If you’ve ever caught yourself instinctively dodging an over-hanging branch or an uneven surface on the road based only on what your peripheral vision is telling you, congratulations, you’ve experienced a strange phenomenon known as your peripersonal space.
Like whiskers on cats tell them where their nose is in relation to their surroundings, our bodies are enveloped by an invisible 'force field', and neuroscientists have figured out how to make us feel it for the first time.
Most of us have a very strong feeling of personal space, and get super weirded-out when someone enters it uninvited, but you’d probably struggle to explain to somewhere where exactly the acceptable and creepy zones around you start and finish.
Fortunately, for scientists working in the field of neuropsychology, it’s their job to define interpersonal awkwardness, so they've have actually mapped out your own invisible bubble like so:
Pericutaneous space: The space immediately outside your body that can feel almost like someone is touching you if they enter it - for example, a feather might not be touching your skin, but if it’s in your percutaneous space, you might still experience the sensation of being tickled if it hovers close enough.
Peripersonal space: The space within arm's or leg's reach. So to be "within arm's length" of you is to be within your peripersonal space.
Extrapersonal space: The space that occurs outside your reach.
So if you wave your arms and legs around, all that is the invisible force field that your brain has created to better perceive the world around it, and neuroscientists from the Karolinska Institute in Sweden have finally figured out how to make people actually perceive this.
As Anil Ananthaswamy reports for New Scientist, hard neuroscientific evidence on the existence of the peripersonal space didn’t exist until the late 1990s, when psychologist Michael Graziano from Princeton University recorded the brain activity of monkeys to discover that certain neurons were firing not only when an object touched their skin, but also when it hovered near it.
"When the researchers directly stimulated such neurons, they found that the monkeys would reflexively move their heads and limbs as if defending themselves – for example, grimacing and closing their eyes, turning their heads to one side and putting up their arms in a protective posture," says Ananthaswamy.
The experiment has yet to be replicated in humans (PhD idea, anyone?), but Arvid Guterstam of the Karolinska Institute has been collecting the evidence that suggests humans have a similar sense of 'self' beyond our physical boundaries.
There’s the strange 'phantom limb' phenomenon that can manifest in patients who have undergone amputations, making them sense the space where their limb used to be, and some stroke patients report losing their sense of peripersonal space on one side of the body.
Guterstam decided to test human perception of peripersonal space by tweaking a classic experiment called the rubber hand illusion, where a person observes a rubber hand being stroked while his or her own hand is also being stroked in exactly the same way (but hidden from view).
In just a few minutes, the person tends to subconsciously 'adopt' the rubber hand as their own, and report 'feeling' brushstrokes that have only been applied to the rubber hand, and not their actual hand.
As a team of Australian researchers pointed out back in 2013, "It is extremely simple to generate in people the illusion that a rubber arm is part of their body."
Interestingly, the rubber hand experiment seems to be limited by how far away the rubber hand is from the person, and that suggested to Guterstam that it’s linked to the peripersonal space.
So he recruited 101 volunteers (60 female, and 89 right-handed), and asked them to sit through a number of experiments that had the researchers stoking the air above the rubber hand - from a distance of 5 cm to 55 cm - at the same time as the participants' real hands were being stroked out of view.
As they watched this happening, the participants appeared to 'adopt' the rubber hand in much the same way as in the traditional rubber hand experiment, except this time, they also sensed something when an object entered the peripersonal space of the rubber hand, or up to about 40 cm above it.
As Guterstam and his team report in the journal Cognition, the participants reported feeling a "magnetic force" or "force field" between the rubber hand and an object moving in mid-air close to the hand.
"We can elicit this bizarre sensation of there actually being something in mid-air between the brush and the rubber hand," Guterstam explained to New Scientist.
The best part is, the experiment is so simple you can recruit a friend and try it yourself at home, and then try this while you're at it..
Of course, it'd be great for future research to observe this kind of sensation using a person's own hand, and with the brain scans to solidify what's actually going on here, but the research is part of a growing body of evidence that's revealing just how much is being taken in by our subconscious minds without us even knowing about it.
Earlier this week, we reported that a researcher in the US thinks he might have finally found evidence of a sixth sense in humans - the ability to subconsciously detect Earth's magnetic field - and researchers in Australia are figuring out how to harness paralysed patients' subconscious thoughts to help them walk again.
So it's looking more and more likely that while us humans have lost much of the survival instincts of our primate ancestors, we can still sense a whole lot more than we see.