Okay, so you know your hand? Five fingers. Assorted grasping and carrying shenanigans. Right. So it turns out that your brain is constantly using sensory information to check in and make sure it still knows what's your hand and what's not. And it can be fooled.
Arvid Guterstam, Giovanni Gentile and Henrik Ehrsson at the Karolinska Institutet in Sweden discovered, while experimenting with the rubber-hand illusion, that people's brains can be tricked into thinking they feel touches on their hand when the hand is just empty air.
In the rubber-hand illusion, developed at Princeton in the 1990s, a person puts their hand on one side of a divider while a coordinator puts a rubber hand on the other side of the divider, lined up with the real hand. When the coordinator rhythmically and repeatedly touches the person's hand at the same time that they are touching the rubber hand the person begins to feel the touch, anything from a tap to a paintbrush stroke, on the rubber hand as well as on their own hand. This shows that all of the sensory information the person's brain is getting about their real second hand is being crowded out by visual information that their second hand is the rubber hand.
The Swedish researchers noticed, though, that the trick can go a step further. While using empty space as a control in studies of mock-hands they noticed that participants were feeling touches on empty air just so long as the spot being touched was a reasonable distance from their body. Ehrsson told NatGeo that:
Almost everybody that has worked with the rubber hand illusion has assumed that the actual rubber hand is kind of very important. But strangely enough the participants started talking about sensing touches on an invisible hand.
Guterstam used other tests aside from participant reporting to be sure that they were really feeling the phantom hand. When he stabbed the "hand" with a sharp object the participants started sweating and when he had them close their eyes and point to a finger on the hand in question, they pointed to a corresponding part of the empty space.
The research shows that the brain interprets and reconciles information from the senses using formulae that are fairly reliable, but can be tricked. The researchers hope to use this information in treatment of pain associated with phantom limbs.
Keep your hands to yourself. [National Geographic]