After spending time in the microgravity environment of space, astronauts often suffer from sensorimotor disturbances affecting posture, locomotion and spatial orientation functions when they return to Earth. In some cases, these disturbances can limit an astronaut's ability to perform tasks associated with quickly exiting a vehicle. NASA's next spacecraft design may require them to land in water. Combined with the return to normal gravity, a water landing could further compromise performance if the crew needs to exit the vehicle quickly, especially in conditions encountered on rough seas.
Dr. Ajitkumar P. Mulavara is leading scientists in a project to develop countermeasures to the effects of sensorimotor disturbances after long-duration spaceflight. Mulavara and his colleagues have two objectives. The first is to study human motor and visual performance during disturbances similar to that encountered in varying sea conditions. The second objective is to develop countermeasures based on the use of imperceptible levels of electrical stimulation to the balance organs of the inner ear to assist and enhance the response of a person's sensorimotor function. These countermeasures could be used to reduce the time an astronaut needs to re-adapt during such gravitational transitions, allowing rapid vehicle egress.
Ajitkumar P. Mulavara, Ph.D.
Universities Space Research Association
A 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.
Specific Aims 1) Investigate performance of motor and visual tasks during varying sea-state conditions simulated using a six degree-of-freedom motion platform (6 DOFMP). 2) Develop a countermeasure based on stochastic resonance that could be implemented to enhance sensorimotor capabilities with the aim of facilitating rapid adaptation to Earth’s gravity 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 (Vestibular SR/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.
The objective of two separate studies that were conducted was to document human visual performance during simulated low frequency vehicular motions. In the first study we examined the changes in accuracy in normals when performing a seated visual target acquisition task in which the location of target was offset vertically during horizontal full-body rotation at an oscillating frequency of 0.8 Hz (peak velocity-160 deg/s). The main finding was that the accuracy of performance degraded by one step size and response times varied with target location when acquiring off-plane targets at perturbing frequencies of 0.8 Hz in the horizontal plane. In the second study we examined 112 normals and 45 patients who had been diagnosed with either unilateral vestibular weaknesses or with post-acoustic neuroma resections. We determined their seated dynamic visual acuity (DVA) task performance while the chair was stationary and during vertical full-body oscillations at perturbing frequencies of 2 Hz (peak-to-peak motion of 5 cm). Scores were worse for both groups during the dynamic condition compared to the static condition. Further, in the dynamic condition patients’ scores were significantly worse than normals’ scores. These results indicate that low-frequency perturbations in the horizontal and vertical planes can cause decrements in visual performance and can exacerbate sensorimotor deficits after spaceflight at low frequencies of dynamic movements.
We studied the efficacy of low imperceptible levels of stochastic electrical stimulation of the vestibular system in three separate studies – In the first study VSR efficacy was evaluated during an otolith-canal conflict scenario in a variable radius centrifuge at low frequency of oscillation (0.1 Hz) on physiological responses (using eye movements) and perceptual responses (using a joystick) to track imposed oscillations. Results show that VSR stimulation significantly reduced the phase difference between both the eye counter roll movements as well as the perceptual tracking responses with respect to the imposed tilt in gravito-inertial vector. These results indicate that stochastic electrical stimulation of the vestibular system can improve otolith specific responses in intra-vestibular conflict scenarios like that experienced during spaceflight. In the second study VSR efficacy was evaluated on the cross-planar improvement in balance performance on an unstable surface while stimulating in the mediolateral (ML) axis only. Low imperceptible levels of white noise based binaural bipolar electrical stimulation of the vestibular system improved cross-planar balance performance. The amplitude of optimal stimulus for improving balance performance was predominantly in the range of 30-210 μA RMS. These results indicate that bipolar binaural stimulation may be sufficient to provide a comprehensive countermeasure approach for improving postural stability. In the third study VSR efficacy was evaluated during locomotion on an unstable surface.
Low imperceptible levels of white noise based binaural bipolar electrical stimulation of the vestibular system improved locomotor performance consistent with the SR phenomenon in normal healthy subjects. These results indicate that VSR may be used to maximize locomotor performance during dynamic movements.
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. We have also been working with other PI’s in the HRP and NSBRI Sensorimotor team. 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.
Ajitkumar P. Mulavara, Ph.D.
Universities Space Research Association
VSR stimulation prostheses have earthbound application in rehabilitation of patients with balance disorders, strokes, spinal cord injury, peripheral neuropathy in the legs or a single hand that has been injured, and low vision, and for fall prevention training among seniors, as well as in both military and civilian occupations that involve shipping across oceans and flying in turbulence. This project will also enhance the efficacy of ground-based rehabilitation and training programs.