A man lies in a brain scanner, with his foot in one end of a long, narrow box, which is divided into six compartments of equal size.
On a screen he watches a tarantula crawling in one of the compartments. A hand reaches in and moves the spider into another compartment, first one further away from the man's foot, then one closer.
Although the subject was led to believe he was viewing the scene unfold in real-time, the man was actually watching a previously recorded video of the tarantula creeping through the boxes, nowhere near the subject. His fear, however, was real.
The scanner -- a functional magnetic resonance imager (fMRI) -- allowed researchers to capture that fear by recording the activity in his brain as he watched the spider, illuminating the hallmarks of the human fear response in the man's brain.
In a study of 20 individuals who watched the same video, researchers report today that our brains evaluate fear in a nuanced way, drawing on several different regions depending on the proximity, trajectory and our expectations of the feared object -- in this case a Brazilian salmon pink tarantula.
By better understanding which of these brain regions fail to function normally when confronted with fear, the authors hope their findings could one day help treat people with phobias.
"I think the take home message is that what we've shown is that in tandem there are a bunch of systems that are working all at the same time," said study leader Dean Mobbs of the Medical Research Council-Cognition and Brain Sciences Unit in Cambridge, United Kingdom.
The study implicates the same regions that have been found to respond to fear in rat studies and in more contrived human studies, said Elizabeth Phelps of New York University. "But it's nice to be able to verify that these same systems are involved in realistic threats."
"This is a collection of regions that adjust our fear response to the appropriate level given the circumstances," she added. In people with phobias or anxiety disorder, these responses are not in proportion to the threat.
The study new study, published in the Proceedings of the National Academy of Sciences, looked at three aspects of the fear response: the proximity of the feared object, its trajectory and the subject's expectation of how scary it's going to be.
In the first, the researchers showed that different parts of brain were more active when the spider was further away versus closer. When the spider was distant from the subject's foot, the prefrontal cortex, involved in judgment, was more active.
But when the spider got closer, the prefrontal cortex became less active while more primitive parts of the brain -- parts of the amygdala and midbrain -- took over.
"When it's farther away, the brain assesses, 'This is a situation we don't want to be in. Move away from the situation, calmly,'" Mobbs said. "But when a threat comes closer, we need to switch into faster responses in the brain. What you're seeing when it's further away is more anxiety. When it comes closer, you're switching into a panic response, into a different system of fear."
The second aspect the researchers considered was what happened when the spider moved further away from its current position versus when it came closer, regardless of the actual distance from the person's foot. In this case, a part of the brain associated with vigilance was active, and the subjects' fear ratings were higher as it advanced versus as it retreated.
To study the third aspect, the researchers asked the subjects to predict how scared they thought they would feel when the spider moved into a specific compartment. About 45 minutes after the scan, they asked the subjects how they had actually felt. Those that were more scared than they thought they would be showed heightened activity in the amygdala.
"I think this finding means that if we have an expectation that something's going to be scary, we prepare ourselves for it," Mobbs said. "If we don't have an expectation, it's drawing all of your resources towards that."
When things go wrong in our fear responses, as in phobias, any of these parts of the system could be involved, he said, or dysfunction in one could bring down the whole network. "It could be like a scaffold effect," he said.
Understanding which parts of the brain are triggered during a fear response could help doctors target therapy for their patients based on how subjects react to certain stimuli. It could also lead to more targeted drug therapy.
For now, the findings give researchers greater confidence in extrapolating what is known from detailed rat studies to realistic human fear.
"In rats we can study the neurochemistry, but we can't in humans," Phelps said. "Looking at this study and knowing that (the findings) apply to these circumstances in humans, we can then take advantage."