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Eyes perceive light, ears perceive vibrations in the air…
and our inner ears perceive gravity.
It’s easy to think of gravity as something that’s just kind of there,
keeping us stuck to the Earth and not much else.
But scientists are learning that it’s also a key player in our perception.
It seems to be baked deep into how our brains process things like motion,
interacting in surprising ways with how we process the world around us.
So much so that scientists are beginning to wonder
how it might affect how we process other worlds, too.
Let’s start with how we sense gravity.
This is primarily done through the vestibular system:
a series of fluid filled canals and sacks in our inner ear.
When we tilt our head, tiny hairs within these structures bend and sway under the
force of gravity, with different hairs responding to bends in different directions.
The brain takes the combined signal from all of these,
and uses it to work out the direction that gravity is pulling us, relative to our skull.
Anyone who’s spun around real fast as a kid… or adult… can attest to this.
You get dizzy, you lose your balance, and gravity feels like it’s playing tricks on you.
We might even perceive the world to be spinning around us.
But that’s not the only way in which how we sense gravity
can mess with our perception of things.
Having a sense of where objects are being pulled,
and how strongly, can affect how well we can interact with them.
Studies have shown that there are differences in our ability to track
moving objects that move according to different strengths of gravity.
One experiment which investigated this published in 2015 tracked participant’s
eye movements as they attempted to “catch” digital falling objects under different
gravity conditions: typical Earth gravity, weightlessness, and double Earth gravity.
For some of the trials, part of the path of those falling objects was hidden.
So the participants’ brains had to work overtime to figure out
when they’d reappear in order to intercept them.
When analyzing those eye movements, the researchers found that the
participants were better able to smoothly track objects falling according to Earth’s
gravity than those moving in weightlessness or double gravity.
Researchers in the field believe that this difference points strongly to the existence
of an internal understanding of gravity in our brains,
which influences the way we expect stuff to behave.
This internal model also doesn’t appear to be influenced by
what we know about the things we see.
In a paper from 2020, researchers showed that, regardless of which way
we expect an object to move, we’re better at judging how quickly they go
when they move towards gravity, compared to away from it.
To do this, they asked participants to view the surface of a virtually generated planet.
On this other-worldly scene, there were two objects,
both displayed to be the same size: a rugby ball,
which, for those of you not into sports-ball, looks like this, and a rocket.
Participants were asked to watch the objects move between two bars,
and then judge whether they were moving faster or slower in subsequent trials.
Now, the key here is that you’d expect a rocket to go up and a ball to go down.
But the results showed that participants were better at judging the speed
of both objects when they were moving downwards.
From this, the researchers concluded that our sense of gravity works
independently of semantic information we already know about objects,
like the fact that a rocket generally goes upwards.
However, there were a few limitations they didn’t rule out, so more study is needed.
Lucky for us, there’s a whole field for that.
And it’s not all about how inanimate things fall.
Research suggests that our sense of gravity
can also change how we process things like other people’s body movements.
In a study published in the European Journal of Neuroscience,
researchers found that it’s not just the movement of objects
that are subject to our sense of gravity.
Bodily motions are too.
This team asked participants to watch a series of point-light movements,
which are basically a cluster of moving dots often used by researchers
to simulate bodily movements, without much unnecessary detail.
Some of these were random, with no particular movement pattern at all,
whereas others were arranged to look like human movements.
The participants watched these both in a relaxed upright position,
and in a chilled-out tilted position on a tilt table.
They were always shown the groups of dots at the same angle relative to their bodies.
But since the participants’ body positions were different,
the two conditions varied relative to where the participants perceived gravity to be.
The participants were asked to pick out whether the point-light movements
they were watching were random or looked like a person moving.
And they were generally pretty good at it.
After all, for most people, it’s not a tricky task.
But crucially, when they were tilted, they were significantly slower
at deciding whether patterns showed biological motion or not.
The researchers believe this could indicate that our ability to perceive biological
motion doesn’t just rely on the visuals of the motion itself, but how much they
match up with where we perceive gravity to be in any given moment.
Knowing all of this gives us a better understanding of how our senses work,
which is always a plus.
Getting a better handle on all the moving parts of our cognition helps
us to not only better understand how we perceive in general,
but also come up with treatments when things go a little wrong.
But many scientists in this field are wondering what this might mean
for humans who travel into space.
Under different gravity conditions,
astronauts could be perceiving things in ways we never expected.
It’s possible that this could lead to some extra difficulties
performing intricate tasks, which scientists may need to look at
mitigating to make sure their jobs are as easy as possible.
Research in this area is still pretty young.
But in the future, knowing how gravity can affect our understanding of the world
around us could make space exploration easier on those doing the exploring.
And maybe back here in Earth’s gravity, too.
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