Feeling fat in the festive season? It’s all in your mind

Feeling thin or fat is an illusion constructed in the brain, according to a new study published in the journal Public Library of Science Biology. The collaborative study led by UCL (University College London) used a trick called the Pinocchio illusion to scan people’s brains while they experienced the sensation that their waists were shrinking. The study reveals which parts of the brain are involved in body image and may shed some light on anorexia and body dysmorphic disorder, where sufferers are overly concerned by a small or imagined defect in their body, and frequently overestimate or underestimate their actual body size.

The study, led by Dr Henrik Ehrsson of the UCL Institute of Neurology, used the Pinocchio illusion in combination with functional magnetic resonance imaging to study volunteers’ brains. For each volunteer, a vibrating device was placed on their wrist to stimulate the tendon and create the sensation that the joint was flexing, even though it remained stationary. With their hand touching their waist, volunteers felt their wrists bending into their body, creating the illusion that their waists were shrinking.

During the tendon exercise, all 17 participants felt that their waist had shrunk by up to 28 per cent. The researchers found high levels of activity in the posterior parietal cortex, an area of the brain that integrates sensory information from different parts of the body. Volunteers who reported the strongest shrinking sensation also showed the strongest activity in this area of the brain.

Dr Henrik Ehrsson, of the UCL Institute of Neurology, says: “We process information about our body size every day, such as feeling thin or fat when we put our clothes on in the morning, or when walking through a narrow doorway or ducking under a low ceiling. However, unlike more elementary bodily senses such as limb movement, touch and pain, there are no specialized receptors in the body that send information to the brain about the size and shape of body parts. Instead, the brain appears to create a map of the body by integrating signals from the relevant body parts such as skin, joints and muscles, along with visual cues.

“Other studies have shown that people with injuries in the parietal cortex area of the brain experience the feeling that the size and shape of their body parts have changed. People who suffer from migraine with aura can sometimes experience a phenomenon called the ‘Alice in Wonderland syndrome’, where they feel that various body parts are shrinking. This could also be linked to the same region of the brain. In addition, people with anorexia and body dysmorphic disorder who have problems with judging the size of their body might similarily have a distorted representation of their body image in the parietal cortex. These are areas which would be worth exploring in future research, to establish whether this region of the brain is involved in anorexia and the rare but peculiar shrinking symptoms of some migraines.”