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鈥檚 faces.
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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鈥攅ven 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鈥檚 Nature(vol 393, p 470).
鈥淲e all know there are certain people we trust,鈥� 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鈥檛 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鈥檚 trustworthiness. But
he suspects that it will prove to be important in assessing other cues used to
make snap judgments, such as tone of voice.
