Space around others perceived just as our own

A study from Karolinska Institutet in Sweden has shown that neurons in our brain ‘mirror’ the space near others, just as if this was the space near ourselves. The study, published in the scientific journal Current Biology, sheds new light on a question that has long preoccupied psychologists and neuroscientists regarding the way in which the brain represents other people and the events that happens to those people.

"We usually experience others as clearly separated from us, occupying a very different portion of space," says Claudio Brozzoli, lead author of the study at the Department of Neuroscience. "However, what this study shows is that we perceive the space around other people in the same way as we perceive the space around our own body."

The new research revealed that visual events occurring near a person’s own hand and those occurring near another’s hand are represented by the same region of the frontal lobe (premotor cortex). In other words, the brain can estimate what happens near another person’s hand because the neurons that are activated are the same as those that are active when something happens close to our own hand. It is possible that this shared representation of space could help individuals to interact more efficiently — when shaking hands, for instance. It might also help us to understand intuitively when other people are at risk of getting hurt, for example when we see a friend about to be hit by a ball.

The study consists of a series of experiments in functional magnetic resonance imaging (fMRI) in which a total of forty-six healthy volunteers participated. In the first experiment, participants observed a small ball attached to a stick moving first near their own hand, and then near another person’s hand. The authors discovered a region in the premotor cortex that contained groups of neurons that responded to the object only if it was close to the individual’s own hand or close to the other person’s hand. In a second experiment, the authors reproduced their finding before going on to show that this result was not dependent on the order of stimulus presentation near the two hands.

"We know from earlier studies that our brains represent the actions of other people using the same groups of neurons that represent our own actions; the so called mirror neuron system", says Henrik Ehrsson, co-author of the study. "But here we found a new class of these kinds of neuronal populations that represent space near others just as they represent space near ourselves."

According to the scientists, this study provides a new perspective that could help facilitate the understanding of behavioural and emotional interactions between people, since — from the brain’s perspective — the space between us is shared.

Size of personal space is affected by anxiety

The space surrounding the body (known by scientists as ‘peripersonal space’), which has previously been thought of as having a gradual boundary, has been given physical limits by new research into the relationship between anxiety and personal space.

New findings have allowed scientists to define the limit of the ‘peripersonal space’ surrounding the face as 20-40cm away. The study is published today in The Journal of Neuroscience.

As well as having numerical limits the specific distance was found to vary between individuals. Those with anxiety traits were found to have larger peripersonal space.

In an experiment, Dr Chiara Sambo and Dr Giandomenico Iannetti from UCL recorded the blink reflex - a defensive response to potentially dangerous stimuli at varying distances from subject’s face. They then compared the reflex data to the results of an anxiety test where subjects rated their levels of anxiety in various situations.

Those who scored highly on the anxiety test tended to react more strongly to stimuli 20cm from their face than subjects who got low scores on the anxiety test. Researchers classified those who reacted more strongly to further away stimuli as having a large ‘defensive peripersonal space’ (DPPS).

A larger DPPS means that those with high anxiety scores perceive threats as closer than non-anxious individuals when the stimulus is the same distance away. The research has led scientists to think that the brain controls the strength of defensive reflexes even though it cannot initiate them.

Dr Giandomenico Iannetti (UCL Neuroscience, Physiology and Pharmacology), lead author of the study, said: “This finding is the first objective measure of the size of the area surrounding the face that each individual considers at high-risk, and thus wants to protect through the most effective defensive motor responses.”

In the experiment, a group of 15 people aged 20 to 37 were chosen for study. Researchers applied an intense electrical stimulus to a specific nerve in the hand which causes the subject to blink. This is called the hand-blink reflex (HBR) which is not under conscious control of the brain.

This reflex was monitored with the subject holding their own hand at 4, 20, 40 and 60 cm away from the face. The magnitude of the reflex was used to determine how dangerous each stimulus was considered, and a larger response for stimuli further from the body indicated a larger DPPS.

Subjects also completed an anxiety test in which they self-scored their predicted level of anxiety in different situations. The results of this test were used to classify individuals as more or less anxious, and were compared to the data from the reflex experiment to determine if there was a link between the two tests.

Scientists hope that the findings can be used as a test to link defensive behaviours to levels of anxiety. This could be particularly useful determining risk assessment ability in those with jobs that encounter dangerous situations such as fire, police and military officers.

Using Virtual Reality an Arm Up to Three or Even Four Times the Length of a Real Arm Can Be Felt as If It Was the Person’s Own Arm

The authors of the article have added another dimension to this illusion of body ownership. Using virtual reality they have shown that a virtual body with one very long arm can be incorporated into body representation. An arm up to three or possibly even four times the length of a person’s real arm can be felt as if it was the person’s own arm. This is notwithstanding the fact that having one such long arm introduces a gross asymmetry in the body. An extended body space (a body with longer limbs occupies more volume than a normal body) affects also the special space surrounding our body that is called peripersonal space — a space that when violated by objects or other people can be experienced as a threat or intimacy, depending on the context.

In the experiment 50 people experienced virtual reality where they had a virtual body. They put on a head-mounted display so that all around themselves they saw a virtual world. When they looked down towards where their body should be, they saw a virtual body instead of their real one. They had their dominant hand resting on a table with a special textured material that they could feel with their real hand, but also see their virtual hand touching it. So as they moved their real hand over the surface of this table they would see the virtual hand doing the same.

The results of the study were analysed by using a questionnaire to assess the subjective illusion that the virtual arm was part of the person’s body; a pointing task, where the arm that did not grow in length was required to point towards where the other hand was felt to be (with eyes shut), and a response to a threat task, in which a saw fell down towards the virtual hand (figure E, F) and it was measured whether people would move their real hand in an attempt to avoid it.

Based on these data, researchers found that people did have the illusion that the extended hand was their own. Even when the virtual arm was 4 times the length of the corresponding real arm, still 40-50% of participants showed signs of incorporation of the virtual arm as part of their body representation. It was also found that vision alone is a very powerful inducer of the illusion of virtual arm ownership — those who experienced the inconsistent condition where the virtual hand did not touch the table, even though the real hand felt the table top, had a strong illusion of ownership over the virtual arm.

These results show how malleable is our body representation, even incorporating strong asymmetries in the body shape, which do not correspond at all to the average human shape. This type of research will help neuroscientists to understand how the brain represents the body, and ultimately may help people overcome illnesses that are based on body image distortions.