Poker players and police investigators alike know to watch the eyes and, now, so do scientists who study learning.
They say they can see a slow buildup in the eye movements of people just about to have a eureka moment.
In fact, the researchers said they saw the insight building even before their subjects knew they were about to discover the secret to playing a complex numbers game.
"We could see our study participants figuring out the solution through their eye movements as they considered their options," said Ian Krajbich, an assistant professor of psychology and economics at Ohio State University, who helped lead the study team.
"We could predict they were about to have an epiphany before they even knew it was coming," Krabjich said in a statement.
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His team recruited 59 students at Ohio State to take part in the game. It involved a somewhat complex mathematical formula. They each played a series of opponents.
The team used an eye movement tracking apparatus while the players looked at a dial of numbers to make their choices.
"We settled on the two-person beauty contest game, in which each of two players chooses an integer from 0 and 10 with the goal of getting closer to 0.9 multiplied by the average of the two numbers," they wrote in their report, published in the Proceedings of the National Academy of Sciences.
"An important feature of this game is that there is an optimal number regardless of what the other player chooses. That number is 0," they added.
"Prior research has shown that most people initially fail to realize that zero is the optimal strategy, but do eventually figure it out."
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In this experiment, 42 percent of the students figured out that zero was a winner.
That realization comes suddenly, they added. "This 'epiphany' learning is characterized by an unexpected moment of insight, often portrayed in cartoons by a light bulb appearing over a person's head," the team wrote.
"There's a sudden change in their behavior. They are choosing other numbers and then all of a sudden they switch to choosing only zero," Krajbich said. "That's a hallmark of epiphany learning."
The researchers could see people's eyes returning to the zero, and other lower numbers, more often as they began to figure it out. They saw something else, too.
"When your pupil dilates, we see that as evidence that you're paying close attention and learning," Krajbich said. And the group of students who figured out the secret to winning had dilated pupils.
"They were showing signs of learning before they made the commitment to zero," Krajbich said. "We didn't see the same results for others."
There was another lesson in the game. A full 37 percent wrongly decided the winning number was one other than zero.
"One thing we can take away from this research is that it is better to think about a problem than to simply follow others," Krajbich said.
"Those who paid more attention to their opponents tended to learn the wrong lesson."