Experiments Focus on Human Balance


A set of seven major experiments designed by MIT scientists and colleagues to study the human balance system will be aboard the Space Shuttle Columbia, scheduled for liftoff on October 14, as part of the Space Life Sciences 2 (SLS-2) mission.

One of the experiments will also be connected to the first "scientific assistant" based on artificial intelligence to go up in space. This system, which was developed by MIT and NASA scientists, will comment on the experiment as it is going on to aid the astronauts conducting the experiment(see accompanying story).

Through the overall set of experiments, the scientists hope to address "questions of basic science and of space operational medicine," said Professor Laurence R. Young of the Department of Aeronautics and Astronautics, who is principal investigator for the balance experiments and alternate payload specialist for the SLS-2 mission (see accompanying story).

Specifically, the scientists plan to explore how the human balance system adapts to a weightless environment and how those adaptations may contribute to space motion sickness, which affects more than 50 percent of all astronauts.

The information gleaned about the balance system could also lead to better diagnostic techniques and treatments for people in the general population with balance disorders. "At some point in our lives, 50 percent of us will suffer from some sort of balance disorder such as dizziness, the inability to stand, the inability to walk and illusory sensations of motion," said Daniel M. Merfeld, acting principal investigator for the experiments and a research scientist at MIT's Man-Vehicle Laboratory. (All of the balance experiments are organized through the Man-Vehicle Laboratory, which is affiliated with both the Center for Space Research and the Department of Aeronautics and Astronautics.)

Several of the balance experiments have flown on other missions, most recently Space Life Sciences 1 (SLS-1) in 1991. Data from those earlier missions "gave us a much deeper understanding of motion sickness" and have led to some research and clinical applications, said Charles M. Oman, a co-investigator for the current set of experiments and director of the Man-Vehicle Laboratory. Nevertheless, because of variables like the necessarily small number of subjects in space and wide variations between those subjects, the scientists are eager to collect more data with new runs aboard SLS-2.

In addition to Drs. Oman and Merfeld, other co-investigators on the balance experiments are Drs. Carlo J. DeLuca and Serge Roy, both of Boston University, Kenneth E. Money of Canada's Defense and Civil Institute of Environmental Medicine, and Doug G.D. Watt of McGill University.

Balance in Space

Human balance is maintained by a variety of different inputs to the brain, such as visual input from the eyes and tactile input from our body parts (feet touching the ground help communicate to the brain that you're standing). In addition, input on gravity and motion are provided by two organs in each inner ear cumulatively known as the vestibular system.

What happens to the balance system, however, when one important input-gravity-is missing? Will the body begin to rely more on other inputs? If so, how long does it take for these adaptations to occur?

The scientists designed a barrage of experiments to answer these and related questions. All focus on how the gravity-sensitive vestibular system adapts to a weightless environment, and interacts in space with other sensory organs related to balance. Three experiments are described below.

Rotating Dome

Many of us have experienced the illusion that this experiment artificially induces. For example, imagine sitting on a train at the station, waiting for a trip to start, when the train next to you begins to move. For a few seconds, you might think that your train is moving, rather than the train beside it. This phenomenon, called vection, occurs because your eyes see motion, and send a cue to the brain that you're moving. However, most of the time cues from the vestibular and other systems kick in to tell the brain that you're not moving, and the eyes are overruled.

But what happens when vection is induced in space? Without gravity, will a person give more importance to visual cues? While scientists have some answers to these questions from other shuttle missions, they would like to learn more.

In the rotating dome experiment, the astronaut puts his or her face into a dome patterned with randomly spaced colored dots. The dome then starts rotating at the command of a second astronaut who runs the experiment. And slowly but surely, subjects begin to feel like they're rotating, rather than the dome. They're experiencing vection.

On Earth, "almost all of our subjects feel like they're rolling down toward their left shoulder," Dr. Merfeld said. However, because input on gravity from the vestibular system tells the subject he's not going anywhere, "most of our subjects on Earth do not feel like they're rotating head over heels."

In contrast, "when you go into space, the visual system still induces vection, but the vestibular system has no gravity to work with," Dr. Merfeld said. As a result, "most astronauts feel that they are rotating head over heels."

The rotating dome experiment measures a number of variables associated with this phenomenon, including astronauts' perceptions of how quickly they're rotating and reflexive responses like eye and neck movements. "The goal is to get enough measurements over the course of the flight to track any adaptations [of the visual system]," Dr. Merfeld said.

For example, he said, "if visual cues become more dominant in space, as is hypothesized (and supported by data from other missions), the astronauts' sense of rotation should get stronger and stronger and stronger over the course of the flight."

As with all other experiments aboard SLS-2, pre- and post-flight tests of the rotating dome will also be conducted. Such studies "give us some idea of what each individual's response is on Earth, which we can compare to what happens to the individual in flight," Dr. Merfeld said. Post-flight tests help the scientists track the astronauts' re-adaptation to gravity. For these reasons, "the pre- and post-flight studies are probably just as important as the inflight tests," he said.

The rotating dome experiment aboard SLS-2 will hold a separate distinction over other experiments because it will be connected to the first "scientific assistant" based on artificial intelligence to go up in space.

The Astronaut Science Advisor (ASA), developed by scientists at MIT and NASA's Ames Research Center, will take the data coming from the rotating dome experiment and make comments on it as the experiment is going on. The astronaut running the experiment can then use those comments to modify the experiment if, for example, unusual data are recorded or there are time constraints.

Rotating Chair

This experiment studies an eye reflex that the scientists believe should work differently in space because the reflex is influenced by the vestibular system, which is sensitive to gravity. This reflex, known as the vestibulo-ocular reflex (VOR), "is what allows us to see while we're moving," Dr. Merfeld said. (Without the VOR, objects around us would appear blurry as we moved.)

To induce the VOR and the eye movements associated with it, an astronaut is either rotated quickly in a chair then suddenly stopped, or, in a variation of this, the astronaut is rotated and stopped, and his or her head is suddenly pitched forward out of the plane of rotation. The VOR is measured for both variations. Earlier runs of this experiment in space (SLS-1 in 1991 and IML-1 in 1992) showed that "some of the dynamics of the VOR do change in weightlessness," said Dr. Oman, who is principal investigator for the experiment.

For example, when the experiment was conducted during the IML-1 mission, eye movements associated with the VOR were found to be more pronounced than they were when the astronauts were tested on Earth. Furthermore, Dr. Oman said, "we found a preliminary correlation between changes in the VOR and space motion sickness. The people that showed the greatest changes in the VOR were most sick over the course of the mission."

With SLS-2, the scientists hope to find "whether these earlier results will be confirmed," he said.

US Laboratory Sled

The sled experiment was designed to directly stimulate one of the two sets of vestibular organs-the otoliths-with the goal of learning more about how these organs work. The scientists are especially interested in the otoliths because these are the vestibular organs that detect gravity.

However, the otoliths also detect linear acceleration, or how quickly you pick up speed while running, etc., and it is impossible on Earth to separate the otoliths' response to these two stimuli.

In space, however, "any measurements the otoliths give are related to acceleration," Dr. Merfeld said. As a result, he asked, immediately after a flight, will an astronaut's otolith organs be more weighted toward acceleration? (Data from other flights on this general question are inconclusive.)

To explore this, astronauts before and after the flight (though not during) will be accelerated horizontally on the US Laboratory Sled and their perceptions of movement and eye reflexes will be measured. The sled, which consists primarily of a chair on wheels that runs along two rails, was developed by NASA based on a prototype built in the Man-Vehicle Laboratory.

"The sled is one of our most important experiments," Dr. Merfeld noted, "because it's directly stimulating the sensory organs that are most directly affected by the absence of gravity in flight."

Key People

Many people at MIT have worked long hours in preparation for the SLS-2 mission.

For example, said Dr. Oman, "a large number of MIT students have been directly involved in training and testing the SLS-2 astronauts." These students, who will also conduct postflight tests of the crew, include senior Michelle Zavada and graduate students Keoki Jackson, Chris Pouliot, Karl Schultz, Scott Stephenson, Corrie Lathan and Tom Klemas. Juan Mendoza and Karla Polutchko, who graduated last June, were also part of the team. Nicolas Groleau, who received his doctorate last fall, worked on the Astronaut Science Advisor while at MIT and joined the ASA team at NASA's Ames Research Center after graduation.

In addition to Drs. Young, Oman and Merfeld, key staff at the Man-Vehicle Laboratory (MVL) include Drs. Alan Natapoff, Lyman Hazelton, Steve Robinson (visiting MVL from NASA Langley this year), James Costello, Sherry Modestino, Bev Linton and Kim Tseko.

Finally, an engineering team from the Center for Space Research led by William F. Mayer designed and built most of the flight hardware for the vestibular experiments. Dr. Mayer's team included Bob Goeke, Pete Tappan and James O'Connor.

Overall, the MIT SLS-2 group is optimistic about the upcoming mission and their experiments, which are funded by NASA. Concluded Dr. Merfeld: "We're really excited and well prepared, and we're going to get great data."

A version of this
article appeared in the
October 6, 1993

issue of MIT Tech Talk (Volume
38, Number
9).


Topics: Aeronautical and astronautical engineering

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