Lifting an arm, picking up your foot, wiggling your fingers – most of us can do these things without giving them a second thought. Once your brain has set the movement in motion how do you know (without looking) that the appendage responded appropriately? It turns out that you can’t really tell. NewScientist.com is reporting today that researchers from the Prince of Wales Medical Research Institute in Sydney, Australia used a simple test to determine that the same signal your brain sends to initiate a movement is also responsible for the sensation of movement you feel, and this feeling occurs regardless of whether or not the actual movement takes place. Their results offer insight into the phantom limb phenomena, a sensation that a missing limb is still attached and is moving appropriately with other body parts, experienced by a majority of amputees.
During the study led by Simon Gandevia, participants had all sensation in one of their hands temporarily disrupted by anesthesia or by the restriction blood flow, thereby effectively creating a “phantom” hand. When researchers asked the subjects to flex their wrists all reported that they felt the movement take place even though the position of their hands had not actually changed. Additionally, the perceived magnitude of the change in position was found to be proportional to the amount of effort they were told to make.
This experiment seems like such an obvious starting point, but for more than 100 years neuroscientists have debated how we are able to determine and keep track of the movement and positions of our bodies and body parts. It was originally thought that feedback from the skin, joints and muscles combined to create the sensation of movement. Gandevia’s work, however, concludes that as far as the brain is concerned, if it has issued a command to move, then the movement has occurred.
The study will be published in the Journal of Physiology.