Human see. Human do. As with monkeys, it’s apparently the same for some nerve cells in the brain.
Macaque monkeys have specialized brain cells — called mirror neurons — that activate when a monkey performs an action involving an object, such as picking up a grape, or when watching someone else do the same task. The discovery of these neurons in 1996 led to speculation that they could be involved in everything from simulating others’ actions to language development to autism. There was only one problem: no one had definite proof that such cells exist in humans.
Now a new study in the Aug. 12 Journal of Neuroscience provides strong evidence that humans have mirror neurons too.
Researchers used functional MRI to examine volunteers’ brains for signs of mirror neurons. While in a scanner, volunteers either performed two different types of grips — a precision grip or putting a finger through a ring and pulling the ring — or watched videos of someone else making the movements. Groups of neurons in a part of the brain called the inferior frontal gyrus responded both to watching and doing the same action, researchers led by James Kilner, a neuroscientist at the Wellcome Trust Center for Neuroimaging at University College London in England, reported.
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Other groups have tried various techniques to discover human mirror neurons, but without success. Those groups had used volunteers perform or imitate that didn’t involve objects, such as playing rock, paper, scissors or making undirected motions. But interactions with objects are necessary to activate mirror neurons in monkeys, says Scott Grafton, a cognitive neuroscientist at the University of California, Santa Barbara. Kilner succeeded in finding the neurons because his experiments explored the interaction between movements and objects, Grafton says.
“This is a knowledge system that captures information about body movements,” Grafton says. Experiments similar to Kilner’s might help determine whether mirror neurons hold information simply about reaching for an object, or if the cells also react to subsequent actions with that same object — such as turning a dial once it has been grasped.
Objects’ crucial role in activating mirror neurons in both monkeys and people “might have some deep important meaning,” says Ilan Dinstein, a neuroscientist at New York University. Kilner’s experiment suggests that humans have mirror neurons for movements that involve objects, but not facial expressions or other types of motion. That could mean that some of the functions ascribed to mirror neurons may not hold true.
“A flood of theories regarding what mirror neurons do in humans came out before anyone proved they exist or not,” Dinstein says. “A lot more groundwork needs to be done before people can talk about these theories of simulation, language and so forth.”
Kilner doesn’t expect his work to be the last word on whether humans have mirror neurons. Until researchers can watch single mirror neurons firing, as with monkeys that have electrodes implanted in their brains, there will still be questions about the existence of human mirror neurons, he says.