Bodily illusions rearrange the sense of touch

The brain is bombarded by information about the physical proportions of our bodies. The most familiar sensations, such as a puff of wind or the brush of our own shirt sleeve, serve to constantly remind the brain of the body's outer bounds, creating a sense of what is known as proprioception.

In a new study, researchers report this week that the brain's ability to interpret external signals and update its sense of bodily self is more dynamic than had been previously thought and that such updates can happen very quickly, altering within a matter of seconds how body parts and individual touch sensations are perceived.

The work is reported by researchers Frederique de Vignemont, Henrik Ehrsson, and Patrick Haggard at University College London.

The information that is integrated in the course of proprioception comes from several different senses, including touch, pain, vision, information from muscles, and so on. The brain must combine all these information inputs to accurately perceive the external world through our body's interaction with it and also to produce a coherent sense of self. Because all these signals carry such different kinds of information, the brain must perform a constant juggling act in order to make sense of the body and the world.

In the new study, the research team used a method called tendon vibration to induce a distortion of healthy volunteers' sense of their own bodies. When the biceps tendon of the right arm was vibrated, the subjects in the experiments felt within seconds that their right elbow was rotating away from the body, even though the arm was actually quite still. If subjects held their left index finger with their right hand while this happened, they felt their left index finger getting longer as they felt their arm move.

The team then tested how these bodily illusions rearranged the sense of touch. They touched subjects with two metal rods on the left index finger, and asked them to judge whether the distance between the rods was greater or smaller than the distance between two additional rods touched on the forehead. When tendon vibration made the index finger seem longer than it really was, subjects overestimated the tactile distance on the index finger, relative to the forehead. The sense of touch was altered within seconds of applying the vibration, suggesting a very strong and fast link between the brain systems for touch and body position.

The observations showed that abnormal input into one of the body's sensory systems produced a rapid and profound change in another sensory system: The brain had changed the way it interpreted the signals from the sense of touch. These links between different bodily senses show that the brain constantly updates its internal map, and they shed light on the dynamic and flexible nature of the body map that is used to interpret each new perception.