Hand study reveals brain's distorted body model

The human brain
The human brain

Our brains contain a highly distorted model of our own bodies, according to new research by scientists at UCL (University College London). A study published today, which focussed on the brain's representation of the hand, found that our model of our bodies is out of sync with reality - with a strong tendency to think that the hands are shorter and fatter than their true shape.

The results of the study, published in the journal , show that the brain maintains a model of the hand in which our fingers are perceived to be shorter and our hands fatter than they are. Neuroscientists suspect the reason for these distortions may lie in the way the brain receives information from different regions of the skin.

Dr Matthew Longo, lead author from the UCL Institute of , said: "The phrase "I know the town like the back of my hand" suggests that we have near-perfect knowledge of the size and position of our own but these results show that this is far from being the case."

"Our results show dramatic distortions of hand shape, which were highly consistent across participants. The hand appears to be represented as wider than it actually is and the fingers as shorter than they actually are - a finding that might also apply to other parts of the body," added Dr Longo.

Participants in the study were asked to put their left hands palm down under a board and judge the location of the covered hand's knuckles and by pointing to where they perceived each of these landmarks to be. A camera situated above the experiment recorded where the participant pointed. By putting together the locations of all the landmarks, the researchers reconstructed the brain's model of the hand, and revealed its striking distortions.

The research, which was funded by the Biotechnology and Biological Science Research Council (BBSRC), aims to find out how the brain knows where all parts of the body are in space even when your eyes are closed - an ability known as 'position sense'. Neuroscientists think that position sense requires two distinct kinds of information. Signals that the brain receives from muscles and joints play an important role in position sense, but the brain also needs a model of the shape and size of each body part. For example, to know where the fingertip is in space, the brain needs to know the angles of joints in the arm and hand, but also the length of the arm, hand, and finger.

It is this model of our body's size and shape that is investigated in the study. "Of course, we know what our hand really looks like" said Dr Longo, "and our participants were very accurate in picking out a photo of their own hand from a set of photos with various distortions of hand shape. So there is clearly a conscious visual image of the body as well. But that visual image seems not to be used for position sense."

The results showed that in this task people estimated that their hands were about two-thirds wider and about one-third shorter than actual measurements.

Neuroscientists suspect that the brain's distorted model of body shape result is due to the way the brain represents different parts of the skin. For example, the size of the representation of the five fingers gets progressively smaller for each finger between the thumb and the little finger, mirroring the relative size of fingers in the body model reported in this study.

"These findings may well be relevant to psychiatric conditions involving body image such as anorexia nervosa, as there may be a general bias towards perceiving the body to be wider than it is. Our healthy participants had a basically accurate visual image of their own body, but the brain's model of the underlying position sense was highly distorted. This distorted perception could come to dominate in some people, leading to distortions of body image as well, such as in eating disorders," said Dr Longo.

More information: 'An implicit body representation underlying human position sense' is published in the Proceedings of the National Academy of Sciences.
Citation: Hand study reveals brain's distorted body model (2010, June 14) retrieved 18 April 2019 from https://medicalxpress.com/news/2010-06-reveals-brain-distorted-body.html
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Jun 14, 2010
An important distinction to keep in mind is between 'body image' and 'body schema'. The reporting here seems to confuse the two. Of course, much of the literature confuses the two, as well.

'Body image' refers to the way the body is represented in conscious experience. 'Body schema' refers to the unconscious representations of the body.

Something else that threw me off was mention of "position sense". I have never heard this term used to refer to our ability to perceive bodily position; generally, the term used is 'proprioception'.

Jun 14, 2010
I think the conclusion is a little presumptuous. It might be that a primitive circuit reasoned that the fact that the hand couldn't be seen simply increased the probability that it couldn't be seen simply because the hand didn't reach that far. The primitive bias was then used as a "correction factor" to the more intellectual estimate of where the hand was. A better experiment would be to distort the hand's position with a prism (both long and short) and then somehow test when the subject detected the distortion.

Jun 15, 2010
The Brain's: "Perception of 'Hand'" ? ? ?
I was 40 years of age before I heard:
"Your hand will do the catching;
"You needn't involve your mind,
"Hands KNOW!"

My 'inner-self' ? ? ? always had to be in control!

Big mistake!
Perhaps a corollary is:
"I must tell my Alimentary . . . 'You have work to do' . . .
"Digest the food I have swallowed!"
The 'Body parts/components KNOW their JOB(s)' . . . ie. "Lead, follow, or get out of the way!"

Normally, We are oblivious to our individual Corpus? Constituants . . .
they function!
When 'Pain/unusual sensation(s) eminate from a normally complacent 'Part', there is usually a reason, and then our 'Brain' kicks in:
"Something Is WRONG!"

Your Hand; knows how to catch!

Roy Stewart,
Phoenix AZ

Jun 15, 2010
Several questions I have that might be answered in the original paper. First how many participants and tries per participant. Then did they do with hand dorso/plantar flexion resp. touching/not touching the "cover board". Then "finger pointing with say index finger, to mark "position", with and without eyes open. Then with eyes open pointing with a "laser pen" attached to say the forehead. Also that version blinded/unblinded. Numerous other alternatives would be interesting to see in further studies, fingers spread out/non spread etc.

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