The perception of one’s own body can be manipulated in such a way that the visual system can also evoke the feeling of touch
The image that a person has of his body includes much more than the look in the mirror reveals. The sensation of being tickled, for example, is completely different depending on whether another person touches the sole of the foot or one’s own finger. Even when we are not looking, the brain recognizes the difference and processes the signals coming in from the touch nerves differently. Evolutionarily, this makes sense: only in this way can an awareness of one’s own physical integrity develop, which is important for survival.
But of course, the brain is wired for normalcy, not for the curiosity of neurologists. The mechanisms involved, researchers have known for a while, can be used for some amazing experiments, such as out-of-body experiences (see art pieces with amputated limbs). But they are also responsible for unpleasant phenomena like phantom pains. Researchers still do not know exactly all the pathways that determine the body image and help in its evaluation and modification. The problem is absolutely relevant medically, because the progress in the construction of artificial limbs is rapid. It would be all the more important to be able to help patients integrate the replacement parts into their body image.
On the way there, researchers now describe an exciting new finding in the journal of the u.S. Academy of sciences (pnas): they were able to prove that the visual system can also take over the function of recognizing a touch – without using the nerves responsible for touches. This is different in the case of the well-known rubber hand illusion: in this case, the brain is given the feeling of being the owner of a rubber hand by someone visibly touching the artificial limb and at the same time the real hand is (invisibly) touched.
In order to observe which brain area reacts when, the researchers had to wire their test subjects – they therefore tested on monkeys. They now played a video of a ball touching a hand to them. At the same time they touched the real arms of the animals. As expected, the brain activated the two areas responsible for touch and visual stimuli. After the training, however, the researchers dispensed with the physical touch – and again both areas switched on. However, this took 50 to 70 milliseconds longer – about as long as it takes to pass information from the visual to the somatosensory cortex. This is good news for patients – it promises that artificial limbs can be integrated into their own body image in a lifelike manner.