Frequency-place map for electrical stimulation in cochlear implants: Change over time

Abstract

The relationship between the place of electrical stimulation from a cochlear implant and the corresponding perceived pitch remains uncertain. Previous studies have estimated what the pitch corresponding to a particular location should be. However, perceptual verification is difficult because a subject needs both a cochlear implant and sufficient residual hearing to reliably compare electric and acoustic pitches. Additional complications can arise from the possibility that the pitch corresponding to an electrode may change as the auditory system adapts to a sound processor. In the following experiment, five subjects with normal or near-to-normal hearing in one ear and a cochlear implant with a long electrode array in the other ear were studied. Pitch matches were made between single electrode pulse trains and acoustic tones before activation of the speech processor to gain an estimate of the pitch provided by electrical stimulation at a given insertion angle without the influence of exposure to a sound processor. The pitch matches were repeated after 1, 3, 6, and 12 months of experience with the sound processor to evaluate the effect of adaptation over time. Pre-activation pitch matches were lower than would be estimated by a spiral ganglion pitch map. Deviations were largest for stimulation below 240° degrees and smallest above 480°. With experience, pitch matches shifted towards the frequency-to-electrode allocation. However, no statistically significant pitch shifts were observed over time. The likely explanation for the lack of pitch change is that the frequency-to-electrode allocations for the long electrode arrays were already similar to the pre-activation pitch matches. Minimal place pitch shifts over time suggest a minimal amount of perceptual remapping needed for the integration of electric and acoustic stimuli, which may contribute to shorter times to asymptotic performance.

Copyright © 2015 Elsevier B.V. All rights reserved.

Figures

Figure 1

Insertion angles for all 12 electrodes in each of the five subjects

Figure 2

Individual air-conduction pure-tone thresholds in the non-implanted ears

Figure 3

Individual frequency-place functions for electrical stimulation in all five subjects at activation. The solid green line represents the spiral ganglion place-frequency as predicted by Stakhovskaya et al. (2007). Only 2 successful pre-activation matches were made for S1 so the two data points for S1 are not connected by a line.

Figure 4

Individual frequency-place functions for electrical stimulation in all five subjects. Each panel represents one of five subjects tested at the different test intervals. The solid green line represents the spiral ganglion place-frequency as predicted by Stakhovskaya et al. (2007). The dashed red line represents the frequency allocation for the corresponding subject.