Have you ever tried to visualize your hands… after closing your eyes? Previous studies had shown that we tend to underestimate our finger length increasingly from thumb to little finger. Researchers at Birkbeck, University of London,  made a new discovery after examing a 38-year-old woman born without a left arm. Your brains do not require visual or sensory input to represent your hands!

New Scientist reports:

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 (Clinical Psychological Science).

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.

Read more about previous research that demonstrated through hand projections how faulty body perceptions work in  anorexia and other eating discorders:

Hand projections demonstrate how anorexia works 

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The pink handfish - a.k.a. brachionichthys

TRIBUTE: THE EVOLUTION OF THE HAND

[tweetmeme source=”handresearch” only_single=false] A few weeks ago (may 2010) Australian researchers Daniel Gledhill & Peter Last from the CSIRO Wealth reported to have discovered 9 new species of the ‘handfish’, in a research that highlights an urgent need to better understand and protect the diversity of life in Australia’s oceans. But the major unanswered question is: ‘two of the handfish fins look like hands, but are they?’

Mr Gledhill described the Handfish as follows:

“Handfishes are small, often strikingly patterned or colourful, sedentary fish that tend to ‘walk’ on the seabed on hand-like fins, rather than swim. Fifty million-years ago, they ‘walked’ the world’s oceans, but now they exist only off eastern and southern Australia“.

One of the newly named species, the Pink Handfish, is known from only four specimens and was last recorded off the Tasman Peninsula in 1999.

Interestingly, if we take a look at some features of red handfish one can notice that it actually isn’t walking with the hands… but with the feet! (See the photos below)

The red handfish - a.k.a. brachionichthys politus!

And in the perspective of evolution it might actually make sense that the hands of the handfish appear to have 4 digits, and the feet have more digits – though 6 digits is really rather remarkable.

Why are 6 digits remarkable?

First of all, the 4-digit hand combined with a 6-digit foot reminds us to a typical characteristics of… amphibians, they are usually featured with 4 digits on the front limbs and 5 digits on the hind limbs!

However, while an amphibian usually spends his live some time on land and some time in the water – there is another rather funny ‘creature’ that has likewise hand- (4 digits) and foot characteristics (5 digits), named: the axolotl – a tiger salamander complex that is living in the waters of Mexico. The difference with the salamander is that the axolotl only lives in the water, just like the handfish!

The axolotl - belonging to the tiger salamander complex!

Most vertebrates have 5 digit limbs!

One should also be aware that most vertibrates (including: mammals, birds, reptiles, amphibians, and various fishes) have 5 digits on each limb. So, from the perspective of evolution it is not a coincidence that us human have 5 finger and 5 toes!

However, in this perspective it is rather remarkable that the handfish appears to have a 6 digit fin!

Would you like to shake hands with a handfish?

Evolution animation: from fish to human!

SUGGESTION FOR FURTHER READING:
The mystery of the five fingers!
Five things that your 5 fingers can tell you!
The difference between the human hand & the hands of primates!

Evolution: from fish to human!