Approximately 18 million people develop symptoms associated with inner ear (vestibular) disorders each year; a number that increases with age. It is estimated that more than 70% of individuals with vestibular loss will not have a resolution of symptoms after two weeks. Given the rate of growth of the elderly population in the United States, it is likely that the incidence and prevalence of the dizziness due to vestibular disorders will increase. One disabling symptom is the inability to see clearly during head motion (poor gaze stability), as when driving a car. For a majority of these individuals, vestibular rehabilitation remains the only treatment option. Unfortunately, vestibular rehabilitation strategies to improve gaze stability have not been adequately investigated. Persons with vestibular hypofunction use different strategies to improve their ability to see clearly during a head motion. The strategies employed are improved when the head motion can be predicted, as in self-generated head rotations. Although many studies have demonstrated the adaptive capacity of the vestibular system to passive head motion, self-generated head motion - which represents a more functional aspect of daily living and is congruent with the vestibular rehabilitation tactics provided by clinicians - has not been thoroughly investigated. In addition, studies of the auditory and optokinetic systems provide compelling evidence that neural plasticity is enhanced when the error signal driving adaptation is adjusted incrementally throughout training. Few studies, however, have investigated the effect of using an incrementally adjusted error signal for vestibulo-ocular reflex (VOR) adaptation in people with unilateral vestibular hypofunction (UVH). We hypothesize that the efficacy of vestibular rehabilitation can be enhanced by presenting an incrementally adjusted error signal during training. We will compare the rate and magnitude of VOR adaptation for self-generated head motion when using an incremental versus a sudden-task demand. The saccadic oculomotor system is also highly modifiable and has been shown to enhance a deficient VOR, yet it is unknown whether saccade adaptation is a useful means to assist gaze stability during head motion. We hypothesize and that the saccadic system can be modified to assist with gaze stability during head rotation and will investigate this in people with UVH. Finally, we will model the various error signals used in the VOR and saccade adaptation paradigms. This is important for the long-term goals of this project, which are to determine the adaptive preferences that individuals with vestibular loss use to improve gaze stability.
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