When shown pictures of unfamiliar faces, adults who were shy toddlers showed a relatively high level of activity in a part of the brain called the amygdala. Adults who were more outgoing toddlers showed less activity in this brain structure, which is related to emotion and novelty. The findings appear in the journal Science, published by the American Association for the Advancement of Science (AAAS).
Scientists have long been interested in finding explanations for differences in temperament, the stable moods and behavior profiles that emerge in infancy and early childhood. One of the most well studied facets of temperament is how people respond to novelty.
Inhibited children tend to be timid with new people, objects, and situations, while uninhibited children spontaneously approach them.
"Now we're suggesting that that same link continues through life," said lead author Carl Schwartz of Massachusetts General Hospital and Harvard Medical School.
"We found that individual differences in temperament are associated with persistent differences in the responsivity of the amygdala, after more than 20 years of development and life experience," he said.
The Science study is the latest chapter in a long-term study of child development. Jerome Kagan of Harvard University (also a co-author on the current study) and colleagues initially categorized a large group of children, around age two, as inhibited or uninhibited. Schwartz's team then studied the children's behavior again, around age 13. Now, approximately nine years later, Schwartz and his colleagues have compared their test subjects' brain activity, using fMRI scans to monitor a subset of 22 individuals, around age 21.
While the authors assumed that the brain differences they found at age 21 would have also existed at younger ages, this hypothesis still needs to be tested.
Some studies have suggested that inhibited children may be more likely to develop social anxiety disorder than their uninhibited peers, and that the disorder in childhood may be an important precursor to depression in adulthood, according to Schwartz. Knowing the biological underpinnings of inhibition may help specialists identify children who may be at risk for the disorder, he said.
"It's only by understanding these developmental risk factors that one can really intervene in the lives of children early, to prevent suffering later in life," Schwartz said.
While social anxiety disorder is currently treated in adults with drugs called SSRIs and behavioral therapy, it's not yet clear what the best interventions would be for children, according to Schwartz.
The study's findings also have important implications for how researchers think about the biological causes of psychiatric disorders, Schwartz said. Two of the adults in the study, who belonged to the inhibited group, had been diagnosed with social anxiety disorder. When the researchers removed these two individuals, there was still a clear difference in brain activity between the inhibited and uninhibited groups.
Researchers have frequently used fMRI to discover differences in brain activity between subjects with a psychiatric diagnosis and a "control group" of healthy individuals. Rather than assuming that these differences underlie the disorder itself, researchers should consider that they may be related to temperament, Schwartz cautioned.
"There is a relationship between temperament and this later [social anxiety] disorder but it's not deterministic. Temperament is not a pathological category, but a flavor that humans come in," said Schwartz, also noting that environmental factors profoundly influence behavior as well.
Schwartz's team showed the test subjects a series of faces, some of which they were seeing for the first time, and some of which they had seen previously, during a "familiarization phase." The researchers analyzed the whole brain, but had hypothesized that differences would turn up in the activity of the amygdala, an almond-shaped structure involved in emotion and novelty detection.
Indeed, the amygdalar response to the unfamiliar faces was stronger in the inhibited group than it was in the uninhibited group. The responses from both groups were roughly the same for the familiar faces, however. The faces used in the study had neutral expressions.
Schwartz proposed that in inhibited individuals, the amygdala has a lower threshold for firing in response to novel stimuli.
Schwartz's co-authors are Christopher I. Wright at Massachusetts General Hospital, Harvard Medical School, and Brigham and Women's Hospital, in Boston, MA; Lisa M. Shin at Massachusetts General Hospital, Harvard Medical School, in Boston, MA and Tufts University in Medford, MA; Jerome Kagan at Harvard University, in Cambridge, MA; and Scott L. Rauch at Massachusetts General Hospital, Harvard Medical School, in Boston, MA The study was supported by the Milton Fund of Harvard University and The Mental Illness and Neuroscience (MIND) Institute.
The American Association for the Advancement of Science (AAAS) is the world's largest general scientific society, and publisher of the journal, Science (www.sciencemag.org). AAAS was founded in 1848, and serves some 265 affiliated societies and academies of science, serving 10 million individuals. Science has the largest paid circulation of any peer-reviewed general science journal in the world, with an estimated total readership of one million. The non-profit AAAS (www.aaas.org) is open to all and fulfills its mission to "advance science and serve society" through initiatives in science policy; international programs; science education; and more. For the latest research news, log onto EurekAlert!, www.eurekalert.org, the premier science-news Web site, a service of AAAS.
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