Fear is a complicated emotion, and scientists have recruited a scary laboratory aide — the Brazilian salmon pink tarantula — to help map out how the feeling is processed in the brain.
Using video of the 8.7-inch-long arachnid, British researchers showed that the human brain engages several different systems when evaluating threats. For instance, the part of the brain that engages when a threat is approaching is different from the part that is activated when a threat is receding, they reported Monday in the journal Proceedings of the National Academy of Sciences.
Previous work had shown that avoiding a threat people were trained to fear in the lab caused brain activity to switch from anxiety-related regions to panic centers. But it was not clear that people would respond in the same way to something they feared naturally.
The researchers chose a spider species for the test because fear of spiders is one of the most common phobias in human beings, said Dean Mobbs, the study's lead author and a neuroscientist at the Cognition and Brain Sciences Unit at the Medical Research Council in Cambridge, England.
"Most of us have not been bitten by a spider, but we seem to have this innate fear of them," Mobbs said. "At just the mention of the word people tend to shake in their boots."
"I'm not a big fan of them myself," he added.
The researchers recruited 20 volunteers and placed them in a magnetic resonance imaging machine to watch which parts of their brains were activated during different stages of the experiment. Each volunteer was asked to place a shoeless foot into a box that was connected to a row of five other boxes.
The researchers asked the subjects how scared they thought they would be when the tarantula was placed into each of the boxes. Then they showed them four-second videos of the creature entering the compartments.
When the tarantula was placed far away from the foot, the MRI machine showed brain activity in the part of the prefrontal cortex involved in recognizing threats. As the tarantula appeared to move closer to the foot, a different fear system kicked in inside a primitive region of the midbrain associated with the fight-or-flight response.
In other words, as the spider advanced toward people's feet — regardless of its absolute distance — their brains switched from anxiety to panic.
In addition, as the tarantula appeared to recede, the MRI machine registered increased activity in the orbitofrontal cortex. Mobbs said this area might emit a safety signal that dispels the fear signal.
The subjects were led to believe that the tarantula was entering a box while they lay in the brain scanner, but in reality there were no spiders present. The video was prerecorded so the scientists could control for the tarantula's movements.
When people underestimated their own fear, they also remembered the tarantula being bigger than those who had a better handle on their own fear responses. That suggests fear affects cognition as well, Mobbs said.
"It's a clever study," said Elizabeth Phelps, a neuroscientist at New York University who wasn't involved in the research.
The researchers tested only people whose fear of spiders was low to middling. But learning where proper fear responses break down in people with phobias or anxiety issues could eventually help researchers develop targeted therapies for them, Mobbs said.