Young adults who had been deemed "inhibited" as toddlers had a heightened response in the brain region, called the amygdala, when viewing unfamiliar faces, researchers report in the journal Science.

The study provides a possible physical explanation for some differences in temperament, according to lead author Dr. Carl E. Schwartz, of Harvard Medical School (news - web sites) and Massachusetts General Hospital in Boston.

"These two temperament types are like two flavors of human beings," Schwartz said in an interview with Reuters Health.

"They're not the only two," he added. "There are many, like the list of flavors at Ben and Jerry's."

Schwartz and his colleagues used an imaging technique called functional MRI to take a closer look at the relationship between temperament and brain function.

The 22 adults who took part in the new research had participated in a study of temperament when they were toddlers, Schwartz explained. At that time, researchers determined whether the children were inhibited or uninhibited using a number of tests.

One of the tests involved a battery-operated robot.

"A grad student made a little R2D2 robot," Schwartz said. "The uninhibited toddlers would walk up and poke the robot in the eye and say 'duh.' The inhibited child would freeze or even run to his mother."

For the new study, Schwartz and his colleagues took brain scans of the 22 participants -- 13 of whom had been categorized as shy toddlers -- as they looked at a series of photographs of six different faces.

Next, participants were shown a series of photographs that included the familiar six plus some new ones.

While all 22 showed activity in the amygdala when new faces appeared, people who had been labeled shy as toddlers had a greater response in the brain region.

Schwartz and his colleagues looked at the amygdala because of its established role in emotion.

"It's known to have a central role in processing of stimuli and experiences that have an emotional content," Schwartz said.

But, he said, this and other research "suggests a somewhat broader role" for the amygdala in detecting novelty and changes in the environment.

SOURCE: Science 2003;300:1952-1953.