Elsevier

Hearing Research

Volume 212, Issues 1–2, February 2006, Pages 160-175
Hearing Research

Research paper
Psychophysical measures in patients fitted with Contour™ and straight Nucleus electrode arrays

https://doi.org/10.1016/j.heares.2005.11.005Get rights and content

Abstract

The objective of this study was to compare the psychophysical performance of patients using the Nucleus Contour™ electrode array with that of patients using the straight banded-electrode array. In particular, we wished to consider how psychophysical parameters would differ for an electrode array positioned closer to the modiolus, and how this might influence both patient benefits and the design of speech processing strategies. Nine subjects participated in the study: four used the Nucleus straight array and five used the Nucleus Contour electrode array. Radiographic analyses found that the Contour array lay closer to the modiolus, was more deeply inserted and spanned a larger fractional length of the basilar membrane than the straight banded-electrode array. The results were analysed in terms of array type and of the position of the individual electrode band, both distance from the modiolus and longitudinal placement. Mean threshold was lower for the Contour array but maximum comfortable level was similar. Whereas threshold varied significantly with distance of electrode band from the modiolus, maximum comfortable level did not. Pitch varied fairly regularly with longitudinal position of the stimulated electrode, with the exception of one Contour subject. The forward masking profiles, using moderately loud maskers, were narrower for the Contour array, indicative of more localized neural excitation.

Introduction

A major development in the design of multichannel cochlear implant electrode arrays has been an emphasis on positioning the array in close proximity to the modiolus and, thereby, to the neural targets. Both physiological (Merzenich and White, 1977, Shepherd et al., 1993) and mathematical modelling (Finley et al., 1990, Frijns et al., 1996) studies have suggested that such modiolar placement should reduce the required stimulating current, increase the dynamic range and result in more localized regions of neural excitation. A number of perimodiolar electrode arrays have been developed, all designed for insertion into scala tympani, and targeting the spiral ganglion cells and their dendrites. Radiographic studies of caderveric temporal bones have confirmed the efficacy of these designs in positioning the electrode contacts close to the modiolar wall (Balkany et al., 2002). However, it remains an important issue to test the predictions of how such perimodiolar placement might influence both the efficiency and precision with which local populations of neurons can be stimulated. In addition to such simple measures as threshold and comfortable level, differences might be found in perceptual measures of loudness, pitch and channel separation.

The Nucleus CI22 and CI24 cochlear implants (Cochlear Ltd., Sydney, Australia) both incorporated an electrode array comprising 22 banded electrodes, mounted on a silicone carrier that was moulded straight and tapered to the tip. Radiographic and temporal bone studies have shown that, after surgical insertion, these electrode arrays in general adopted a position along the outer wall of scala tympani. In contrast, the Contour electrode array (Parkinson et al., 2002, Saunders et al., 2002) comprises 22 half-banded electrodes, mounted on a silicone carrier that is moulded into a tight pre-curved shape. The Contour array is designed to assume a perimodiolar position after surgical insertion and withdrawal of the stylet wire, which initially holds the array straight.

Prior to the development of the Contour array, an experimental pre-curved perimodiolar electrode array was combined with the Nucleus CI22 receiver-stimulator and implanted in three adult patients in Melbourne (Cohen et al., 2001b). Although there were no control subjects in this study, the considerable variation in the distance from the modiolus of the individual bands in one subject enabled useful conclusions to be drawn. The findings suggested that most of the predicted benefits could be achieved by placement close to the modiolus.

It has been confirmed that the Contour array adopts a position considerably closer to the modiolus than did the straight array (Richter et al., 2001, Tykocinski et al., 2001, Balkany et al., 2002). Several studies have evaluated the psychophysical performance (mostly limited to thresholds and comfortable levels) of subjects using the Contour relative to subjects using the straight array (Tykocinski et al., 2001, Parkinson et al., 2002, Saunders et al., 2002). These studies confirmed that both threshold and comfortable levels were lower, on average, for the Contour subjects. However, the dynamic ranges were found to be similar.

In assessing the performance of the Contour electrode array, it is important not only to consider whether the predicted improvements have been delivered but also to determine whether there are differences in other aspects of performance. For example, the pitch percept is known to vary regularly, and essentially monotonically, with longitudinal position of the stimulated electrode band in most patients fitted with the standard “straight” arrays (Busby et al., 1994, Cohen et al., 1996a). However, the modelling work of Frijns and colleagues (Frijns et al., 2001) has suggested that, in some circumstances, cross-turn or “ectopic” excitation may be possible, where axons descending from more apical locations are excited in addition to the local, target neurons. This would give rise to anomalously low pitch percepts, especially at high levels. Indeed, such behaviour has been reported by Cohen et al. (1996a) for bipolar stimulation of a straight array. Frijns et al. (2001) have predicted that this would be more likely to occur for a medially placed electrode band positioned beyond the basal turn. It is important, therefore, to investigate how the pitch percepts elicited by the Contour array vary with longitudinal site of stimulation. Furthermore, current commercially-available speech processing strategies have been developed primarily using electrode arrays positioned at the outer wall of scala tympani, and having certain characteristics regarding the growth of loudness with stimulation current. If more medial placement of the array was found to result in very different loudness growth functions, then speech processing strategies might need to be modified accordingly.

The objective of the present study was to compare the psychophysical performance of patients using the Contour and straight electrode arrays. This was to be done, first, in light of the benefits predicted from perimodiolar placement of an electrode array within scala tympani. Here, we investigated whether the Contour array provided reduced current requirements, increased dynamic range and narrower spread of neural excitation. The second area of consideration was whether there were other differences between the two arrays which might suggest revision of speech processor coding strategies for the Contour array or which might otherwise impact speech perception performance in Contour patients. This area comprised investigation of the longitudinal position of the array, the variation of pitch with longitudinal site of stimulation and the growth of loudness with stimulus current.

The study considered, in addition to the effect of electrode array type per se, the effects of the radial distance of the individual electrode band from the modiolus (Riw) and the longitudinal position of the band (Pl). The effect of Riw within the Contour electrode group was important to consider, in addition to the effect of array type, because it was the fundamental variable giving rise to differences between the electrode arrays. Consideration of the effects of both array type and Riw would, therefore, tend to reduce the limitations imposed by our small numbers of subjects. The influence of the maximum comfortable level on various quantities was also considered.

This work was conducted under the ethics approval of the Human Research and Ethics Committee of the Royal Victorian Eye and Ear Hospital, under Projects 93/226H and 92/193H.

Section snippets

Subjects

Nine profoundly hearing-impaired adult subjects from the Melbourne Cochlear Implant Clinic participated in the study. The subjects were selected primarily on the basis of their availability for the study. The histories are summarized in Table 1. All of the subjects were implanted with the Nucleus® 24 cochlear prosthesis (Cochlear Limited): the straight array was used in subjects S1–S4 and the Contour™ array in subjects C1–C5.

The electrode array was implanted using standard clinical procedure,

Estimation of positions of electrode bands from radiographs

Reconstructed radiographs are shown in Fig. 1 for all nine subjects. The electrode bands spanned mean angular ranges of 72° (s.d. 25°)–379° (s.d. 42°) for the straight array and 37° (s.d. 11°)–408° (s.d. 40°) for the Contour array. These mean placements corresponded to characteristic frequency ranges of approximately 7720–915 Hz and 11,580–790 Hz, respectively (making no allowance for the angular offset between basilar membrane location and associated spiral ganglion cells). Thus, for the Contour

Discussion

The main questions posed in comparing the psychophysical data for the Contour and straight electrode arrays were: (1) did the arrays differ? and (2) did a given psychophysical quantity vary with radial distance (Riw), longitudinal position (Pl) or MCL? In addressing these questions, an obvious limitation, especially regarding the array difference, was the small subject numbers in each group.

The comparison of psychophysical outcomes for subjects using the straight and Contour arrays was made in

Acknowledgements

We wish to acknowledge support provided by the Commonwealth of Australia through the Co-operative Research Centre for Cochlear Implant and Hearing Aid Innovation. Thanks are due to our subjects, to Cochlear Limited, to Dr. Peter Busby for reading the manuscript and providing helpful comments, and to the staff of the Cochlear Implant Clinic at the Royal Victorian Eye and Ear Hospital, Melbourne, for their assistance.

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