SAN DIEGO--Fear, though unpleasant, often keeps us away from dangerous situations that could get us killed. Now a study presented here 11 November at the Society for Neuroscience meeting suggests that for humans, what's really scary is frightened or angry people. A group of researchers reports that the brain structure responsible for fear reacts more strongly to pictures of terrified faces than to the scary images themselves.
Sudden, intense fear generates a "fight-or-flight" response, not just in humans but in many animals. The brain's central fear processor is a structure called the amygdala, which is primarily responsible for pumping up the heart rate, heightening awareness, and sending a surge of adrenaline that prepares an animal to respond to danger.
Using functional magnetic resonance imaging, neuroscientist Ahmad Hariri and his colleagues at the National Institute for Mental Health in Bethesda, Maryland, imaged the brains of 12 subjects while they viewed pictures of faces with angry or fearful expressions. They also imaged the subjects' brains as they looked at pictures of fearsome or threatening situations, both natural (snakes, spiders, sharks) and humanmade (explosions, car wrecks, guns). Both sets of pictures elicited a strong response from the amygdala, but the response to faces was stronger. The team also measured skin conductance, a measure of how much someone is sweating, during the photo viewing and found that the facial expressions caused a much larger change in skin conductance. Hariri suggests that the stronger response to facial expressions is adaptive; facial stimuli help us detect human threats as well as lions and tigers and bears.
Neuroscientist Adam Anderson of Stanford University says the evolutionary link may be weak, though, because facial expressions don't help lower animals. "Lizards don't have an amygdala because they're supposed to be responding to lizard facial expressions," he says.