Gravity is the unseen force that dominates our entire lives.
It’s what makes walking uphill so difficult and what makes parts of our body eventually point downhill. It is unyielding, everywhere, and a force that we battle with every time we make a move.
But exactly how do people account for this invisible influence while moving through the world?
A new study in Frontiers in Neuroscience used virtual reality to determine how people plan their movements by “seeing” gravity using visual cues in the landscape around them, rather than “feeling it” through changes in weight and balance. PhD Student Desiderio Cano Porras, who worked in Dr. Meir Plotnik’s laboratory at the Sheba Medical Center, Israel and colleagues found that our capability to anticipate the influence of gravity relies on visual cues in order for us to walk safely and effectively downhill and uphill.
In order to determine the influence of vision and gravity on how we move, the researchers recruited a group of 16 young, healthy adults for a virtual reality (VR) experiment.
The researchers designed a VR environment that simulated level, uphill, and downhill walking. Participants were immersed in a large-scale virtual reality system in which they walked on a real-life treadmill that was at an upward incline, at a downward decline, or remained flat.
Throughout the experiment, the VR visual environment either matched or didn’t match the physical cues that the participants experienced on the treadmill.
Using this setup, the researchers were able to disrupt the visual and physical cues we all experience when anticipating going uphill or downhill.
So, when participants saw a downhill environment in the VR visual scenery, they positioned their bodies to begin “braking” to go downhill despite the treadmill actually remaining flat or at an upward incline.
They also found the reverse – people prepared for more “exertion” to go uphill in the VR environment even though the treadmill remained flat or was pointing downhill.
The researchers showed that purely visual cues caused people to adjust their movements to compensate for predicted gravity-based changes (i.e., braking in anticipation of a downhill gravity boost and exertion in anticipation of uphill gravitational resistance).
However, while participants initially relied on their vision, they quickly adapted to the real-life treadmill conditions using something called a “sensory reweighting mechanism” that reprioritized body-based cues over visual ones.
In this way the participants were able to overcome the sensory mismatch and keep walking.
“Our findings highlight multisensory interactions: the human brain usually gets information about forces from “touch” senses; however, it generates behavior in response to gravity by “seeing” it first, without initially “feeling” it,” says Dr. Plotnik.
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Dr. Plotnik also states that the study is an exciting application of new and emerging VR tech as “many new digital technologies, in particular virtual reality, allow a high level of human-technology interactions and immersion.
We leveraged this immersion to explore and start to disentangle the complex visual-locomotor integration achieved by human sensory systems.”
The research is a step towards the broader goal of understanding the intricate pathways that people use to decide how and when to move their bodies, but there is still work to be done.
Dr. Plotnik states that “This study is only a ‘snapshot’ of a specific task involving transitioning to uphill or downhill walking. In the future we will explore the neuronal mechanisms involved and potential clinical implications for diagnosis and treatment.”
Gravity as a universal force
Today, scientists know of four forces — things that attract (or repel) one object to (or from) another. The strong force and the weak force operate only inside the centers of atoms.
The electromagnetic force rules objects with excess charge (like electrons, protons, and socks shuffling over a fuzzy carpet), and gravity steers objects with mass.
The first three forces largely escaped humanity’s notice until recent centuries, but people have long speculated about gravity, which acts on everything, from raindrops to cannonballs.
Ancient Greek and Indian philosophers observed that objects naturally moved toward the ground, but it would take a flash of insight from Isaac Newton to elevate gravity from an inscrutable tendency of objects to a measurable and predictable phenomenon.
Newton’s leap, which became public in his 1687 treatise Philosophiæ Naturalis Principia Mathematica, was to realize that every object in the universe — from a grain of sand to the largest stars — pulled on every other object.
This notion unified events that appeared totally unrelated, from apples falling to Earth (although it probably didn’t inspire his breakthrough, Newton did work near an apple tree) to the planets orbiting the sun. He also put numbers to the attraction: Doubling the mass of one object makes its pull twice as strong, he determined, and bringing two objects twice as close quadruples their mutual tug. Newton packaged these ideas into his universal law of gravitation.
Gravity as the geometry of space
Newton’s description of gravity was accurate enough to detect the existence of Neptune in the mid-1800s, before anyone could see it, but Newton’s law isn’t perfect. In the 1800’s, astronomers noticed that the ellipse traced by Mercury’s orbit was moving more quickly around the sun than Newton’s theory predicted it should, suggesting a slight mismatch between his law and the laws of nature. The puzzle was eventually resolved by Albert Einstein’s theory of general relativity, published in 1915.
Before Einstein published his groundbreaking theory, physicists knew how to calculate a planet’s gravitational pull, but their understanding of why gravity behaved in such a way had advanced little beyond that of the ancient philosophers.
These scientists understood that all objects attract all others with an instantaneous and infinitely far-reaching force, as Newton had postulated, and many Einstein-era physicists were content to leave it at that. But while working on his theory of special relativity, Einstein had determined that nothing could travel instantly, and the pull of gravity should be no exception.
For centuries, physicists treated space as an empty framework against which events played out. It was absolute, unchanging and didn’t — in any physical sense — really exist. General relativity promoted space, and time as well, from a static backdrop to a substance somewhat akin to the air in a room.
Einstein held that space and time together made up the fabric of the universe, and that this “spacetime” material could stretch, compress, twist and turn — dragging everything in it along for the ride.
Einstein suggested that the shape of spacetime is what gives rise to the force we experience as gravity. A concentration of mass (or energy), such as the Earth or sun, bends space around it, like a rock bends the flow of a river. When other objects move nearby, they follow the curvature of space, as a leaf might follow an eddy around the rock (although this metaphor isn’t perfect because, at least in the case of planets orbiting the sun, spacetime isn’t “flowing”). We see planets orbit and apples fall because they’re following paths through the distorted shape of the universe. In everyday situations, those trajectories match the force Newton’s law predicts.
Einstein’s field equations of general relativity, a collection of formulas that illustrate how matter and energy warp spacetime, gained acceptance when they successfully predicted the changes in Mercury’s orbit, as well as the bending of starlight around the sun during a 1919 solar eclipse. [In Photos: Einstein’s 1919 Solar Eclipse Experiment Tests General Relativity]