In space, our bodies don't quite operate as they're supposed to. Before we make the next great leap, we must appreciate how weightlessness toys with our perceptions.
It’s here! The first manned mission to Mars. The year is 2029, and Jen, a healthy young adult, has been selected for the arduous journey. The outward voyage is projected to take eight months — that’s if everything goes to plan. This mission has pushed physicists, engineers and the project leaders to the brink. But despite all this planning, at the end of the day the success of the mission will depend on Jen’s ability to handle the isolation and cope with the physiological demands of weightlessness. Her body will be pushed beyond the limits she believed existed, and her senses shocked by the unfamiliar stimuli to which she is exposed.
Different species are specialised to detect different environmental signals; hence, they have different experiences of the world. For instance, sounds at rock concert levels cause pain in humans, whereas whales can belt out and listen to tunes another 20 decibels louder. In turn, whales are adapted to weightlessness and may not have the receptors to appreciate gravity the same way as we do. In the early 1900s, German biologist Jakob von Uexküll coined the term umwelt to describe an animal’s perceptual world — its mental map of nearby space.
For decades, neuroscientists have sought to manipulate the human umwelt. In 1969, Paul Bach-y-Rita conducted the first studies on sensory substitution, a technique used to restore sensation in people with sensory loss. His device converted visual input from a camera to touch input on the tongue, enabling the blind to crudely see their surrounds. Many devices have been produced since this time, but they are often expensive or invasive to the user. Fast-forward to 2017, David Eagleman from Stanford University is developing a low cost device that takes sound and converts it to a pattern of touch on the back, which has proved effective for the deaf.
Eagleman is also tinkering with the umwelt using an exciting new paradigm known as sensory addition. This augmentation is achieved by wiring our nervous system to wearable devices that detect signals that are otherwise undetectable by humans. The brain can learn to understand these new inputs. His team is working on a project where pilots receive information about their aircrafts’ pitch and yaw as touch stimuli, and learn to use it to fly by ‘feel’. He thinks that, in the future, a system like this could replace the need for displaying huge amounts of data in the cockpit.
Jen’s spacecraft is now hurtling through outer space, a zero gravity environment. Almost immediately she feels clumsy, a problem further compounded by the bulkiness of her spacesuit. She drops her coffee cup, but uncannily it doesn’t plummet to the floor; it is suspended in space. Can she relearn how to control her body while in space?
Whenever we move objects around, it is necessary to estimate their heaviness. An object has no weight in a zero gravity environment, only a mass dependent on the amount of matter it contains. To feel this yourself, take two similar objects and try to judge which one is heavier by sliding them back and forth along a flat surface. Difficult? Now lift them up and down. Easier? In fact, experiments on Earth show that we can’t accurately estimate the weight of an object when only information on its mass is available.
These results were replicated in microgravity, a condition that can be transiently created in an aircraft taking a parabolic path. Researcher Helen Ross designed the experiments for testing this hypothesis in the Spacelab, a collaboration between the American and European space agencies. In space, astronauts estimating the weight of an object made errors that were double those on Earth. Further, they did not improve at all during the nine-day flight. This suggested that in space, we use inertial cues — an object’s resistance to movement — to judge its weight.
Can the mind be trained to get a grip in space? Anecdotally, we know that we can relearn something if sufficient time is available, such as writing with our non-dominant hand or learning to ride a bike with flipped steering. Psychologists have learnt a lot about object manipulation from the size-weight illusion, in which the visually smaller of two objects of equal weight is perceived as heavier. This may be an evolutionary adaptation dating back to our hunter-gatherer days to help us select objects for their throwability.
To see if the illusion could be unlearnt, scientists devised a clever experiment. Participants lifted a set of objects in which visually larger objects weighed less, yet lifting the objects over a thousand times had minimal influence on the size-weight illusion. Three days of conditioning attenuated the size-weight illusion, suggesting that adapting to zero gravity may require extensive training to unlearn a lifetime of experience.
According to Gavin Buckingham, an expert in object manipulation at the University of Exeter, there are two things we need to learn to interact with objects in environments with new dynamics. “First,” he says, “we need to learn to move our limbs in such a way as to cancel out the dynamics of the new environment”. Buckingham’s studies show that this typically takes between 20 and 100 movements, depending on the task difficulty. “Second, users need to adjust their gripping forces to account for the changes in loading forces brought about by the changes in the novel environment.” Grip force adaptation requires as few as ten movements.
Buckingham also studies how well we can learn new movement patterns by simply observing others. The activation of mirror neurons in the brain, which are active when we watch others, may be responsible for these effects. Buckingham believes that although observational learning has not been tested in microgravity, “Zero gravity training videos showing people making errors or moving clumsily in microgravity might well be a very cost-effective way to ‘kick-start’ the learning of effective object interaction in space”. Indeed, recent studies suggest that watching people make errors, rather than watching Nadia Comaneci score a perfect 10 in gymnastics, are most effective as learning aids. Buckingham suggests this is “unsurprising, as errors are what give you the most information about what has changed in the new environment”.
Drifting around in a spaceship bound for Mars, Jen finds that — on top of moving about like a newborn lamb — she also has no sense of which way is up and which way is down. She can float where she wishes, but must look out the window towards Earth to find ‘down’. Early in the flight, she experiences the nausea and dizziness associated with motion sickness. She eventually finds her ‘space legs’ as the journey progresses, and can estimate the vertical as long as her eyes are open.
In space, the perception of body orientation and self-motion is impaired due to the absence of the usual reference cues. On Earth, we know vertical by integration of information from vision, balance organs in the inner ear, muscle receptors, and touch receptors on our feet. On Earth, we can create the illusion of self-rotation by viewing a field of rotating dots. This sensation takes a while to develop, until the brain is overcome with evidence that the body must be rotating. The illusion was much stronger when recreated in the Spacelab. Furthermore, the effect of touch inputs to the feet diminished during the flight, as if the astronauts had optimally adapted to rely on visual cues.
Our understanding of weightlessness and self-orientation has practical applications, not just in space but also back on Earth. Elderly people are particularly prone to falling over, with an annual cost in the hundreds of millions of dollars for New South Wales alone. As a result, there is a strong research presence surrounding self-motion, falls and balance in Australia.
In some sufferers of vertigo, brain lesions disrupt the ability to combine information from vision and the inner ear. People with symptoms of vertigo often describe it as a debilitating condition accompanied by a “feeling that everything is spinning” even when their visual surrounds appear stationary. Scientists from Neuroscience Research Australia have developed a training program that corrects this problem in some people. Patients track a green dot that is projected onto a wall, a task that some people are unable to perform due to impaired integration of visual and vestibular (balance) signals. The difficulty level is adjustable so that patients are continuously challenged to improve. A device that patients can take home to perform the training is currently under development. Fifteen minutes of training with the device per day may be sufficient to substantially improve balance in vertigo patients.
Similarly, when Jen rises each day — if there is a concept of time in space — she completes an intense battery of exercises. These are designed to prevent the postural dysfunction that many astronauts experience upon return to Earth. These days, NASA pairs astronauts preparing for space flights with trainers specialised in strength and conditioning for space. These training protocols have been informed by research findings surrounding the damaging effects of living in a weightless environment.
Roberta Bondar, the first neurologist to enter space, found that, in astronauts not able to stand within 10 minutes of returning to Earth, the regulation of blood flow to the brain was impaired. Numerous methods of combatting this impairment have been proposed. The in-flight artificial gravity depicted in the film 2001: A Space Odyssey may seem farfetched, but the idea is under serious investigation by scientists as a countermeasure.
In 2014, Japanese scientists sent fish to the International Space Station Aquatic Habitat to study, at the molecular level, how the skeletal system adapts to reduced gravity. To do this, they needed a way to remotely track the activity of the cells responsible for remodelling bone in real time. They created transgenic lines of fish in which these cells would glow under the light of a fluorescent microscope. Within a single day, spaceflight increased the numbers of osteoblasts and osteoclasts — the cells responsible for building and breaking down bone, respectively. These cellular changes were met with the upregulation of a whole host of genes in the bone as early as day two of the flight. This was the first study to show how rapidly these processes unfold. The capability to study in real time the effects of weightlessness on biological systems may open the doors to developing interventions that enable man to reach Mars in one piece.
The many human research projects being conducted at the International Space Station will be crucial to guiding astronauts, like Jen, to Mars. Can we get people to Mars unscathed? As a society, how are we to select people for these missions? These problems have to be weighed against the difficulties with unmanned flights. For now, all we have been able to do is get a chunk of metal from our pale blue dot to the Red Planet, but this chunk of metal has generated mountains of useful data, and sets the bar that we’ll no doubt endeavour to surpass.
Edited by Andrew Katsis and Ellie Michalides