You're only allowed one left hand

In previous posts I’ve asked how we know where our hands are and how we combine information from our senses. Today’s paper covers both of these topics, and investigates the deeper question of how we incorporate this information into our representation of the body.

Body representation essentially splits into two parts: body image and body schema. Body image is how we think about our body, how we see ourselves; disorders in body image can lead to anorexia or myriad other problems. Body schema, on the other hand, is how our brain keeps track of the body, below the conscious level, so that when we reach for a glass of water we know where we are and how far to go. There’s some fascinating work on body ownership and embodiment but you can read about that in the paper, as it’s open access!

The study is based on a manipulation of the rubber hand illusion, a very cool perceptual trick that’s simple to perform. First, find a rubber hand (newspaper inside a rubber glove works well). Second, get a toothbrush, paintbrush, or anything else that can be used to produce a stroking sensation. Third, sit your experimental participant down and stroke a finger on the rubber hand while simultaneously stroking the equivalent finger on the participant’s actual hand (make sure they can’t see it!). These strokes MUST be synchronous, i.e. applied with the same rhythm. The result, after a little while, is that the participant starts to fell like the rubber hand is actually their hand! It’s a really fun effect.

There are of course limitations of the rubber hand illusion – a fake static hand isn’t the best thing for eliciting illusions of body representation, as it’s obviously fake, no matter how much you think the hand is yours. Plus it’s hard to do movement studies with static hands. The researchers got around this problem by using a camera/projection system to record an image of their participant’s hand and playing it back in real time. They got their participants to actively stroke a toothbrush rather than having the stroking passively applied to them, and then showed two images of their hand to the left and right of the actual (unseen) hand position.

The left, right or both hands were shown synchronously stroking; the other hand in the first two conditions was shown asynchronously stroking by delaying the feedback from the camera. The researchers asked through questionnaires whether participants felt they ‘owned’ each hand. You can see these results in the figure below (Figure 3B in the paper):

Ownership rating by hand stroke condition

For the left-stroke (LS) and right-stroke (RS) conditions, only the left or right image respectively was felt to be ‘owned’ whereas in the both-stroke (BS) condition, both hands were felt to be ‘owned’. This result isn’t too surprising; it’s a nice strong replication of the rubber hand results other researchers have found. Where it gets interesting is that when participants were asked to make reaches to a target in front of them they tended to reach in the right-stroke and left-stroke conditions as if the image of the hand they felt they ‘owned’ was actually theirs. That is, they made pointing errors consistent with what you would see if their real hand had been in the location of the image.

In a final test, participants in the both-stroke condition were asked to reach to a target in the presence of distractors to its left and right. Usually people will attempt to avoid distractors, even when it’s just an image or a dot that they are moving around a screen, and the distractors are just lights. However in this case participants had no qualms about moving one of the images through the distractors to reach the target with the other, even though they claimed ‘ownership’ of both.

This last point leads to an interesting idea the authors explore in the discussion section. While it seems to be possible to incorporate two hands simultaneously into the body image, this doesn’t appear to translate to the body schema. So you might be able to imagine yourself with extra limbs, but when it comes to actively move them the motor system seems to pick one and go with that, ignoring the other one (even when it hits an obstacle).

To my mind this is probably a consequence of the brain learning over many years how many limbs it has and how to move them efficiently, and any extra limbs it may appear to have at the moment can be effectively discounted. It is interesting to see how quickly the schema can adapt to apparent changes in a single limb however, as shown by the pointing errors in the RS and LS movement tasks.

I wonder if we were born with more limbs, would we learn gradually how to control them all over time? After all, octopuses manage it. Would we still see a hand dominance effect? (I’m not sure if octopuses show arm dominance!) And would we, when a limb was lost in an accident, still experience the ‘phantoms’ that amputees report? I haven’t touched on phantoms this post, but I’m sure I’ll return to them at some point.

Altogether a simple but interesting piece of work, which raises lots of interesting questions, like good science should. (Disclaimer: I know the first and third authors of this study from my time in Nottingham. That wouldn't stop me saying their work was rubbish if it was though!)

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Newport, R., Pearce, R., & Preston, C. (2009). Fake hands in action: embodiment and control of supernumerary limbs Experimental Brain Research DOI: 10.1007/s00221-009-2104-y

Image copyright © 2009 Newport, Pearce & Preston