The Past Present and Future of Bionic Devices Restoring Cranial Nerve Function
While cochlear implants have achieved widespread clinical application, with >120,000 devices in use worldwide, other potential applications for neuroelectronic interfaces in the head & neck have so far received less attention. Considering the important role cranial nerves play in communication, navigation, and other vital or life-enhancing functions, neural interfacing applications in the head and neck are especially attractive targets for development of new neuroelectronic devices. This lecture will briefly review common and contrasting features of several such devices under development, then provide a detailed review focusing on one such effort the Johns Hopkins Multichannel Vestibular Prosthesis (MVP) designed to restore semicircular canal sensation (inner ear transduction of head rotation). Bilateral profound loss of vestibular sensation can be disabling, with affected individuals suffering illusory drift of visual fields during head movement, chronic disequilibrium and postural instability caused by failure of vestibulo-ocular and vestibulo- spinal reflexes.1 Sensory substitution devices can augment input to postural reflexes, but there is currently no clinically available treatment that can restore a normal vestibulo-ocular reflex (VOR) in patients who have lost all semicircular canal function. A neuroelectronic prosthesis that emulates normal sensory transduction of head rotation could significantly improve quality of life for these patients. Like the semicircular canals in a normal ear, such a device should excite a pattern of activity on the implanted labyrinth’s three ampullary nerves to encode 3 orthogonal components of head rotation. Drawing on classic experiments by Suzuki, Cohen and others that demonstrated the ability to evoke eye movements via the VOR through electrical stimulation of ampullary nerves,2,3 Gong and Merfeld created a single-channel head-mounted vestibular prosthesis able to measure head movement in one direction and elicit eye movements in canal-plugged animals.4-8 The Johns Hopkins Vestibular NeuroEngineering Lab’s goals are to extend this approach toward clinical application by (1) creating a multichannel vestibular prosthesis able to restore the 3-dimensional (3D) VOR for all directions and speeds of head rotation, (2) evaluating the device’s performance in animals that accurately reflect the condition of human patients with bilateral vestibular loss due to aminoglycoside ototoxicity, and (3) developing a model of electrical current flow in the implanted labyrinth to facilitate optimal design of electrodes and stimulus protocols.
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