Mirror system gets an assist

In amputee, brain systems work in tandem to understand observed task

When a woman born without limbs watches someone else sew, copycat regions in her brain activate even though she can’t hold a needle herself. Additional brain regions also lend support, demonstrating how flexible the brain is when it comes to observing and understanding the actions of others.

Scientists have known for over a decade about the mirror system, a network of brain regions usually activated by watching and performing an action. But just how the brain smoothly and quickly intuits what other people are doing, particularly when the action isn’t something the observer can do, has been unclear, says study coauthor Lisa Aziz-Zadeh of the University of Southern California in Los Angeles. 

In the study, a middle-aged, healthy woman born with no arms and legs underwent brain scans as she watched videos of people performing actions such as holding and eating an apple slice, sewing with a needle and tapping a finger. Actions that the woman was capable of performing herself activated the mirror system, including parts of the brain that control movement. Mirror areas kicked in even for tasks the woman accomplishes in a different way, such as picking up food using her mouth instead of hands. (The participant had prosthetics briefly as a teenager but hadn’t used them in the past 40 years.)

When the woman witnessed actions that were impossible for her, such as using scissors, her brain’s mirror system still kicked in, but additional brain regions were recruited to help. These extra regions aren’t normally needed when people watch a task they’re able to perform, the researchers write in an upcoming Cerebral Cortex. These regions are thought to be involved in a process called “mentalizing,” in which a person tries to understand what someone else is thinking.

“What’s interesting is that even when she can’t do it, when it’s impossible for her, she still recruits her mirror system, but she additionally recruits these mentalizing regions,” Aziz-Zadeh says.

By suggesting that the mentalizing system kicks in for this woman when she cannot copy an action, the new study helps clarify how these two brain systems work together, says cognitive neuroscientist Marcel Brass of Ghent University in Belgium.   

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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