Scientists at Oxford and UCL have tricked subjects in an experiment into believing that a rubber hand was part of their own body.
The findings about the brain’s awareness of its own body will hopefully lead to better understanding of disorders of self-perception, as for example in brain-damaged patients who no longer recognise their own limbs after stroke, or amputees who still experience pain from limbs that are no longer there.
Volunteers in the experiment, which was a collaboration between psychologists at UCL and Oxford, hid their real hand beneath a tilted table, while a rubber hand was placed in front of them in a plausible position for it to be part of their own body. The rubber and real hands were then simultaneously stroked with a paint brush. The subjects, who could feel their own arm being brushed as they saw the rubber hand being stroked, experienced the vivid illusion that the fake hand was actually part of their own body.
The researchers used functional magnetic resonance imaging to scan brain activity during the task. The results, published last month in Science, show that one area of the brain, the premotor cortex, integrates information from all the senses to produce a feeling of ‘ownership’ of parts of the body.
If the information from the various senses is inconsistent, as in the experiment, sight will tend to dominate – and so the brain starts to ‘believe’ what it sees rather than what it feels.
On average it took subjects 11 seconds to start feeling that the rubber hand as part of their own body. When asked to point to their hand after the experiment, most pointed to the fake hand. The stronger the feeling of ownership, the greater the activity recorded in the premotor cortex.
‘Distinguishing oneself from the environment is a critical, everyday problem that has to be solved by the central nervous system of all animals,’ said Henrik Ehrsson, the study leader. ‘If the distinction fails, the animal might try to feed on itself and will not be able to plan actions that involve both body parts and external objects, such as reaching for a banana.’
Picture: a volunteer lies with his real hand concealed and a rubber hand in view. Credit: Dr Henrik Ehrsson, University College London.