The researchers at the Wellcome Trust Centre for Neuroimaging at UCL, London said that their development could help people suffering from panic attacks.
Using a "Pac Man"-like computer game wherein a volunteer is chased by an artificial predator, the researchers showed that the fear response moves from the strategic areas of the brain towards more reactive responses as the artificial predator approaches.
To explore what happens in the brain in such a situation, the researchers scared volunteers with a game, in which subjects were chased through a maze by an artificial predator. If caught, they received a mild electric shock.
Simultaneous brain scans measuring blood flow showed that when the predator was far-off, lower parts of the prefrontal cortex area of the brain behind the eyebrows were active. This region is associated with complex decision-making, such as planning an escape.
But when the predator moved closer, activity shifted to the periaqueductal grey area, responsible for quick-response survival mechanisms such as fighting, flight or freezing. This region is also associated with the body's natural pain killer, opioid analgesia, preparing the body to react to pain.
"Without fear, animals would not react to threats. This is a poor survival strategy and makes it more likely that the animal will be eaten and not pass on its genes," Dr Dean Mobbs from UCL, lead author on the study, said.
"The most efficient survival strategy will depend on the level of threat we perceive. This makes sense as sometimes being merely wary of a threat is enough, but at other times we need to react quickly. The closer a threat gets, the more impulsive your response will be - in effect, the less free will you will have," he added.
Although this natural defence mechanism is beneficial in evolutionary terms, Dr Mobbs believes that malfunctions in the system might help make clear why some people suffer from anxiety disorders and panic attacks.
"When our defence mechanisms malfunction, this may result in an over-exaggeration of the threat, leading to increased anxiety and, in extreme cases, panic. Although brain-imaging studies like ours cannot directly help to cure such disorders, they do improve our understanding of how the emotional system operates. This is the first step to helping people with anxiety-related disorders," Dr Mobbs said.
The findings of the study are published in the journal Science.