He looked shifty . . .


By Alison Motluk THE same area of the brain that controls the fear response also keeps you from buying policies from dodgy insurance salesmen or from talking indiscriminately to anyone who happens to be standing at the bus stop, according to scientists in Iowa. The amygdala, a walnut-sized knot of neurons deep in the brain, matches our first impressions of people with our knowledge from past experiences, giving us the ability to make these sorts of judgments, the researchers claim. Ralph Adolphs, Antonio Damasio and their colleagues at the University of Iowa College of Medicine in Iowa City studied three patients with damage to both sides of their amygdalas. Previously, such patients have been shown to have unusually poor memories for emotionally charged events, and to be incapable of reading expressions of fear or anger on other people’s faces. The researchers showed the patients 100 photographs and asked them to rate the trustworthiness and approachability of the people shown. They asked the patients whether they would trust the people pictured with their money or their life, and whether they would want to strike up a conversation with them. The same pictures were also shown to patients with damage to only one side of their amygdalas, to others with damage elsewhere in their brains, and to volunteers without any form of brain damage. The patients with amygdalas damaged on both sides of their brains stood out from all the others, giving all the faces high ratings—even those judged least trustworthy and approachable by the rest of the volunteers. When asked to choose between the face rated lowest by people with undamaged brains and the one with the highest rating, they could not tell the difference, the researchers report in this week’s Nature(vol 393, p 470). “We all know there are certain people we trust,” says Damasio. But this is the first time anyone has identified the underlying neural circuitry. He believes the amygdala will also turn out to be important in autism, in which sufferers fail to establish normal social relationships. Although there is no firm evidence of this from human studies, monkeys with amygdalas damaged as infants seem to develop social impairments. But the amygdala isn’t needed to assess trustworthiness and approachability if enough information is provided. When the researchers asked the volunteers to judge these qualities in people on the basis of short biographies, the patients with severely damaged amygdalas did as well as the others. Adolphs believes the amygdala is unnecessary if we have sufficient information to make an analytical assessment of someone’s trustworthiness. But he suspects that it will prove to be important in assessing other cues used to make snap judgments,
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