Reading the intentions of others is a valuable social skill. If John sees Mary pick up an apple, for instance, he'd like to know whether she intends to share it with him or hurl it at his head. Now neuroscientists say they've identified a class of neurons involved in figuring out what those around us intend to do next.
In the 1990s, researchers studying monkeys discovered neurons that appear to track the movements of others. The neurons reside in a part of the brain that guides arm reaches, but they also fire bursts of electrical activity when a monkey sees another monkey--or a scientist--reach for a piece of fruit, for example. Using functional magnetic resonance imaging (fMRI) and other techniques, researchers have since found similar "mirror" neurons in humans.
In the new study, neuroscientist Marco Iacoboni at the University of California, Los Angeles, and colleagues tested a hunch that mirror neurons detect intentions as well as movements. They recruited 23 volunteers, who slid into a fMRI scanner and watched short video clips. In one clip, volunteers saw a setup for a small tea party--a teacup and pot alongside a plate of cookies and other goodies. In another, they saw the messy aftermath, complete with cookie crumbs and a used napkin. At the end of both clips a hand reached in from offscreen and grabbed the teacup. The grasping movement was identical in each clip, but the background scenery suggested different intentions: grasping the cup to drink tea in the first clip, versus cleaning up in the second.
The volunteers' brains registered a difference, the researchers report in the March issue of PLoS Biology. An area of the right frontal cortex previously shown to have mirrorlike responses was more active when subjects saw the "drinking" clip than when they saw the "cleaning" clip. And both clips elicited more mirror neuron activity than a clip of a hand grabbing a teacup from a blank background. The findings indicate that neurons in this region are interested not just in the motion, but the motivation behind it, Iacoboni says.
The study "adds an important piece to the puzzle of understanding how we understand the actions of others," says Christian Keysers, who studies mirror neurons at the University Medical Center Groningen in the Netherlands.