Your brain sees your hands as short and fat

By Ed Yong | June 14, 2010 3:00 pm

Hands

Knowing something like the back of your hand supposedly means that you’re very familiar with it. But it could just as well mean that you think it’s wider and shorter than it actually is. As it turns out, our hands aren’t as well known to us as we might imagine. According to Matthew Longo and Patrick Haggard from University College London, we store a mental model of our hands that helps us to know exactly where our limbs are in space. The trouble is that this model is massively distorted.

To keep track of where your various body parts are, your brain maps your posture by processing information from your muscles, joints and skin. Close your eyes and move around a bit, and you’ll still have a good idea of what position you’re in even if you can’t see or touch yourself. But there’s no such direct signal that tells your brain about the size and shape of your body parts. Instead, your brain stores a mental model with those dimensions mapped out.

To visualise this model, Longo and Haggard asked volunteers to hide their hand under a board and use a baton to indicate the position of ten landmarks – the tip and base knuckle of each finger. Their answers were surprisingly inaccurate.

Hand_measurements

They underestimated the lengths of their fingers by anywhere from around 5% for their thumb and over 35% for their ring and little fingers. In contrast, they overestimated the width of their hand by around 67%, and particularly the distance between their middle and ring knuckles. Our mental hand is a shorter, wider version of our real one. Longo and Haggard found the same thing if they asked the recruits to angle their hands at 90 degrees under the board, and if they tested the right hand as well as the left.

These distortions actually reflect how sensitive each part of the hand is. The skewed mental map is remarkably similar to another map called Penfield’s homunculus, which charts the areas of the brain’s somatosensory cortex (the bit that processes touch information) that is devoted to each body part. Regions that have a more acute sense of touch correspond to larger parts of the homunculus, but they also loom bigger in our mental map. Regions that are less sensitive are smaller on both charts.

As we move from the thumb to the little finger, our digits become less sensitive and the mental map increasingly underestimates their true size. The back of the hand is more sensitive to movement across it than movement along it; accordingly, our mental map depicts a wider, shorter hand.

And we have no idea about this. Consciously, the volunteers had a pretty good appreciation of the size and shape of their hands. When Longo and Haggard showed them a selection of hand images and asked them to select the one that best matched their own, they did so very accurately. But even though they passed this test, they still failed to place the baton in the right place when their hands were hidden.

If we hold such a distorted depiction of our own hands, how is it that we ever grasp things successfully? It’s possible that our motor system uses a different model but Longo and Haggard put forward two more plausible ideas: that cues from vision are strong enough to override the warped map; and that we learn to correct for the misshapen model. Only by removing both of these factors did they finally reveal how skewed our perceptions actually are.

Reference: PNAS http://dx.doi.org/10.1073/pnas.1003483107

Image: Hands by Toni Blay

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Comments (13)

  1. This is so cool! Now I have a great science experiment to use with kids to help them see that they may not ‘know’ completely what they think they do!

  2. Evan Harper

    Is this why high people are fascinated by looking at their hands?

  3. Tracy Koehler

    This article is a good example of why you’re better than chocolate, fairy lights and kittens chasing yarn. (Christine Ottery’s list of Yong-attributes seemed fitting…though I may have overestimated their relevance…and fully acknowledge that your actual attributes may be shorter and fatter than her list suggests.)
    Personally, I love reading about research that shows us how our brains perceive and interact with the world — and the many subtle factors that combine to show us that even the most mundane activities we engage in on a daily basis…such as anything we do with our bodies…and what doesn’t that apply to?!…offer a multitude of discoveries just waiting to be made. How many people took baths before Archimedes and failed to relate the “mundane” rising of the water to anything outside of bathing? How many times have we used our hands and not considered that our very perception of them is “obviously” affected by our mental models of them? What other “everyday mundane” aspects of our brains and bodies are we failing to notice because they’re so common and obvious as to go unquestioned?
    Probably loads of them. Which is one of the reasons science is so eternally interesting as it probes deeper and more subtly into us and the world around us. And hey, thanks Mr. Yong for being a wicked-groovy source of info about those discoveries. May you live long, and may your perceptions be taller and thinner with each passing year. (Er…WTF?!)

  4. Caryn

    It would be interesting to see if populations of pianists or violinists made different estimates.

  5. Great article as always.

    One question that leaps out at me: Are we ANY good at estimating things with our eyes closed? Humans, by nature are visual creatures. It may be that the ‘resolution’ for us to work in touch is very different compared to when we use our vision. One easy demonstration (which I’m sure has been done somewhere by someone) would be to ask someone to estimate 10cm with their eyes open. Then ask them to do it with their eyes closed. And I guess you would be able to see the increase in error as the distance got bigger and bigger.

  6. Chris M.

    Hm.

    I strongly suspect it has to do with how we actually use our hands. We are consummate tool users, and the majority of our precision work occurs when our hands are closed around a tool, which means it has something much closer to the proportions the article cites. I’d be very interested to see whether we do better at determining our hand position when it’s in use.

    @PopSciGuy, I can write with my eyes closed. We’re really just bad at gauging distances, because they aren’t strictly relevant. We’re a lot better at comparing things, eyes open or closed, since the ratio is what often matters most. Anyway! That ratio thing continues up along the scale, because it’s far and away the best way to get a good estimate while covering a good range of data. Can you imagine what it would be like to estimate a mile with the same accuracy as a centimeter? Say you can recognize ten different distances; do you want 1-10cm, or 1cm, 10cm, 100cm, etc.? There’s a trade-off being made between how precise your sensory system is and how many different ranges it can deal with.

    Me, I’ve got good visual accuracy for short ranges, but it just gets shot to hell at long ranges, because I’m nearsighted. My eyes don’t cover the most useful range for a human, but I see over a similar log-scaled range that someone with good eyes does.

    I digress. Anyway, similar trade-offs are made within our proprioceptive system, and I suspect being tool users has caused us to misconstrue our own hands out flat, in favor of precision with our hands the way they are when we actually use them.

  7. Hi Chris,

    I think you’re making my point. As size/distance estimation increases, the error would increase (ie. ratio). And yes, to the proprioceptive system distance doesn’t matter so much, whereas it does to the visual system. Hence, our ability to resolve distance with our eyes closed (purely based on touch ) wouldn’t be as good as when we have our eyes open.

    As a separate issue, estimating size based on the touch of your tongue would be an altogether different thing. The tongue has a huge representation within the somatosensory system and might overrepresent size – which might explain why my occasional cavity feels so big!

  8. JuJu

    I like the article, but couldn’t it be that we perceive our hands differently, not because of relative differences in sensitivity, but because when we first learned to use them as a babies, they had different shape than we do now and the brain hasn’t gotten to remapping these perceptions?

  9. Marissa

    I’m intrigued by this article, but I would like to suggest a secondary experiment to remove a variable. I’m wondering if the same results would appear in subjects who are blind. If we daily live without the aid of sight, would our “mind’s eye” be more accurate? This may bring up a whole new set of testing criteria (if the subjects could/could not recognize their own hands) however I still think it would be interesting to see what sort of results would become of a specialized population of subjects!

  10. JMW

    Interesting. I just tried the experiment twice. The first time with my palm down. I got the base knuckles about right, but I placed my finger tips about a half-inch shorter than where they actually were.

    About 20 minutes later, I tried it with my palm up. Again, the base knuckles were pretty good, and this time I placed my finger tips only about a quarter-inch shorter of where they should be.

    Now, to be truly indepedent of bias, this experiment should be done with 50 people doing the plam-down first, and 50 doing the palm up first. I could have been influenced by knowing that I had been short the first time.

  11. Chris M.

    @PopSciGuy, definitely. There are some specific reasons for it, though; I wasn’t able to find the graph that I was thinking of, but it turns out different sensory modalities have much different scaling laws. Pain, for example, scales in a very nonlinear way, such that you get a nice thresholding effect, unlike light, where you get a wide-ranging gradient. I don’t know that anyone’s done this scaling for proprioception. I mean, it’s likely, but I haven’t run into it.

  12. Torbjörn Larsson, OM

    they overestimated the width of their hand by around 67%,

    No, from the figure and data that doesn’t seem to be the case. (Stupid paywall, so no original paper handy.) Perhaps the mean overestimate of individual positions is ~ 70 %.

  13. Matt C.

    JuJu: nah, you recalibrate everything constantly, so there’s no way this could be a hangover from infancy.

    PopSciGuy and Chris M: Perception of an object’s size is pretty much independent of perception of distance. If you get people to estimate the size of objects at various distances, then they give pretty much the right answer consistently (i.e. they don’t think something far away is smaller than the same object nearby). This seems to hold true in both vision and touch – if you extend touch perception by giving people rods with which to feel objects at a greater distance than normal, they still estimate the size accurately. I would personally go so far as to suggest that size and shape of objects is represented in much the same way by both vision and touch, but that’s perhaps getting a bit off-topic…

    Anyway, there are some tasks that have been done with things like length estimation which would be a lot like what you’re suggesting, PopSciGuy – I’ll try to dig out some references when I’m not in such a rush!

    JMW: people in Haggard’s group have been doing something along those lines (localization of different kinds of stimulus (a simple touch, or a nociceptive stimulus, things like that) with palm up and palm down, and there seem to be slightly different biases – nothing published yet, just seen the poster at a conference last week.

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