Crewmember adapted to the microgravity state may need to egress the vehicle within a few minutes for safety and operational reasons after g-transitions. During exploration class missions the interactions between a debilitated crewmember during re-adaptation to gravity and the prevailing environmental constraints imposed during gravitational transitions may lead to disruption in the ability to perform functional egress tasks. At present, no operational countermeasure has been implemented to mitigate this risk. Therefore, the overall goals of this project are to: 1) investigate performance of motor and visual tasks during simulated perturbation conditions and 2) to develop a countermeasure based on stochastic resonance to enhance sensorimotor capabilities with the aim of facilitating rapid adaptation during gravitational transitions following long-duration spaceflight. Stochastic resonance (SR) is a mechanism whereby noise can assist and hence enhance the response of neural systems by detecting sub-threshold signals. SR thus enables the enhanced detection of relevant sensory signals. SR stimulation using imperceptible noisy vibratory or electrical stimulation has been shown to improve balance function in normal young and elderly subjects, stroke patients, and in the rehabilitation of functional ankle joint instabilities. This project specifically has used imperceptible levels of electrical stimulation of the vestibular system (VSR) as the proposed countermeasure to improve performance in egress tasks. The project has also conducted a series of studies to document human visual performance during simulated low frequency dynamic perturbations and further investigate the efficacy of VSR stimulation on physiological and perceptual responses during otolith-canal conflicts and dynamic perturbations. Goal 1: The objective of two separate studies that were conducted was to document human visual performance during simulated wave motion in the 0.1 to 2.0 Hz range. The main findings of both studies showed that dynamic visual acuity (DVA) is reduced in the vertical plane at frequencies of 2 Hz and in the horizontal plane at frequencies of 0.8 Hz. DVA varies with target location, with acuity optimized for targets in the plane of motion. Thus, low frequency perturbations in horizontal and vertical planes can cause decrements in visual performance that may be exacerbated after long-duration spaceflight. Goal 2: For determining efficacy of VSR stimulation on physiological and perceptual responses during otolith-canal conflicts and dynamic perturbations we have conducted the following series of studies: 1. We have shown that imperceptible binaural bipolar electrical stimulation of the vestibular system across the mastoids enhances balance performance in the mediolateral plane while standing on an unstable surface. We have followed up on the results of this previous study showing VSR stimulation improved balance performance in both mediolateral and anteroposterior planes while stimulating in the mediolateral axis only. 2. We have shown the efficacy of VSR stimulations on enhancing physiological and perceptual responses of whole-body orientation during low frequency perturbations (0.1 Hz) on the ocular motor system using a variable radius centrifuge (VRC) on both physiological (using eye movements) and perceptual responses (using a joystick) to track imposed oscillations. The variable radius centrifuge provides a selective tilting sensation that is detectable only by the otolith organs providing conflicting information from the canal organs of the vestibular system (intra-vestibular conflict). These results indicate that VSR can improve performance in sensory conflict scenarios like that experienced during spaceflight. 3. We have showed the efficacy of VSR stimulation to improved balance and locomotor control on subjects exposed to continuous, sinusoidal lateral motion of the support surface while walking on a treadmill while viewing perceptually matched linear optic flow. 4. We have developed and tested a practical methodology of finding the optimal amplitude of VSR stimulation using perceptual thresholds indicated by seated subjects using a game pad in response to applied electrical vestibular stimulation with sinusoidal signals of varying peak amplitudes. Preliminary analysis of these data indicated that the optimal amplitude of stimulation was found to be in the range of 10 to 20% of their maximum probability of detecting the signal. 5. We have developed a methodology to detect the functional vestibular cortex using a magnetic resonance imaging (MRI) compatible device and this study is ongoing to determine the effects of VSR stimulation on brain function. 6. We have shown the safety of short term continuous use of up to 4 hours of VSR stimulation and its efficacy in improving balance and locomotor function in Parkinsonian Disease patients. Thus, maximizing postural, locomotor, and perceptual performance during dynamic movements will have a significant impact on development of vestibular SR as a unique system to aid recovery of function in astronauts after long-duration spaceflight or in people with balance disorders. The data obtained in this project will aid in the design of a countermeasure system used for improving functional tasks during and after g-transitions. The VSR methodology developed in the current project is being integrated with the sensorimotor adaptability (SA) training modalities being developed by Dr. Bloomberg and his team to improve its efficacy. The operational version of this countermeasure will be available as a skin patch vestibular prosthesis during spaceflight that will further act synergistically along with the pre-and in-flight SA training and provide an integrated, multi-disciplinary countermeasure capable of fulfilling multiple requirements making it a comprehensive and cost effective countermeasure approach.