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Predicting Sensorimotor Adaptation to Altered Gravity by Measuring Vestibular Perceptual Thresholds (First Award Fellowship)

Principal Investigator:
Torin K. Clark, Ph.D.

Massachusetts Institute of Technology

In space exploration missions, astronauts experience a series of altered gravity environments to which they must adapt their sensorimotor systems. While all astronauts eventually adapt, there appear to be large differences between individuals in how quickly and effectively adaptation occurs, which are poorly understood. Being able to predict these differences is critical to help develop personalized training countermeasures and reduce the likelihood sensorimotor impairment impacts mission success and safety.

Dr. Torin K. Clark and colleagues aim to better understand and predict individual differences in sensorimotor adaptation to altered gravity. Specifically, the researchers will test if an individual’s perceptual thresholds (i.e. how small of a motion stimulus can be correctly identified as being motion to the left or right), which are a measure of sensory noise, can predict their adaptation capacity. It is believed that higher sensory noise levels may slow adaptation because the brain cannot as easily distinguish changes in the environment that require adaptation.

NASA Taskbook Entry

Technical Summary

Astronauts must maintain appropriate sensorimotor function for tasks such as piloted landing or docking, as well as vehicle egress and extra-vehicular activity. However, the altered gravity environments regularly experienced during space exploration missions require sensorimotor adaptation. While nearly all astronauts eventually adapt there appears to be substantial individual differences in how quickly and effectively this adaptation occurs which are not well understood. It is thought that sensory conflict – unexpected sensory signals – when sustained, result in adaptation. However, even in an unchanged environment sensory signals are noisy and will lead to sensory conflict. The central nervous system must avoid inappropriately adapting to noisy sensory signals when the environment has not changed. Presumably if the conflict is consistent and sustained, the central nervous system can become more certain the conflict is due to a change in the environment and not created by sensory noise.

Thus, we hypothesize that vestibular perceptual "noise" is a limiting factor in sensorimotor adaptation. If noise levels are high, a more sustained sensory conflict is required before adaptation is appropriate. We have extensive experience measuring perceptual "noise" through vestibular perceptual thresholds (i.e. how small of a stimulus can be correctly identified when being moved to either to the left or right). Substantial inter-individual differences exist in vestibular perceptual thresholds which may help explain the poorly understood differences in altered gravity adaptation. Thus we aim to experimentally test whether individuals with higher "noise" levels, as measured by perceptual thresholds, are indeed more challenged to adapt to an altered gravity environment.

A hyper-gravity environment on our Eccentric Rotator centrifuge is utilized as an altered gravity test-bed to study adaptation. Future experiments may extend to study hypo- and microgravity adaptation environments. In the hyper-gravity environment, subjects are then tilted about their roll axis. Operationally relevant tilt perception and manual control tasks will serve as metrics to track adaptation. Initial exposure to hyper-gravity causes perceptual overestimation and manual control errors, however over time the central nervous system adapts to the novel environment and performance improves. The adaptation will be characterized for each subject and compared to their vestibular perceptual thresholds. Thresholds will be measured for interaural translation, roll rotation, and roll tilt to better understand along which pathways (otolith, semicircular canal, or more central) the noise level is most predictive of adaptation.

We aim to deliver a prediction tool to identify individuals which may have difficulty adapting to altered gravity environments and may benefit from sensorimotor training. The tool will require less than one hour of test time and will involve benign vestibular perceptual threshold tests. The proposed set of experiments will greatly enhance our understanding of the drivers and limiting factors for sensorimotor adaptation to altered gravity environments, and may inform the development of individualized sensorimotor training countermeasures.

Earth Applications

As much as 40% of the US population seeks medical attention for dizziness at least once in their lifetime. In particular, 25-35% of individuals over the age of 65 years fall at least once per year due to functional balance deficiencies. Sensorimotor adaptability programs have Earth applications in the rehabilitation of patients with balance disorders and to help reduce falls in the elderly. A greater understanding of how sensorimotor adaptation is limited and a capability to predict an individual's adaptability could be critical for designing rehabilitation programs. Thus the sensorimotor adaptability programs developed here for astronauts in altered gravity environments remain relevant for different clinical populations.

This project's funding ended in 2015