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On The Other Other Hand: How to Finally Get That Extra Limb You've Always Wanted

The mind, it seems, is not constrained by the four-limbed body plan it’s used to growing up with: there’s room in our brain’s blueprint of the body for more stuff.

Feb 28 2011, 7:30pm

It's rare that a neuroscience research article grabs you with its title. "Differential Targeting of Optical Neuromodulators to Ganglion Cell Soma and Dendrites Allows Dynamic Control of Center-Surround Antagonism" is not the snappiest headline. But there are the occasional exceptions; I couldn't look away from a funky study out of the Karolinska Institutet in Stockholm released last week, entitled "The Illusion of Owning a Third Arm."

The article (available for open access here) details an ingenious set of experiments that actually convinced individuals that a prosthetic "third limb" was of their own flesh. While the traditional "phantom limb" experiments used mirrors to trick people into thinking a prosthetic arm was their own, this experiment put the fake one right out on the table next to the real thing.

The researchers, led by Henrik Ehrsson, prepared the third-arm-illusion by synchronously brushing both the fake and real right hands of the subjects. After this psychological primer, it was knives out:

Hands both real and fake were threatened with knives, and the subjects' skin conductance responses (SCRs) were measured using electrodes. The SCR is a commonly used measure of psychological and physiological arousal, occurring in response to important environmental stimuli (i.e. your hand about to get lopped off).

Surprisingly, SCRs were comparable in reaction to both hands being threatened: when the fake was threatened, the real one responded exactly as if it was threatened itself. Weird.

Several measures were needed to elicit the curious illusion: The fake hand had to be placed near the real one and positioned similarly, it had to be a hand (they tried using a rubber foot as a control and it didn't work), and the synchronized brushing of the real and fake hands was a necessary prelude. When all these elements are in place, the authors argue, the subject's brain essentially "matches" the sensory information about the two hands, treating the fake hand as a normal body part.

What makes this particular study special is that people’s bodies effectively "added" a limb to the equation, rather than merely getting tricked into replacing real with fake. The researchers write:

…The illusion reported here is qualitatively different from the traditional rubber hand illusion as it is characterised by less disownership of the real hand and a stronger feeling of having two right hands. These results suggest that the artificial hand 'borrows' some of the multisensory processes that represent the real hand, leading to duplication of touch and ownership of two right arms. This work represents a major advance because it challenges the traditional view of the gross morphology of the human body as a fundamental constraint on what we can come to experience as our physical self, by showing that the body representation can easily be updated to incorporate an additional limb.

The mind, it seems, is not constrained by the four-limbed body plan it's used to growing up with – there's room in our brain's blueprint of the body for more stuff. Whether it's just room for an extra hand, or room enough for anything (legs? toes? eyes?), remains unknown.

This study has important implications for prosthetics, but i can't help thinking about consumer products for human augmentation: imagine a Vishnu-like human on the floor of the NYSE, or a team of Goros) working an assembly line, and you can see why this study might come in handy.