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Feet That Know What Hands Are Doing

13 July 2007 (All day)
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Gazzola et al., Current Biology

Scanning for hands.
In brain scans from both a composite of normal volunteers (bottom) and of an individual born without hands (top), motor circuits light up while watching others move their hands. This suggests that the disabled volunteer is mapping the motions onto other motor pathways.

Monkeys imitate what they see, but so do humans, only more discreetly. Whether any muscles actually flex, our brains fire up the same pathways needed to perform any action we observe another person perform. But new work on disabled volunteers indicates that the brain instead activates alternate circuits when faced with an action its body cannot physically copy. The research suggests that the brain's motor system may be wired to work toward a goal rather than just duplicating a movement.

Every time you watch someone press a computer key or pick up a cup, regions of your brain unconsciously respond, mapping what you see onto the motor pathways you would use to carry out that same motion. Researchers believe that this so-called mirror neuron system, which consists of a subclass of motor neurons, is critical to learning new behaviors, and perhaps for developing skills like recognizing facial expressions. But neuroscientists have long wondered how the brain reacts if the body lacks the ability to replicate the action.

In a study reported 12 July in Current Biology, a team from the Netherlands, Italy, and France showed videos of hands performing simple actions, like grasping a cup, to 16 "normal" people and two aplasic people born without hands or arms. While the volunteers watched the video, the researchers spied on their brain activity using MRI.

The team found that when observing the hand motions, all the normally developed people activated motor pathways that would have allowed them to hold a cup. The aplasic people activated motor pathways too, but not the same ones, says neuroscientist and senior author Christian Keysers of the University of Groningen in the Netherlands. Their brains lit up in regions needed for moving the feet. The results underscore that the mirror neuron system isn't mindlessly imitating, but working toward a goal, he says. The two people without hands or arms recognized they could lift a cup with their feet--and their brain lit up accordingly.

According to Marco Iacoboni, a neuroscientist at the University of California, Los Angeles, the study nicely demonstrates that "the role of mirror neurons is to let us mirror people, not just actions." However, he points to a great deal of variability in the level and location of brain activity in both groups. Without a larger study, he says, it's impossible to determine whether these phantom movements are universal.

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