My real hand is slimmer (Image: Milton Montenegro/Getty)

EVER wanted to know what an invisible hand looks like? Well, it is slightly wider than a real hand, and it has shorter fingers too. For the first time, the perceived shape of a phantom limb has been measured. This should make it possible to learn more about how the brain represents what we look like.

The illusion of a phantom limb can kick in after an amputation or in people missing limbs from congenital disease. The result is the sensation that the limb is, in fact, present.

One theory suggests people with phantom limbs take cues from those around them to work out what their missing body part looks like. Another theory is that the sensation of an invisible limb reflects brain activity in regions that map our body in space.


To clarify the sensory origins of phantom limbs, Matthew Longo at Birkbeck, University of London, and colleagues enlisted the help of CL – a 38-year-old woman born without a left arm, who periodically feels she has a phantom hand. They asked her to place her right hand beneath a board and indicate where she believed her fingertips and knuckles were. She then repeated the exercise imagining that her phantom left hand was beneath the board instead.

Previous studies have shown that we tend to underestimate our finger length increasingly from thumb to little finger. This mirrors differences in the sensitivity and size of areas in the brain’s somatosensory cortex that are thought to represent each digit, probably by making use of visual, mechanical and tactile feedback. The thumb is represented by a larger area of the cortex than the little finger.

As expected, CL reported these characteristic distortions when indicating the dimensions of her right hand. But she made the same errors when describing her phantom hand, implying she perceives both hands in the same way (Psychological Science, DOI: 10.1177/0956797612441219).

That suggests there is “some structural representation in the brain of a body part that has never existed”, says Patrick Haggard at University College London, another member of the team. This implies that the somatosensory cortex does not require visual or sensory input to represent a body structure.

Understanding how the brain perceives the body could have broader implications. Longo says there are a few studies showing that people with eating disorders may inaccurately judge the size of their body from tactile feedback. Those results suggest there may be some relation between somatosensory representations of the body and our conscious feelings of what our body is like,” he says.