You know what your personal space is. Uninvited strangers invading it invoke an immediate sense of unease. A bird flying over but too close to your head may cause you to reflexively duck. It’s like we don’t just end where our bodies end, but a little bit beyond.

Now researchers have figured out a way to make you "feel" the invisible bubble that wraps around you. In a new study published in the journal Cognition on June 24, neuroscientists at the Karolinska Institute in Stockholm describe how they did this using an altered version of the so-called rubber hand illusion.

In the classic version of the rubber hand illusion, a person is shown a fake rubber hand placed in front of them while their own hand is hidden behind a screen. If both real and fake hands are simultaneously stroked by a brush for a few minutes, the person's brain starts to accept the fake hand as part of the body. To test this, just bring a knife or hammer close to the fake hand and watch how someone reacts. Here’s a video:

In the new experiments, the researchers repeated this procedure but never actually touched the rubber hand. Instead, they applied brushstrokes in mid-air above it. Simultaneously, they stroked the participant’s hidden real hand with real touch. A few minutes later, the participants started to feel the rubber hand was their own. What’s more, they started to sense what felt like a “magnetic force” or a “force field” between the brush and the rubber hand, according to the study.

“We can elicit this bizarre sensation of there actually being something in mid-air between the brush and the rubber hand,” Arvid Guterstam of the Karolinska Institute, a co-author of the study, told New Scientist.

The finding plugs into research stretching to the 1990s that has suggested the brain has a mechanism to recognize the immediate space around the body, said neuroscientist Michael Graziano of Princeton University, who wasn’t involved with the new study but has researched the topic.

“The space around the body is processed like a jello mold. Like a thick layer of space around the body, deforming and moving as the limbs move,” Graziano told the Huffington Post.

The illusory effect in the experiments started to fade if the brushstroke went higher than about 15 inches above the rubber hand. In other words, that’s as far as your “force field” goes.

Your “Peripersonal” Space

Our brains contain a high-resolution map of our bodies. There are patches of neurons that respond to touch on your toes, their neighboring neurons respond to touch on your leg, and so on. But there are also groups of neurons in charge of recognizing the immediate space around the body, which is called the peripersonal space.

That space has several purposes, but its most important function is defense. This extended body representation in the brain may help us quickly detect approaching dangers. It also seems essential for our ability to move our limbs to pick up an object without knocking everything else over, walk through a door without hitting the door frame or make it through a crowd without bumping into people.

The discovery of peripersonal space goes back to animal studies conducted by Graziano and his colleagues at Princeton University in early 1990s.

And it was a somewhat serendipitous discovery.

“We were trying to study visually responsive neurons in an obscure part of the brain -- the claustrum -- but we ended up finding more interesting signals in a nearby part of the brain, the basal ganglia,” Graziano said.

The researchers stumbled on neurons that responded to both touch and vision. The same neuron would become active if you touched the body in a specific area and when you moved a visible object into a region of space near that area. Other scientists had described similar neurons in other brain regions, so the Princeton researchers followed the trail.

“At that time, nobody knew the function of these neurons,” Graziano said. “It was not until our experiments in the early 2000s that we realized they primarily emphasized the protection of the body. When we electrically stimulated those neurons, revving them up, the stimulation would induce a complex, coordinated behavior that resembled ducking or blocking.”

For example, Graziano explained, if the neurons responded to touching the right side of the face and also to the sight of objects looming toward the right side of the face, then electrical stimulation of those neurons would cause the subject to squint, turn the head toward the left, hunch the right shoulder and lift the right hand to a blocking posture near the face. This movement was so automatic that it could be evoked even under anesthesia.

“We now understand these peripersonal space neurons are important in monitoring the margin of safety around the body,” Graziano said.

Shifting Boundaries

Most data on these neurons comes from monkeys, but brain scans of humans have revealed similar groups of neurons in our brains. And sometimes, people with certain types of brain damage lose their ability to process the peripersonal space.

“These people have no trouble processing distant space, but lose a sense of peripersonal space,” Graziano said.

There's reason to think that our fluid force field is also extendable, Graziano said. Just think about how, when driving a car, you have a sense of where the car's boundaries end so you don't hit other objects. Or put on a tall hat and see how it feels after a few hours.