It is believed that our bodies are fixed and unchangeable except through the slow process of growing and aging. Over recent years there have been research results that defy this common sense view -- it seems that the human brain will quickly accept gross changes in the body -- incorporating external objects such as a rubber arm into the body representation, and even whole bodies seen in virtual reality.

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.

For 10 of the participants although their real hand touched the table top, their virtual hand did not -- and this was done to create an inconsistency between what they felt and what they saw. This group always saw their virtual arm at the same length as their real arm. For another group who also saw the virtual arm at the same length as their real one, there was no inconsistency -- the real hand touched the surface of the table, and the virtual hand was seen to do the same.

This same consistency was kept for three other groups of 10 people each -- but one where the table moved away to double the length of the real arm, and the virtual arm stretched to double its length (figure B), another 10 where the virtual arm stretched to three times the true length (figure C), and another group where it stretched to four times the true length (figure D).

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.