Research exposing people to spiders has shown that the brain's response to fear is highly complex and involves a balancing act between raising the alarm and signalling the all clear.
New research from the University of Cambridge , published in today's Proceedings of the National Academy of Sciences, suggests that when these systems malfunction, we can develop anxiety problems such as post-traumatic stress disorder.
Dean Mobbs and colleagues used tarantula spiders to induce various levels of fear in study participants, while scanning their brains using magnetic resonance imaging (MRI).
As the participants lay in the MRI tunnel, they were asked to put their foot inside a box and shown a supposedly live video feed of a spider being placed somewhere in the same box.
The volunteers were told that the spiders were at various distances from their foot, from less than 18 centimetres to as much as 90 centimetres. Some spiders were allowed to approach the foot while others were moving in the opposite direction.
The volunteers were actually seeing pre-recorded spiders, although they didn't know this.
Spiders activate a complex fear circuitry
The brain scans revealed a complex cascade of fear responses, involving a number of brain structures.
As the tarantula moved closer, certain areas of the brain lit up, corresponding to increased fear. But as the spider retreated, a completely different part of the brain kicked in, signalling that all was safe.
The brain scans also showed that no matter how close the volunteers thought the spider was, they were less afraid if it was moving away than moving towards them. The spider's movements were monitored in the brain by the amygdalae - almond-shaped structures responsible for regulating the emotions, and commonly associated with fear and anxiety - and the connected bed nucleus of the stria terminalis (BNST).
The BNST is a band of nerve fibres acting as an information highway for anxiety and stress signals to reach various parts of the frontal lobe, where the information is assessed.
The researchers also noted that when the spider moved further away, a part of the brain called the orbital and medial prefrontal cortex (omPFC) became active. This is where functions including perception, motor control, self-awareness and cognitive functioning are dealt with. It's also the part of the brain which sends out natural opiates.
The researchers believe this region of the brain might be helping us cope with a distant threat, calming us down while we wait to see what happens next.
When the spiders were close, there was greater activity in the periaqueductal grey (PAG) area of the brain. Stimulating the PAG can provoke defensive responses characterised by freezing immobility, jumping, racing heartbeat, and increases in blood pressure. They are all typical responses to terror, but normally people's sensitivity is reduced with exposure.
Brain region malfunctions in phobics
The researchers say a malfunction in the PAG may be responsible for the extreme fear that phobics have, and may be the reason why they never get used to the things that make them so anxious.
Neuroscientist Professor Pankaj Sah from the Queensland Brain Institute says that studying the circuitry of fear can help understand some phobias.
"The response to the fear that you get is basically an anxiety attack. So if you understand the circuitry, you have some understanding of what goes on in, for example, post-traumatic stress," he says.
"In this study, they were looking for which part of the brain turns on when something is not threatening any more."
"We are seeing the orbitofrontal cortex turning on [when the threat moves away]. This activity may be suppressing the fear. Maybe this is the bit that says 'relax, there's no need to fear now.' In terms of treating phobias, it's suggesting that the orbitfrontal cortex is something you should think about. If someone is so disabled by their phobia and nothing is working, you might want to think about stimulating the OFC [with deep brain stimulation] to give them some relief."
He said the new study exposed participants to a much more realistic form of fear than previous research.
"Those studies just showed people nasty pictures of car crashes, but spiders are something we are hardwired to avoid."