Recent studies have shown that reflexes and perception evoked by vestibular cues can utilize qualitatively different mechanisms; yet there is a paucity of knowledge regarding vestibular perception. This may be why large percentages of vestibular patients report perceptual deficits that are undiagnosed. Regardless of explanation, the underlying causes of many perceptual deficits are not diagnosed. With the exception of the subjective visual vertical test, quantitative clinical tests focus exclusively on reflexive responses. However, the best way to assay undiagnosed motion perception symptoms is via perceptual testing. Therefore, the clinical goal for the research proposed herein is to establish precise efficient methods to quantify motion perception in general and, more specifically, to assay perceptual thresholds. To achieve these clinical goals, we propose patient testing alongside basic science studies. Specifically, one basic science goal of this proposal is to determine thresholds for rotation, translation, and tilt as functions of frequency. To our surprise, tilt thresholds evoked by dynamic tilt stimuli have never been reported. Furthermore, our knowledge of rotation and translation thresholds as a function of frequency - crucial both for clinical testing and for a scientific understanding of the dynamics of vestibular perception - is limited. Using threshold tests, we also propose to study another fundamental characteristic of vestibular perception. Specifically we propose to study the coordinate frame(s) utilized for motion perception. While we focus on perception, we also propose to measure VOR thresholds. To accomplish all of the above goals, we propose a comprehensive set of studies using patients, monkeys, and normals to achieve the following specific aims: SA 1. Measure perceptual thresholds in normal humans for A) rotations about an earth-vertical axis (canal only), B) for translations (otolith only), and C) for tilts (canal & otolith) across a wide range of frequencies. SA 2. Measure both perceptual and VOR thresholds for yaw rotation (canal only), inter-aural translation (otolith only) and roll tilt (canal and otolith) in rhesus monkeys. SA 3. Measure perceptual thresholds for rotation, translation, and tilt in patients suffering known vestibular disorders.