Excuse me, that's my hand! (... but is it really?)

About 15 years ago, a one-page Nature study shook the scientific community. Researchers from the University of Pittsburg showed with a simple experiment that people could feel that a fake rubber hand was in fact their own- they called it the ‘rubber hand illusion’. It goes like this: place a fake hand on a table in front of you and your own hand just next to it. Then block your hand from your view, stare at the fake hand, and get someone to stroke both hands in the same way for a few minutes. Now close your eyes and point at your hand. Most people will point at the fake hand, and so should you.

Credit: melodi2/everystockphoto

Since this intriguing discovery, neuroscientists have been trying to understand how the brain combines visual, touch and position information to create the feeling of body ownership, or in another words, the awareness that our body parts belong to ourselves. A new study led by Anna Berti’s team at the University of Turin now shows that the embodiment of an alien limb, like someone else’s hand, can be so deeply rooted in our neural circuits that it affects motor control.

When we try to perform a different motor task with each hand at the same time, let's say drawing a circle with one hand and a straight line with the other, both hands somehow get it wrong. In this circle-line example, we would end up with two oval doodles, because one hand’s task interfered with the other. This is called ‘bimanual coupling’ effect, and it happens because our brains find it hard to cope with different motor directives simultaneously (it is possible, but it requires a lot of practice).

Berti and colleagues explored this idea to ask whether believing that an alien hand doing something is our own (in this case, it was the researcher’s hand), could affect how our own real hand performs a task. So they asked volunteers to draw lines and circles in different experimental set ups. As expected, when healthy subjects watched an alien hand draw circles, they could draw straight lines with their own hand as well as if the alien hand wasn’t there because they knew it didn’t belong to them.

What happens to people who are convinced the alien hand is their own? To answer this question, Berti’s team tested brain-damaged people with the left side of their body paralysed and who also suffered from asomatoagnosia. Patients with this rare condition can deny, forget, ignore or misperceive their paralysed limbs. The patients in this study were convinced that the alien hand drawing circles was their own, as if they had a spontaneous rubber hand illusion, and this interfered with their motor control- they couldn’t draw a straight line with their healthy hand. These patients’ brains fully integrated the alien hand into their sensory and motor neural circuits.

As weird as it may sound, our body self-awareness is a mental representation created by the brain, which can be tricked so convincingly that someone else’s hand, or even a rubber hand, can completely replace our own.

Reference:

Garbarini F., Pia L., Piedimonte A., Rabuffetti M., Gindri P. & Berti A. (2013). Embodiment of an alien hand interferes with intact-hand movements, Current Biology, 23 (2) R57-R58. DOI: 10.1016/j.cub.2012.12.003

This article was published in Lab Times on 2-03-2013. You can read it here.