Elsevier

Hearing Research

Volume 242, Issues 1–2, August 2008, Pages 141-153
Hearing Research

Current focusing and steering: Modeling, physiology, and psychophysics

https://doi.org/10.1016/j.heares.2008.03.006Get rights and content

Abstract

Current steering and current focusing are stimulation techniques designed to increase the number of distinct perceptual channels available to cochlear implant (CI) users by adjusting currents applied simultaneously to multiple CI electrodes. Previous studies exploring current steering and current focusing stimulation strategies are reviewed, including results of research using computational models, animal neurophysiology, and human psychophysics. Preliminary results of additional neurophysiological and human psychophysical studies are presented that demonstrate the success of current steering strategies in stimulating auditory nerve regions lying between physical CI electrodes, as well as current focusing strategies that excite regions narrower than those stimulated using monopolar configurations. These results are interpreted in the context of perception and speech reception by CI users. Disparities between results of physiological and psychophysical studies are discussed. The differences in stimulation used for physiological and psychophysical studies are hypothesized to contribute to these disparities. Finally, application of current steering and focusing strategies to other types of auditory prostheses is also discussed.

Introduction

Over several decades, refinements of cochlear implant (CI) technology and changes in candidacy criteria have created a state in which current CI candidates can expect to understand speech over the telephone after receiving an implant. Despite this high level of overall success, there remains a subset of CI users for whom speech recognition is poor. In addition, speech reception with a cochlear implant is diminished in the presence of background noise and competing acoustic signals, even for users who are able to perform well in the absence of competing sounds. Goals of current CI research are to identify factors that contribute to this decreased performance and to develop technologies and stimulation strategies that can overcome these deficiencies. For example, speech reception for CI users is related to their ability to perceive spectral shapes (Henry and Turner, 2003, Henry et al., 2005, Litvak et al., 2007b, Won et al., 2007). The ability to discriminate spectral shapes across the electrode array might be limited in part by current spread in the cochlea, and in part by the limited number of distinct stimulation sites in the intracochlear arrays used in contemporary implants and stimulation strategies. That is, spectral contrast may be limited by the broad current spread or spread of activation and by the limited dynamic range typical of monopolar stimulation. The number of distinct stimulation sites may be limited by the number of electrodes in the intracochlear array. Stimulation methods that can stimulate a larger number of distinct locations in the cochlea, and do so more specifically, should lead to improvement in spectral shape perception, and so result in better speech recognition. Consequently, methods to increase contrast by current focusing and to increase the number of stimulation sites by current steering have become of great interest in CI research.

Current steering and current focusing are each a subset of a broader category of field shaping strategies – the modulation of intracochlear electric fields by simultaneous application of current to multiple CI electrodes. The overall goal of field shaping is to precisely control the region of the auditory nerve (AN) array that will be activated at any given moment. The specific goal of current steering is to stimulate AN regions that are centered between physical CI electrodes, and that of current focusing is to stimulate AN regions that are narrower than those stimulated using traditional monopolar strategies. An example of a simple form of field shaping is used in bipolar and tripolar stimulation strategies.

Among the challenges to field shaping strategies are the increased power consumption resulting from concurrent stimulation of multiple electrodes, the requirement for multiple independently controllable current sources in implanted CI circuitry, and the possibility that shunting of current through the conductive perilymph in the scala tympani (ST) may make it impossible to reach comfort or threshold levels without exceeding either the compliance limit of current source circuitry or the safety limit of intracochlear electrodes. In a research setting, limitations on circuitry and power consumption can be overcome by using percutaneous connectors with multiple external current sources. Some contemporary implantable processors1 have incorporated multiple independent current sources, and consequently enable use of field shaping strategies employing more than two electrodes. As further advances in electronic hardware increase compliance limits and improve battery capacity in CI processors, field shaping will become a realizable goal.

In this paper, we will review research exploring current steering and current focusing stimulation paradigms. We will begin with an overview of field shaping strategies by describing results from computer and physical models of steered and focused stimulation. Then we will review the results of physiological studies using steered and focused fields. And finally, we will describe the psychophysical and perceptual consequences of steered and focused stimuli in CI users, including their effects on speech reception.

Section snippets

Models of field shaping

As mentioned earlier, the goal of field shaping strategies is to accurately and precisely specify the region of AN neurons activated at any moment. Analysis and design of these strategies begins with modeling to determine current levels for intracochlear electrodes that are needed to achieve desired patterns of AN stimulation. This modeling has three steps: the first is determination of the distribution of the potential field within the cochlear tissues, the second is prediction of the location

Physiological studies of current focusing and current steering

Several physiological studies of field shaping have been conducted with the specific goal of determining the tonotopic location and distribution of auditory nerve fibers activated by different intracochlear electrical stimulation strategies. Most of these studies have concentrated on current focusing; only a few have examined the effects current steering (Bonham et al., 2006, Miyoshi et al., 1997, Miyoshi et al., 1999, Miyoshi et al., 1996).

Moreover, most focusing studies have limited

Evidence of focusing with bipolar and tripolar stimulations

In experimental animal models, stimulation using “focused” configurations appears to reliably produce neural activation patterns that are more specific than those produced by monopolar stimulation. It is less clear, however, whether such stimulation also produces narrower excitation patterns in human cochlear implant users. Although animal and human experiments have been reported to be contradictory (e.g. Chatterjee et al., 2006), care must be taken in comparing the two. In fact, the degree of

Physiology and psychophysics of monopolar stimulation

One surprising result from studies of single- and multi-unit responses recorded from central structures in experimental animals has been the extremely broad excitation patterns in response to monopolar stimulation [e.g. Snyder et al. (2007)]. When specificity is observed with monopolar stimulation (e.g. Fig. 4, σ = 0), it is usually maintained only as long as the amplitude of stimulation is less than 3–4 dB above threshold of the most sensitive fibers. However, for most human listeners, excitation

Acknowledgements

The authors would like to thank Russell Snyder for his extensive and valuable assistance and discussion throughout preparation of this manuscript, and for his contributions to the neurophysiological studies, and Monita Chatterjee for her valuable comments on an earlier version of this manuscript. This research has been funded by NIH/NIDCD DC-02-1006 and HHS-N-263-2007-00054-C, and Hearing Research Inc. (BHB), and Advanced Bionics Corporation (LML).

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