A Pac-Man-like computer game that delivered electric shocks to gamers has been used to shed light on how the brain reacts to imminent danger.
An MRI scan shows the active areas of the brain
Scans showed the different regions of the brain used by volunteers as the level of threat in the game increased.
The scans showed that activity switched from the front of the brain to the middle as anxiety turned to panic.
Writing in the journal Science, the researchers said the change was crucial for an animal's survival.
"Without fear, animals would not react to threats," said Dr Dean Mobbs of University College London and one of the authors of the paper.
"This is a poor survival strategy and makes it more likely that the animal will be eaten and not pass on its genes."
Fight or flee
The team asked volunteers to play a computer game in which they had to move a blue triangle through a 2D maze while avoiding a red dot "predator". If the predator caught the triangle, the volunteer received an electric shock.
As the volunteers played the game, Dr Mobbs used an MRI scanner to monitor their brain activity. The scanner showed which regions of the brain were receiving the highest flow of blood.
The higher the blood flow to an area of the brain, the more active that area is, explained Dr Mobbs.
As long as the predator was some distance away, blood flowed most strongly to the prefrontal cortex in the forebrain, Dr Mobbs found.
The forebrain is active during periods of anxiety, and helps coordinate escape strategies to avoid the threat, he said.
But when the computer game predator moved nearer, blood flow switched to the midbrain.
The midbrain is a primitive area of the brain, and it controls gut-level reflexes such as the decision to fight or flee, said Dr Mobbs.
"When a fast response is needed, the midbrain may inhibit the prefrontal cortex," he said.
"The closer a threat gets, the more impulsive your response - in effect, the less free will you have," he added.
The prefrontal cortex is much larger in modern humans than it was in our ancestors, and so we may have evolved to be more adept at avoiding threatening situations, thinks Dr Mobbs.
"We are probably better survival machines now," he said.