Successful social communication is based, above all, on the ability to understand the actions of other people. But how can we imagine what other people are thinking, or what intentions they have? Psychologists and neuroscientists trace it back to a kind of simulation that goes on in our brain as soon as we observe a person acting.
The actions of the observed person are, so-to-speak, internally imitated. Indeed, researchers at the Max Planck Institute for Cognitive and Neurosciences in Munich, in cooperation with scientists from the University of Bournemouth in England and Rutgers University in Newark, New Jersey, have shown that we understand the actions of another person, apparently, on the basis of our own "action inventory". In other words, our own mind and body give us the foundation to understand what other people are doing, thinking, or feeling. Evidence for this comes out of an experiment involving two patients that, because of an extremely rare illness, lost the ability to perceive their own body. (Nature Neuroscience, October 2005)
In the recently published study, Simone Bosbach and Wolfgang Prinz showed, with their colleagues, that two specific patients have deficits in their ability to interpret the actions of other people. These two patients are currently the only known cases worldwide with this kind of clinical picture. Its psychological consequences are dramatic. Both patients report that, at the beginning of their illness, most of all, they had the feeling that they had "lost" their entire body. Since then, they have learned to carry out simple body movements. However in order to do that they have to be able to see their body. In the dark, the patients lose complete control over their bodies, because they are no longer able to determine, for example, the position of their arms and legs relative to the body, with the help of the sensory receptor cells in the joints and muscles.
Normal people can do this without any problems, thanks to the self-perception of their own body (proprioceptive feedback). This self-perception also lets our brains know when, and in which range, muscles contract or expand, and to which extent joints bend or stretch. This sense makes us able to pose in certain body positions and to carry out movements, and it is also decisive for the psychological consciousness of having a body.
Bosbach and her colleagues confronted the patients with short video films in which people are asked to lift boxes. Each box was a different weight. Both patients were given the task, in the first part, of guessing the weight of the box that the person in the film was lifting. The patients received no other clues; they had to guess the weight of the box solely from the motion sequence of the lifter. It turned out that the patients were able to complete the task as correctly and unerringly as the control subjects. Apparently they were able solve the problem using their knowledge that, for example, a slow body movement signifies a heavy load and a faster movement, which gives the impression the subject was unloading something, suggests a lower weight.
In the second part of the task, the patients also saw videos of people who were lifting boxes. However, this time, in some cases, the people in the film were deceived about the actual weight of the boxes. So the actor, for example, received the information before lifting the box that he was lifting 18 kilograms - when indeed the box weighed only three. The patients then had to state whether the person in the video had the right or the wrong expectation regarding the weight of the box. Again, the only source of information for the patients to make their judgment was body movement. If the people in the film were deceived about the weight of the box, they tended to show a characteristic discrepancy in the movement, between the phases in which they prepared themselves to lift the box (expecting a heavy one) and the phase in which they were actually lifting the box (which was clearly lighter than expected). This discrepancy was not present when the person had a correct expectation of the weight.