A Cochlear Nucleus Auditory Prosthesis Based on Microstimulation

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Principal Investigator	Affiliation	Contract Number	Link
Doug McCreery, Ph.D.	Huntington Medical Research Institutes	N01-DC8-2102

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Project Title: A Cochlear Nucleus Auditory Prosthesis Based on Microstimulation, July 1997

1. Background

Research on auditory prostheses for deaf individuals is supported by the National Institute on Deafness and Other Communication Disorders (NIDCD). The cochlear implant is not a viable option for providing auditory information to patients with severe sensorineural hearing loss due to poor eighth nerve survival. Instead, investigators have implanted electrodes on the surface of the cochlear nucleus in patients with bilateral acoustic neuromas. In these patients the electrodes were connected to speech processors similar to those used in cochlear implants. Most of the patients have found that their cochlear nucleus implants provided essentially the same limited information that single channel cochlear implants provide patients with good auditory nerve survival.

Using speech processors custom designed for cochlear nucleus implants, deaf subjects have demonstrated substantial increases in auditory recognition test scores and usefulness of their devices for speech recognition as compared to their initial speech processors. However, the benefits are still considerably lower than those achieved by many multichannel cochlear implant users.

The NIDCD has been supporting, under contract, a study in animals and in human cadaver material of the feasibility of a multichannel auditory prosthesis based on microstimulation of the ventral cochlear nucleus with penetrating microelectrodes. These contract studies have shown that microelectrode arrays can be accurately placed in the cat ventral cochlear nucleus, that safe levels of stimulation exist for chronically stimulating this structure, and that these levels of stimulation are effective in activating cochlear nucleus neurons. In addition, using human cadaver material and computer models, surgical approaches to the human cochlear nucleus have been investigated and a trans-labyrinthine approach selected.

A cochlear nucleus auditory prosthesis would be useful to two classes of deaf individuals. One is the group of individuals who have no auditory nerve fibers such as those who have had tumors removed from the 8th nerve. In addition, there is a significant group of individuals who have an insufficient number of auditory nerve fibers remaining to benefit from a cochlear implant.

This study is a competitive renewal of a contract supported by NIDCD entitled "The Feasibility of a Cochlear Nucleus Auditory Prosthesis Based on Microstimulation". A copy of a bibliography of publications and quarterly progress reports resulting from this contract (NO1-DC-5-2105) can be obtained free of charge by writing the Neural Prosthesis Program, N.I.H., Federal Building, Room 8A13, Bethesda, MD, 20892.

2. Objectives

The objective of this research is to complete animal testing of the safety and effectiveness of chronic microstimulation of the cochlear nucleus in animals. In addition, the development of microelectrodes and a microelectrode inserter for use in humans will be completed. It is anticipated that animal testing and microelectrode development will be successfully completed and feasibility testing in humans will be initiated.

3. Work to be Performed

Independently, and not as an agent of the Government, the Contractor shall furnish all the necessary services, qualified personnel, material, equipment and facilities, not otherwise provided by the Government as needed to perform the statement of work. The Contractor shall study the feasibility of an auditory prosthesis based on microstimulation of the ventral cochlear nucleus and initiate feasibility studies in deaf humans who cannot benefit from a cochlear implant.

A. Using non-human, non-deafened, mammals evaluate the effects of chronic microstimulation of the ventral cochlear nucleus with penetrating microstimulating electrodes.

(1) Fabricate or obtain individual discrete wireactivated iridium microelectrodes. These microelectrodes should have exposed contact areas ranging from 200-2000 square microns.

(a) Determine the safe and physiologically effective ranges of stimulation by implanting these microelectrodes into the ventral cochlear nucleus and electrically stimulating for periods of at least 6 hours per day for at least 15 days.

(b) Utilize electrically evoked potentials and/or single unit recordings to monitor the effectiveness and possible damaging effects of stimulation at different sites within the ventral cochlear nucleus. If neuronal depression occurs with continuous, high frequency stimuli, evaluate lower frequency stimuli, intermittent stimulation and methods of conditioning neurons that begin with low charge and/or low frequency stimuli to increase the tolerance of the neurons to chronic stimulation.

(c) Examine the tissue surrounding the electrodes histopathologically to determine the exact location of the electrodes, the most likely cell types activated, and the extent of tissue damage due to implantation and stimulation.

B. Obtain multiple contact site, multiprong, planar, silicon microelectrodes from the Project Officer [For purposes of planning, assume that up to 20 silicon microelectrodes will be available, each of which will require attachment of leads and insulation of the lead attachment site. Ref. - Anderson, D.J., Najafi, K., et. al. Batch-fabricated thin-film electrodes for stimulation of the central auditory system. IEEE Trans. BMEE 36:693-704, 1989.]

(1) Implant these microelectrodes into the most promising sites in the ventral cochlear nucleus for at least 3 months to evaluate the effects of the insertion, the animal's head movement and the passive presence of multiple electrodes under conditions in which the electrode volume is a significant percentage of the tissue volume defined within the penetrating electrode array (e.g., up to 1%).

(2) Examine the tissue surrounding the arrays histopathologically and attempt to determine the cause of any tissue damage.

(3) If significant tissue damage or evidence of lack of positional stability exists, suggest methods for the redesign of the electrode array in a manner to reduce these effects.

(4) If passive implantation of the electrode probes does not produce unacceptable tissue damage or evidence of excessive electrode movement, repeat the electrical stimulation testing, as performed above with discrete wire arrays, but limit stimulus parameters to levels just below those determined to be safe.

(a) As with the discrete wire microelectrodes, stimulate for periods of at least 6 hours per day for at least 15 days.

(b) Examine the tissue within and around the arrays histopathologically and attempt to determine the cause of any tissue damage.

(c) Examine the silicon microelectrodes for evidence of substrate breakage, insulation damage, or corrosion of the electrode contacts.

C. Obtain fresh, unfixed human brainstems.

(1) Insert both discrete wire and silicon planar microelectrodes as above into the ventral cochlear nucleus.

(2) Evaluate the insertion forces required and any damage to the microelectrodes.

(3) Determine methods of effectively penetrating the glial limitans without damage to the microelectrode. Suggest changes in the design of the microelectrode if this appears to be necessary.

D. Prepare for microstimulation studies of the ventral cochlear nucleus of a deaf human based on the aforementioned animal and human cadaver studies.

(1) Design human stimulation microelectrodes including specifications for optimal electrode contact geometry, size, spacing, number and orientation relative.

(2) Design a tool or set of tools suitable for use with the above designed human microelectrodes that will permit safe and convenient insertion of microelectrodes into the ventral cochlear nucleus while simultaneously assuring alignment of the microelectrode contacts to take advantage of tonotopic gradient and the anticipated surviving neuron populations.

(3) Develop methods for microelectrode insertion, for handling their leads, and for making interconnections between microelectrodes, leads and connectors.

(4) Prepare protocols for determining the subjective perceptions of stimulation, range of threshold as a function of microelectrode position, and the minimum allowable spacing of microelectrode contacts and arrays.

E. Test the microstimulation system in a deaf human who must undergo surgery for removal of an acoustic neuroma.

(1) Determine the feasibility of a ventral cochlear nucleus auditory prosthesis by utilizing the aforementioned tools and protocols.