People can spend lots of time thinking about their bodies, but how does the brain know what the body is really like? By measuring the brain’s perceptual maps of a ‘phantom limb’, researchers have shown that our understanding of body image could be the product of innate hardwiring within the brain, rather than the result of perceiving what our body is actually like.
A ‘phantom limb’ is the term used to describe the sensation that an amputated or missing limb is still attached to the body. The research, published in the journal Psychological Science, explains how scientists from Birkbeck, University of London, and University College London worked with interdisciplinary artist Catherine Long to map the ‘phantom’ left hand she vividly experiences after being born without a left arm.
Catherine was asked to judge the location of the knuckles and fingertips on her phantom left hand, using a special apparatus which allowed her to point to each of these landmarks. Catherine’s actual right hand was then concealed and the experiment repeated as she attempted to pinpoint the location of her hidden digits. By combining the locations of all the landmarks, the researchers were able to reconstruct the brain’s model of both the phantom and actual hand, finding similar patterns for each.
Dr Matthew Longo, lead author of the study from the Department of Psychological Sciences at Birkbeck, said: “The existence of phantom limbs shows that our ‘body image’, as we call the conscious experience we have of our body, is constructed inside the brain, and does not depend on sensory signals reaching the brain from the body.
“Historically, phantom limbs have been difficult to study experimentally because they can’t be measured directly. However, the phantom feels like it occupies a distinct location in space, and we used this fact to measure its size and shape before reconstructing the brain’s internal model of the missing hand,” continued Dr Longo.
Co-author Professor Patrick Haggard, of the Institute of Cognitive Neuroscience at UCL, said: “We reasoned that the brain’s model of the phantom hand could come from one of three sources. It could come from Catherine’s experience of seeing her own intact right hand; it could come from looking at other people’s hands; or it could come from some internal representation of the hand which the brain maintains, even though the hand itself is absent.”
Previous research by the authors demonstrated that people represent their hands as wider than they actually are, with fingers shorter than they actually are. These distortions may come from the organisation of the ‘somatosensory’ brain areas that represent the body and the same characteristic distortions were found in Catherine’s perception of both her intact right hand and phantom left hand.
“If Catherine’s experience of a phantom hand came from looking at her own, intact hand – or from other people’s hands – we would expect to see an accurate map of the phantom, without any distortion,” said Dr Longo. “However, the map of the phantom hand contains the same distortions as the map of her intact hand, suggesting that the same brain processes are at work in both cases.
“The fact that Catherine’s brain clearly retains a body image for her absent left hand suggests that our brains come hardwired with a map of the body.”
View from interdisciplinary artist Catherine Long about her involvement in the research:
“I decided to participate in the study for a very specific reason. Because I have a body that is regularly objectified and labelled by individuals and society, I wanted to engage directly with what goes largely unseen. My aim was to investigate the experience I have of the appearances of a left arm and urges to use a left hand. I wanted to work with the neuroscientists to discover if these urges were directly being triggered by brain impulses, or if the presence of the limb was due to external influences.
“This investigation could furnish a discourse of my own body that resists it becoming a vessel of loss for others in the culture, and that resists my own experience disappearing. The work at the Institute of Cognitive Neuroscience fuelled my desire to delve into how the cultural construction of ‘wholeness’ and ‘loss’ has influenced my own specific bodily experience. I’ve been able to explore the difference between how I experience the appearance and presence of my left arm at unexpected moments, and what it meant to bring the arm into a scientific context and use it as a tool, with conscious intent.”