Project Summary Bilateral loss of vestibular sensation is disabling, with affected individuals suffering chronic disequilibrium, increased risk of falls, and inability to maintain stable vision during head movements typical of daily life. While most individuals with milder loss compensate through rehabilitative strategies enlisting other senses, those with severe loss who fail to compensate have no good therapeutic options. When the vestibular nerves are anatomically intact, as is true in most such cases, electrical stimuli encoding head rotation can drive nerve activity and partially restore vestibular sensation, much as a cochlear implant partially restores auditory sensation. In an on-going first-in-human early feasibility study of six adults disabled by bilateral vestibular hypofunction after ototoxic hair cell injury, we found that vestibular implantation and motion-modulated prosthetic stimulation targeting the implanted ear's three semicircular canals is a feasible, safe and effective treatment for ototoxic loss, as evidenced by directionally-aligned vestibulo-ocular reflexes reliably elicited during >3 years of continuous use, improvements in objective measures of posture and gait performance, and improvement of patient-reported dizziness handicap and vestibular-related disability. On the strength of those results, the United States Food & Drug Administration (FDA) has invited a request for humanitarian device exemption for treatment of ototoxic loss; however, FDA advised that additional data would be required to support expanding availability of this treatment to individuals with idiopathic loss, who make up the largest proportion of bilateral vestibular hypofunction cases. Drawing on a well-established design, intact study team, and protocol that yielded highly impactful results in the early feasibility study of subjects with ototoxic loss, the proposed research program will extend this approach to adults disabled by idiopathic adult-onset bilateral vestibular hypofunction. Results of this research are highly likely to yield broad, sustained impact, either through support of early regulatory approval (if results of vestibular implantation for treatment of idiopathic loss are as favorable as the results already obtained for ototoxic loss) or by providing the necessary foundational data to support design of a subsequent, large-scale pivotal trial of vestibular implantation for idiopathic loss.