Spiders have long given man the jitters. Though relatively unfazed by distant arachnids that do not encroach on their personal space, human beings seem to come equipped with an innate urge to avoid contact with even the tiniest creepy crawlie so long as it sports more than six legs. According to one estimate, over half of women and almost a fifth of men experience arachnophobia. Given that a close encounter with some of the nastier venomous species may have meant death for humanity's ancestors, some evolutionary psychologists believe this to be an evolved predisposition.
All this makes spiders the ideal scary stimulus with which to test human responses to perceived threats. Dr Mobbs and his colleagues were particularly interested to see how the brain distinguishes between how close the threat is on the one hand, and whether it is approaching or retreating on the other. They report their findings in Proceedings of the National Academy of Sciences.
The team asked 25 volunteers to lie down in a functional magnetic resonance imaging (fMRI) scanner with one foot in the first compartment of a purpose-built "imminence box". The box contained another five compartments, each 1' centimetres long, all separated by sliding partitions. The participants were then shown a video feed of a live tarantula being placed randomly into one of these five slots. In each subsequent trial, the subjects were told which compartment the tarantula would enter next and asked to predict how frightened they would be when it did so. They then saw the live tarantula creeping into the indicated slot and asked how afraid they actually felt. (The video had in fact been prerecorded to control for the spider's precise movements; since five of the volunteers appeared to have cottoned on to this, their data were discarded.)
The researchers found that the pattern of their volunteers' brain activity changed, depending on which compartment the tarantula was put in. If a long way from the volunteer's foot, the basic fear network, centred on an area called the amygdala, was quiet and the prefrontal cortex, the bit involved in abstract reasoning, busy. The opposite held true when the compartment was close to the volunteer's foot. Regardless of which compartment the spider was in, though, if it began to crawl towards the volunteer's foot, a specific subsection of the fear network, thought to be the brain's panic circuit, sparked up. Since this happened irrespective of how far the tarantula was from the foot, the fMRI data suggest human brains have a specialised mechanism for determining a looming threat's direction of motion. The researchers also found that when participants' predicted pluckiness did not live up to expectations, the neural mechanisms involved were the same as those that spring to life when someone is faced with any other sudden shock.
Dr Mobbs hopes that by providing a more detailed blueprint of the brain's fear circuitry, his research will prove useful in diagnosing and treating all manner of phobias including those which, like the fear of snakes, heights or confined spaces, as well as spiders, may stem from the evolutionary past. It remains moot whether it will help allay Roosevelt's imponderable fear of fear.
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