While it's easy to forget what was on last Sunday night's dinner menu, some events, like your own wedding or a close encounter with a grizzly bear, become permanently engraved in your mind. In the March issue of Nature Neuroscience, researchers report how a region of the brain called the amygdala helps cement emotionally charged events into long-term memory.
The amygdala, an almond-shaped part of the temporal lobe, has long been linked to emotions. Numerous studies in animals have shown that the amygdala facilitates the long-term memory of traumatic and frightening events, and a handful of human studies have hinted at the same. Lead author Stephan Hamann, a cognitive neuroscientist at Emory University in Atlanta, wondered if the amygdala was also involved in remembering happier events.
To find out, Hamann's team asked 10 male volunteers to look at a dozen pictures. Some of these featured sexually arousing scenes or appetizing food, meant to elicit positive emotions, while others, such as photos of mutilated bodies and lethal violence, were designed to arouse strong negative feelings. The rest of the images lacked an emotional element but varied from unusual and visually interesting (like people in exotic costumes) to downright boring. While participants scanned the photos, the research team measured activity in their amygdalas by tracking blood flow in the brain using positron emission tomography (PET) scanning.
To confirm that the subjects were experiencing the intended emotions, researchers monitored their heart rates and asked them to categorize their feelings about each photo. Pictures that had drawn any strong emotional response triggered an increase in amygdala activity and were remembered better even a month later; photos lacking emotional content, whether unusual or not, caused little amygdala activity and were mostly forgotten.
The study confirms the amygdala's involvement in remembering emotional events, says Larry Cahill, a neurobiologist at the University of California, Irvine. It also deserves merit for discounting the notion that novelty--rather than emotion--might be responsible for improved recall of some events. "It's not every day that you almost get hit by a bus," says Cahill, "but this study shows it's the emotional aspect, not the unusualness of the event, that commits it to memory."