Temporal response properties of the auditory nerve: data from human cochlear-implant recipients

Abstract

The primary goal of this study was to characterize the variability in auditory-nerve temporal response patterns obtained with the electrically evoked compound action potential (ECAP) within and across a relatively large group of cochlear-implant recipients. ECAPs were recorded in response to each of 21 pulses in a pulse train for five rates (900, 1200, 1800, 2400, and 3500 pps) and three cochlear regions (basal, middle, and apical). An alternating amplitude pattern was typically observed across the pulse train for slower rates, reflecting refractory properties of individual nerve fibers. For faster rates, the alternation ceased and overall amplitudes were substantially lower relative to the first pulse in the train, reflecting cross-fiber desynchronization. The following specific parameters were examined: (1) the rate at which the alternating pattern ceased (termed stochastic rate), (2) the alternation depth and the rate at which the maximum alternation occurred, and (3) the average normalized ECAP amplitude across the pulse train (measure of overall adaptation/desynchronization). Data from 29 ears showed that stochastic rates for the group spanned the entire range of rates tested. The majority of subjects (79%) had different stochastic rates across the three cochlear regions. The stochastic rate occurred most frequently at 2400 pps for basal and middle electrodes, and at 3500 pps for apical electrodes. Stimulus level was significantly correlated with stochastic rate, where higher levels yielded faster stochastic rates. The maximum alternation depth averaged 19% of the amplitude for the first pulse. Maximum alternation occurred most often at 1800 pps for basal and apical electrodes, and at 1200 pps for middle electrodes. These differences suggest some independence between alternation depth and stochastic rate. Finally, the overall amount of adaptation or desynchronization ranged from 63% (for 900 pps) to 23% (for 3500 pps) of the amplitude for the first pulse. Differences in temporal response properties across the cochlea within subjects may have implications for developing new speech-processing strategies that employ varied rates across the array.

Copyright © 2012 Elsevier B.V. All rights reserved.

Figures

Fig. 1

Schematic illustrating the subtraction method used to resolve the ECAP. Left panel: traditional forward-masking technique, used to resolve the response to the first pulse in the train. Right panel: modified forward-masking technique, used to resolve the responses to pulses 2–21 (number of pulses indicated by subscript n). MPI = masker-probe interval. In both panels, A = probe alone, B = masker plus probe, C = masker alone, D = zero-amplitude pulse for system artifact. Probe pulses are indicated in bold and labeled “P”; masker pulses are thinner lines labeled “M.”

Fig. 2

Top: Individual example illustrating how alternation depth (vertical bolded solid line at right) and average normalized ECAP amplitude across pulses 2–21 (horizontal light solid line) were calculated. Data are from subject N5, electrode 20, for an 1800-pps pulse train. Bottom: Corresponding ECAP waveforms for the data in the top panel. For clarity, only the first 10 samples of each trace are shown.

Fig. 3

Individual example of temporal response patterns for five rates (top row: 900 pps, bottom row: 3500 pps) across basal (left), middle (middle), and apical (right) cochlear regions. In each panel, ECAP amplitudes were normalized to the amplitude for the first pulse in the train, and are plotted as a function of number of pulses. Data are from subject R2. For each electrode, the stochastic rate (rate at which the alternation was no longer statistically significant) is underlined and bolded.

Fig. 4

Individual example of temporal response patterns for five rates across basal, middle, and apical cochlear regions for subject C29. Data are plotted as in Fig. 3.

Fig. 5

Individual examples of triplet amplitude pattern. Each graph represents data from a different subject. Subject number, electrode, and rate are indicated on each graph.

Fig. 6

Bar graph illustrating the range in stochastic rates across electrodes and subjects. Data from basal (white bars), middle (striped bars), and apical (black bars) electrodes are shown for each subject. Subject C24 had no measurable ECAP responses for an apical electrode.

Fig. 7

Histogram of stochastic rates for each electrode region, as a function of pulse-train rate. Data are from all electrodes/subjects tested.

Fig. 8

Left: Normalized ECAP amplitudes as a function of pulse number for Subject R4, electrode 11. Each panel represents a different rate: 900 pps (top), 1200 pps (middle), and 1800 pps (bottom). Right: Corresponding ECAP waveforms for the plots on the left. Data for the two fastest rates (2400 pps and 3500 pps) were similar to the plots shown for 1800 pps.

Fig. 9

Histogram of rates at which maximum alternation occurred for each electrode region, as a function of pulse-train rate. Data are from all electrodes/subjects tested.

Fig. 10

Box-and-whisker plots showing average alternation depth across pulses 2–21 (odd-numbered pulses minus even-numbered pulses) for each rate and electrode region. Each grouping of three plots represents data for basal (left/white bars), middle (middle/light gray bars), and apical (right/dark gray bars) electrodes. Boxes represent the 25th and 75th percentiles, whiskers represent the 10th and 90th percentiles, and filled circles represent outliers. Means and medians are represented by thick and thin horizontal lines, respectively.

Fig. 11

Box-and-whisker plots showing average normalized ECAP amplitude across pulses 2–21 for each rate and electrode region. Data are plotted as in Fig. 9.

Fig. 12

Example of robustness of temporal response patterns pre and post re-implant. Data are for subject F8, electrode 20, across all five rates. Filled circles represent original recordings with a Nucleus Freedom device; open circles represent data obtained 17 months later following re-implantation with a Nucleus CI512 device.