The researchers got about the same response from everybody when the subjects where shown an image with an unpleasant or threatening expression. The amygdala always lit up.
But it was a different story for a happy face.
Which Comes First?
The extroverts always responded, as revealed by the amygdala, but the shy folks showed little response.
Now, 15 is not a lot of participants, "but the results were extremely robust," Gabrieli says.
"We got this very clean association between amygdala reaction to two different kinds of emotional facial expressions," says Canli.
What that means, he adds, is "the more outgoing someone is, the more his or her brain is likely to react to something perceived as pleasant."
What's still debatable, however, is who's giving the orders.
Is the amygdala calling the shots, telling some of us to react and others to remain stoic, or is the personality telling that crucial part of the brain what it wants to do.
It is, as Gabrieli puts it, the old "chicken and egg thing." The study doesn't tell us where personality comes from.
We know genetics plays a role, because a shy infant tends to remain shy at least though childhood. But "why is it that some people are so different from one another in what they find enjoyable or threatening?" Gabrieli asks.
The Stanford study adds another building block to our understanding, thanks to some amazing new tools that are helping researchers close in on the biological processes that are so closely linked with personality.
But we still have a long ways to go before we will understand why each of us became what we are.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.