For the trust experiment, funded by the Brown Foundation Inc. in Houston, the researchers often paired a volunteer in a brain scanner at Baylor with one at Caltech, more than 1,300 miles away. The researchers at Baylor and Caltech have conducted the experiment with 144 people — the largest interactive brain-imaging study ever.
"In this game, trust builds up, and it must exist somewhere in the brain," said Caltech neuroscientist Cedric Anen. "But there is not one event where we can say, 'That is trust.' We don't know when it starts, how it builds up or what is involved."
The results, so far unpublished, reveal that financial dealings seem to engage neural networks in the cingulate cortex, an area of the brain involved in switching between tasks, monitoring errors and short-term memory.
In sprays of light on a computer screen, the researchers could see how levels of activity shifted. Men typically showed the greatest activity in the seconds before making an investment decision, women in the moments before they revealed their decision to their trading partner.
In Belur's and Tang's paired brains, the offers and counter- offers — signaled by pushing buttons inside their linked scanners — triggered heightened activity along a crescent-shaped strip of brain tissue in the cingulate that appears to track responsibility for social interaction.
With each round of negotiations between the two women, a reputation for fair dealing took hold in their neural tissues.
"Trust is one of those few notions that underlies everything from individuals making decisions together to huge policy questions between nations," said Steve Quartz, director of Caltech's social cognitive neuroscience laboratory. "For a long time, we thought this was a state beyond measurement.
"The brain scanner is beginning now to put a yardstick up against it, to provide a measure for it."
In deconstructing the biology of trust, other researchers have determined that the brain appears to prize that bond between two people biochemically, secreting a powerful hormone to cement working relationships.
The act of trust correlates with elevated levels of a brain hormone called oxytocin, the same chemical released during breast-feeding and uterine contractions, according to experiments done by researchers at Claremont Graduate University.
"It literally feels good to cooperate," said Paul J. Zak, director of the Center for Neuroeconomics Studies at Claremont. As the hormone level rose, people also were more likely to reciprocate trust. "The stronger the trust, the more the oxytocin went up, and the more trustworthy you were.
"Interestingly, participants in this experiment were unable to articulate why they behaved the way they did," Zak said. "But nonetheless their brains guided them to behave in 'socially desirable' ways — that is, to be trustworthy."
Inside the Baylor scanner, Belur invested another $20.
She signaled her decision, then awaited Tang's next move.
Was trust its own reward?
When a decision forms, the brain moves faster than self-awareness.
The brain unconsciously prepares to act a measurable length of time — up to 500 milliseconds — before a person consciously decides to act.