We propose to develop technology that will better convey both the spectral and the temporal features of sound to persons with cochlear nucleus auditory prostheses. An auditory prosthesis at the level of the cochlear nucleus (""""""""Auditory brainstem implant, ABI"""""""") can restore useful hearing to persons who lack functional auditory nerves, but their speech perception is much poorer than that of most users of cochlear implants, and is especially poor for persons afflicted with Type 2 Neurofibromatosis (NF2), the most prevalent etiology for bilateral loss of the 8th nerves. The NF2 patients also exhibit high modulation detection threshold which is the only psychoacoustic variable that has been found to distinguish the NF2 users of the ABIs from those whose deafness is of other etiologies. The available data suggests that all users of ABIs, including those with NF2, would benefit from improved modulation detection. Most ABIs utilize an array of macroelectrodes on the surface of the brainstem, and this array allows some, but limited access to the tonotopic organization of the cochlear nucleus. In animal studies, penetrating microelectrodes are better able to convey the spectral information of sound, and NF2 patients whose auditory brainstem implants includes arrays of surface and penetrating electrodes derive benefit from the hybrid array. However, our experience with the existing penetrating array has revealed several issues that we will address in the proposed studies. We will design an array of 96 microstimulating electrode sites on 24 multisite silicon substrate shanks microelectrodes, that will insure placement of at least 16 microstimulating sites within the human ventral cochlear nucleus, even with the known uncertainly as to where to position the array after removal of the 8th nerve tumor. The probes comprising this array will be fabricated by deep reactive ion etching (DRIE) photolithography, which yields probe shanks that are sufficiently durable to penetrate the glia limitans overlying the human cochlear nucleus. We will verify the mechanical durability of the array by repeated insertions into cat spinal cords. Arrays of DRIE probes will be implanted chronically into cats'cochlear nucleus to evaluate possible tissue damage during implantation of the mechanically robust DRIE probes. The standard of comparison will be Michigan-style probes that are much thinner than the DRIE probes. Also in a cat model, we will determine if and how modulation detection by Type 1 multipolar cells of the ventral cochlear nucleus can be enhanced, relative to that with a 250 Hz charge-balanced pulsatile stimulus used in the present clinical ABI systems. We will evaluate the merits of a higher stimulus pulse rate (500 and 1000 pps) and also of analog electrical stimulation. To support the activities described above, we will develop a 64-site, 4-shank recording array suitable for chronic implantation into the cats'inferior colliculus.
An auditory prosthesis implanted at the level of the cochlear nucleus (an """"""""Auditory brainstem implant, ABI"""""""") can restore useful hearing to persons who lack functional auditory nerves, but their speech perception and recognition of environmental sounds is much poorer than that of most users of cochlear implants, and is particularly poor for persons afflicted with Type 2 Neurofibromatosis (NF2), the most prevalent etiology for bilateral loss of the auditory nerves. Typically, the nerves are destroyed during surgical resection of each of the bilateral vestibular schwannomas that are typical of this condition. The ABIs now in use do not efficiently convey to the users the temporal modulation of sound and persons with NF2 fare particularly poorly in this respect. In an animal model, we will compare several protocols for encoding sound into the electrical stimulation that is delivered to the stimulating electrodes. Our objective is to develop an improved method of conveying the temporal features of sound to ABI users. Most ABIs utilize an array of macroelectrodes on the surface of the brainstem that allows some, but limited access to the tonotopic organization of the cochlear nucleus. In animal studies, penetrating microelectrodes are better able to convey the spectral information of sound and NF2 patients whose auditory brainstem implants includes an array of surface electrodes and an array of penetrating electrodes derive benefit from the hybrid array. Also, the penetrating electrodes have proved useful in those instances when the patient does not receive auditory percepts from the surface electrodes. However, our experience with the existing version of the penetrating array has revealed a number of issues that we will address in the proposed studies. In view of the prevalence of NF2 relative to that of other causes of bilateral loss of the auditory nerves, it is most unfortunate that persons with NF2 have not obtained as much benefit from their auditory brainstem implants as have patients whose deafness is of other etiologies. However, the prevalence of NF2 is approximately 1 in 40,000 live births with a high probability of bilateral acoustic tumors, and so while the condition fortunately is quite rare, in developed countries alone, there are many thousands of persons who can benefit from these devices, and with minimal risk and discomfort in addition to those related to the surgical removal of the tumors, since the devices are implanted into the cochlear nucleus during the same surgical procedure in which the tumor is surgically removed. Thus there is a need for improvements to the implants themselves and for methods of encoding sound into electrical stimulus that are optimized for an auditory prosthesis implanted in the cochlear nucleus.
