectively. Electrode configuration is indicated by black for MP and gray for pTP stimulus configurations. Data from the high and low threshold channels are indicated by the fill of the symbol (open or striped for the low threshold channels and filled for the high threshold channels). The pTP fraction used for each subject is indicated in the top right of each panel. In the legend for each panel the number in parentheses indicates the subjective rating of the loudness of the highest level tested for each stimulus configuration and channel. EABR threshold was taken as the level for which the amplitude growth function reached 0.1 μV and the current level at threshold is indicated by arrows below the abscissa. A least-square error best fit line is shown in bold from threshold to the highest tested level.

Figure 4

A) The difference between amplitude growth function slopes (μV/dB) for monopolar and partial tripolar stimuli (abscissa) are shown for each subject for the low (diagonal stripes) and high (stippled) threshold channels. B) The difference between amplitude growth function slopes (μV/dB) for high and low threshold channels (abscissa) are shown for each subject for the MP (open) and pTP (grey filled) configurations.

Figure 5

Relation of behavioral threshold (abscissa) and EABR threshold (ordinate) for MP (left) and pTP (right) stimuli. As in previous figures, the symbol and fill of the data indicate the subject and the high or low threshold status of the channel, respectively. The solid line indicates a least-square error best fit line to the data. The statistics are based on the non-parametric Spearman’s rank correlation coefficient.

Figure 6

Relation of the relative EABR threshold and sharpness of psychophysical tuning curves. EABR thresholds are relative to the average for each subject and are plotted on the abscissa. The apical slope of the psychophysical tuning curve in dB/mm is plotted on the ordinate. Symbol and fill indicate subject and electrode configuration. The solid line indicates a least-square error best fit line to the data. The statistics are based on the non-parametric Spearman’s rank correlation coefficient.