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Kathleen Cullen believes we all have a sixth sense. Or at least this is how the Department of Physiology researcher thinks of the vestibular system, the sensory system located in the inner ear. "The vestibular system tells us how our heads are moving in space," explains Cullen. When perceiving the world around us, we have to integrate information from all our senses. A problem with this integration occurs when our vestibular sense disagrees with our visual sense, called "sensory conflict," which causes carsickness and a newer phenomenon, space sickness. As glamorous as space travel may seem, the harsh reality is that astronauts spend a large part of their time among the stars feeling nauseous. To understand how to prevent space sickness and how our vestibular sense works with other systems, Cullen has been recruited by NASA as a member of the National Space Biomedical Research Institute.
The vestibular system consists of two types of sensors in the ear that detect either rotational or linear movement. Three semicircular canals sense which way the head is turning, while sac-like organs, called otoliths, sense linear movement. The canals and otoliths are filled with fluid. When our heads move, the motion of these fluids results in the bending of hair-like structures called cilia, which are attached to cells; this stimulates the cells and communicates the movements to our brains.
Carsickness occurs if a passenger's vestibular system detects movement while their eyes, looking at objects inside the vehicle, simultaneously tell our brains that they are stationary. On the other hand, drivers — who direct their gazes outside of the car — never get carsick because their vestibular and visual cues agree. In space, astronauts experience sensory conflict called an "inversion illusion." "An astronaut will feel that he is upright and then will see another astronaut who is oriented the other way and this is very provocative for motion sickness," says Cullen. Now space agencies mark top and bottom within the shuttle. But in early space flights the importance of reference points wasn't recognized until NASA realized that astronauts were getting violently ill.
The weightless environment has other disorienting effects on the vestibular system. On earth, gravity exerts a constant force on the sensors in the otolith sacs, so that if we tilt our heads or lie down, we know that our head positioning has changed. "During space flight, astronauts experience disorientation and nausea from visual stimulation without corresponding gravitational vestibular stimulation," explains Cullen. Many of us have experienced the same sensation right here on Earth after a few beers, but for slightly different reasons: the alcohol causes vestibular stimulation without corresponding visual stimulation. Alcohol decreases the density of blood, which causes the cupola, a piece of tissue in the vestibular canals, to float and stimulate the sensors. Our vestibular system senses movement but our visual system does not, which causes the dizziness we associate with excessive imbibing. A good night's sleep may cure drunkenness, but for astronauts, the suffering continues.
There is hope, however. Through Cullen's research we now understand better how the brain combines cues from the vestibular system with other sensory information. She has demonstrated that the brain can distinguish between actively and passively generated head movements. "An active movement is one that we generate ourselves, like nodding, and a passive movement occurs as a result of something or someone else's actions, like when your head moves through space as a passenger in a car," says Cullen. She has shown that these movements can be differentiated by certain neurons that receive information directly from vestibular sensors. This finding was surprising because in both active and passive movements, the actual stimulation to the vestibular sensors may be the same, but there are differences at another level — for example, an active movement is preceded by the intent to move. Thus the brain is able to integrate information from the vestibular system with information from other systems to generate a specific response.
Astronauts will feel better in space if they can rely more on all their other senses and less on their miscuing vestibular sense. Researchers are creating training programs to equip astronauts with the neurophysiological tools needed for space travel. Such training has already proved successful for pilots, who can suffer from a number of vestibular illusions. One of the most common illusions occurs when pilots are flying in a circle at a constant speed. "The vestibular system is only sensitive to acceleration, so at a constant velocity, the pilot doesn't sense motion anymore," explains Cullen. This is because the brain quickly adapts to the signal as the fluid in the ear begins to move at the same rate as the plane and no longer stimulates the sensors. "As the pilot starts to bank out of their rotation, he has a sense that he is actually turning in the opposite direction. He may try to compensate for this, which could cause a graveyard spiral," says Cullen. Pilots train on flight simulators to prevent these types of accidents. Similar training for astronauts might be the key to relieving space sickness.
In addition to flight simulators, specific exercises prepare astronauts for space travel. Astronauts may train by running through obstacle courses while wearing goggles that distort their perception. The goal is to make the vestibular system extremely adaptable so that the space environment is less uncomfortable. This is a high priority for NASA, which hopes to have a manned flight to Mars after the success of the Rover landing. "A lot of the time, our astronauts don't feel so good while they're in space. They're nauseous and this really hurts the productivity of the flight," says Cullen. Research efforts that lead to greater understanding of the vestibular system is only good news for future astronauts.
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