Considerations for Design of Future Cochlear Implant Electrode Arrays: Electrode Array Stiffness, Size, and Depth of Insertion

INTRODUCTION Cochlear implants (CIs) have been used successfully for more than two decades as a rehabilitative aid for severe to profound hearing loss. Over this period, the expectation for increased communication capability with these devices has grown dramatically. The earliest CI recipients reported substantial benefits in lipreading performance and recognition of environmental sounds but little or no recognition of speech using only the auditory information provided by the implant [1]. As multichannel CIs were introduced, several studies demonstrated that subjects using these devices could discriminate speech without assistance from visual cues. Improvement in CI performance has continued to the present; many current CI recipients routinely communicate via the telephone, and congenitally deaf children who are implanted as infants or toddlers often develop language skills sufficient to allow them to attend mainstream schools. With the dramatic success achieved to date, one might ask what direction future research and development efforts should take to increase the performance of CIs and benefit subjects with a wider range of hearing impairments. A CI operates as an integrated system that includes one or more microphone inputs, a software-controlled digital signal processor, a transcutaneous link, and an intracochlear stimulating electrode array. In this study, we focus on the mechanical design of the electrode array by evaluating five different devices that have been widely implanted in human subjects and three prototype electrode designs in order to ascertain how specific mechanical properties of each device relate to the incidence of damage. Three widely accepted goals for the development of future CI electrode arrays are (1) deeper insertion into the scala tympani (ST) to access lower frequency cochlear neurons; (2) greater operating efficiency, defined as a reduction in the stimulus charge required to produce a comfortable loudness level; and (3) reduced intracochlear damage associated with surgical insertion. Deeper Insertion CI subject testing and acoustic simulations in hearing subjects have shown that speech recognition is degraded when the frequency bands presented to a listener do not approximate the normal acoustic frequency represented at the cochlear place of stimulation [2-8]. Because the tono-topic locations representing the primary speech formant frequencies are located further along the cochlear spiral, i.e., at lower frequency locations, than most fully inserted implant electrode arrays, a significant mismatch occurs for most users between the processed frequency band assigned to each stimulus channel and the cochlear place that it excites. Thus, electrodes with mechanical characteristics that facilitate deeper insertion may be advantageous. Until recently, determining the optimum depth of insertion and distribution of processed frequency information has been impeded by the lack of an accurate frequency--position map of the human spiral ganglion as well as the lack of a clinical method to assess where each CI stimulating site is located in relation to that map in an individual subject. Recent studies have determined the relationship between the progression of characteristic frequencies along the basilar membrane [9] and the comparable frequency versus position map of neurons in the spiral ganglion [10-11]. Using a different experimental approach, two recent studies of CI patients with residual hearing compared the pitch percepts produced by stimulation of individual implant channels with percepts produced by acoustic stimulation of varying frequency in the nonimplanted ear [12-13]. Clinical methods using modern high-resolution imaging methods to better estimate the frequency location of the implanted electrode array in individual subjects have also been proposed [10,14]. These anatomical, psychophysical, and imaging studies should help to direct the development of electrode arrays and fitting techniques that will result in a more accurate correspondence between the frequency spectra of processed sounds and the location of electrical stimulation. …