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.

Comparison of Place-versus-Pitch Mismatch between a Perimodiolar and Lateral Wall Cochlear Implant Electrode Array in Patients with Single-Sided Deafness and a Cochlear Implant

“…The range of observed mismatches is in line with our previous report (-2.0 octaves for 10 SSD patients with CI422 [Peters et al, 2016]), and also concordant with other series evaluating various CI types in SSD patients [Boëx et al, 2006;Dorman et al, 2007;Baumann and Nobbe, 2006;Schatzer et al, 2014;Vermeire et al, 2015]. There are several other explanations proposed for the observed mismatch, including methodological shortcomings in the pitch matching procedures [Devocht et al, 2015], tonotopical reorganization after CI activation [McDermott et al, 2009;Vermeire et al, 2015], the electrical current between the electrode contacts and the extracochlear reference electrode following the trajectory of least resistance, which may differ from the histologically observed radial fiber trajectories [Peters et al, 2016;Stakhovskaya et al, 2007], and finally the degree of contralateral residual hearing [Vermeire et al, 2008]. One paper observed no mismatch between their electric-acoustic pitch match comparisons and the Stakhovskaya reference line [Carlyon et al, 2010].…”

“…In spite of the task difficulty, the average results were stable over the observed time (average interval between Test and Re-test experiments was 4.3 months) for both groups. The good reproducibility is in line with the only other report investigating electric-acoustic pitch matches over time [Vermeire et al, 2015].…”

“…5a), that is, with increasing hearing loss, the mismatch becomes smaller. We investigated a group of SSD patients with moderate (mainly high-frequency) hearing loss; when comparing our mismatch results to the results of groups with severe to profound (high-frequency) hearing loss [Boex et al, 2006;Dorman et al, 2007;Baumann and Nobbe, 2006;Schatzer et al, 2014;Vermeire et al, 2015], the mismatch found is about equal. Apparently, the effects of distorted pitch perception due to high-frequency hearing loss are small compared to those of the, still unknown, cause(s) of the mismatch.…”

“…In pitch matching experiments in SSD patients implanted with a CI, the perceived pitch is 1 to 2 octaves lower than the pitch predicted by the amended place-pitch relation [Boëx et al, 2006;Dorman et al, 2007;Baumann and Nobbe, 2006;Schatzer et al, 2014]. Some of this mismatch may be due to the fact that in several studies [Boëx et al, 2006;Baumann and Nobbe, 2006;Dorman et al, 2007;McDermott et al, 2009;Schatzer et al, 2014;Vermeire et al, 2008;Vermeire et al, 2015] the determination of the electrode array's position was inaccurate, due to the use of planar X-ray imaging [Xu et al, 2000]. Furthermore, the better ear in these patients was moderately or severely impaired, resulting in possible distorted pitch perception [Gaeth and Norris, 1965].…”

“…Patients with single-sided deafness (SSD) and a CI must fuse the electrical input from their CI with the acoustic input from their better ear. A possible mismatch in pitch between ears may have detrimental effects on the fusion of the signals from the two ears, which may limit performance [Buchman et al, 2014;Vermeire et al, 2015].…”