Astronauts love doing zero-G stunts on the International Space Station, but only after the urge to vomit from space sickness has faded. Now fish, snails and other animals could help understand whether living in space can create long-term or even permanent damage in the inner ear.
Scientists found that the inner ears of toadfish have high sensitivity to even the slightest movements, and that the toadfish brain can both boost and reduce signals from the sensitive inner ear. Because humans have very similar ear structures to these and other animals, toadfish could provide clues about how astronauts' inner ears adapt to spaceflight.
"You can drop a fish's inner ear right into a human and it fits right in there," said Richard Boyle, a biologist at the NASA Ames Research Center in Moffett Field, Calif.
Humans do eventually adjust to living in a weightless environment. But their inner ears have to go through a second round of readjustment to full Earth gravity once they return — and scientists still don't know how easily the inner ear can make that switch after longer space missions.
Boyle's work is detailed in a study published in the February issue of the journal Proceedings of the National Academy of Science. His co-researchers included lead author Stephen Highstein, a marine biologist at the Marine Biological Laboratory in Woods Hole, Mass., and Richard Rabbitt, a bioengineer at the University of Utah in Salt Lake City.
Getting your space bearings
Living beings evolved inner ears with hair cell sensory organs that can detect sounds as well as movements of the head. The balance sensory organs include tiny ear stones made of calcium carbonate that act as small weights because of gravity.
When the head moves, the inertial lag of such stones creates force on the hair cells — not unlike how car passengers will feel the press of inertial lag when their vehicle jerks forward from a standstill. That signal gets amplified so that the brain automatically registers even the smallest head movements. The inner ear similarly detects bigger events such as the sudden drop when a person steps off a curb.
The system works beautifully on Earth, but quickly leads to disorientation and nausea for spaceflyers who first experience weightlessness in space.
"When you're up in space, you still have mass but no weight," Boyle told SPACE.com. "So you can't detect gravity, and the structures sensitive to inertial acceleration and orientation with respect to gravity lose their properties."
Inner ears go haywire for a few days before the brain takes charge to regain a sense of balance. The nervous system also begins boosting the signal strength from the inner ear, so that the human or animal becomes hypersensitive to movement.
To adapt or not adapt
That works until astronauts return to Earth and become incredibly sensitive when just taking a step or turning their heads. Boyle has seen a similar hypersensitivity in snails that have returned to Earth after launching aboard Russian space missions.
Humans' ability to adapt quickly to the feeling of zero-G has proved a blessing for now, even if it baffles scientists. Our species has necessarily adapted to changes in predators and climate throughout history, but there's no obvious reason for why it should adapt so quickly to changes in gravity.
"The brain probably begins right away," Boyle said. "It's amazing when you think that for all of human history on Earth, gravity has always remained the constant."
Boyle also noted the darker possibility that the brain's eagerness to adapt to the lack of Earth's gravity may prove harmful in the long run. Perhaps a point of no return exists where the inner ear and brain adjusts permanently to zero-G, and the body simply breaks down and absorbs the ear stones.
Some human patients on Earth already suffer from dizziness and other conditions because their ear stones begin breaking down. But scientists remain uncertain whether astronauts could suffer the same fate from living in space too long.
The current record for living in space goes to cosmonaut Valeri Polyakov, who spent almost 438 days aboard the Russian Mir space station. Yet Boyle noted that scientists often lack access to astronauts after they return to Earth, and so it's difficult to carry out long-term health studies on space travelers.
Living long and prospering in space
The recent study on toadfish necessarily took place on Earth because of its complexity in monitoring the animals' brain signals. But scientists hope to someday see similar experiments take place in space.
"The experiment I'd like to do right now is a short-duration launch profile done on a suborbital flight, where you record neural activity during the acceleration of launch, the initial periods of microgravity, and during the return," Boyle explained.
Fish might also prove a good candidate for longer studies that send animals to Mars or beyond, given that they can live for 40 or 50 years. Lab rats or mice would have long since died.
Doing such studies before sending humans on longer-duration space missions would seem prudent at the very least, according to Boyle.
"When something goes wrong [with the inner ear], patients in the clinic are on the ground and they don't know if they're up or down or swimming or flying," Boyle said. "In the wild, an animal would be dead."