This study represents the extension of an ongoing project on the evaluation of the many effects of vestibular dysfunction in man. It will attempt to clearly characterize the disturbances in vestibulo- ocular and vestibulo-spinal function that arises from both acute and chronic loss of portions (or all) of one (or both) vestibular nerves. The magnitude of various functional abnormalities and the time course of their partial or complete recovery will provide insights into the extent of compensation for injury of portions of this sensory system. Comparison of test results against a symptom scale will hopefully clarify the role of various subportions of the vestibular system in the genesis of the common human complaints of vertigo and postural imbalance. Utilization of a large group of subjects with known post-surgical lesions of a labyrinth, complete vestibular nerve, superior vestibular nerve, or both vestibular nerves will also allow the assessment of the specificity and sensitivity of standard and newer vestibular function tests in predicting the side and extent of vestibular damage. Methodology will include caloric tests of canal function, the ocular countertorsion reflex (OCR) as an indicator of otolith function, galvanic stimulation of the vestibular nerve, a quantitative ataxia test battery and other methods. Of special emphasis will be the development of tests of the long-neglected otolith organs with comparison of the suitability of the OCR, parallel swing and an auditory evoked cortical potential response test in assessing their integrity. Pharmacologic studies on normals and vestibular-damaged subjects will also define the site of action of various anti-vertigo medications. This combined assessment of the effects of vestibular dysfunction and reliability of various methods in assessing functions of subportions of the vestibular organs will hopefully improve the diagnostic precision with "dizzy" patients and refine selection methods for the surgical relief of vertigo.