The changes occur in the amygdala, a region of the brain that serves as an "alarm" to activate a cascade of other biological systems to protect the body in times of danger, said Matthew D. Lieberman, assistant professor of psychology at UCLA and lead author of the study.
The findings will be published May 8 in the online version of Nature Neuroscience, and later in the print version.
Five out of eight African Americans (63 percent) responded with significantly more amygdala activity when presented with expressionless photographs of African Americans than when they were shown expressionless photographs of Caucasians, Lieberman and his colleagues found. Seven of 11 Caucasians (64 percent) in the study also responded with greater activity in the amygdala when viewing the African American photographs.
Although a third of participants in each race did not show this effect, no participant in the study responded with greater amygdala activity to the Caucasian photographs than to the African American photographs, Lieberman said.
"We didn't see any differences in amygdala activity between the racial groups," Lieberman said. "From looking at the amygdala, you couldn't tell if the scans were from African American or Caucasian participants.
"Many people of either race may not be happy to find out that a part of their brain involved in responding to potential threats responds more to African Americans than Caucasians," Lieberman said. "Even people who believe to their core that they do not have prejudices may still have negative associations that are not conscious."
Why do African Americans have this amygdala response?
"One theory," Lieberman said, "is that people are likely to pick up the stereotypes prevalent in a society regardless of whether their family or community agrees with those stereotypes. Several social psychologists have found evidence for this view. From an early age, cultural views, media portrayals and even the body language of authority figures may train our brains, whether we consciously agree or not."
Previous research has shown that Caucasians show an increased amygdala response to African American photos to the extent that they hold nonconscious negative attitudes towards African Americans, Lieberman said.
Co-authors on the study are Johanna Jarcho, a UCLA graduate student in Lieberman's laboratory; UCLA graduate student Naomi Eisenberger; Susan Bookheimer, professor of psychiatry and biobehavioral sciences at UCLA's David Geffen School of Medicine; and Ahmad Hariri, assistant professor of psychiatry at the University of Pittsburgh School of Medicine and a former UCLA graduate student.
The researchers also studied whether adding a verbal label (such as "African American") when viewing African American photos changes the amygdala response, and found it does.
"When people look at an African American and think of the word 'African American,' we no longer see the amygdala response," Lieberman said. Instead, the researchers found changes in a second region of the brain: the right ventrolateral prefrontal cortex. This region of the brain is located behind the forehead and eyes, and has been associated with thinking in words about emotional experiences; it also is associated with inhibiting behavior, impulses and emotions.
"This region is especially active when you add the verbal label to the face," Lieberman said. "The people who show the most activity in the right ventrolateral prefrontal cortex show the least activity in the amygdala.
"We found that when the right ventrolateral prefrontal cortex gets turned on, the amygdala does not," he added. "When you engage in verbal labeling, that partially turns off or disrupts the amygdala response. The right ventrolateral prefrontal cortex was significantly active only when people were looking at African Americans and choosing the word 'African American.'"
These results suggest that "thinking about the race of others in words may regulate some of the threat experienced when confronting unfamiliar or feared others," Lieberman said. "It is possible this emotional 'benefit' of using race-related words may have inadvertently contributed to the widespread use of race-related words and stereotypes."
Lieberman and his colleagues used functional magnetic resonance imaging (fMRI) to examine brain activity for this study, conducted at UCLA's Ahmanson-Lovelace Brain Mapping Center.
The research was supported by the National Science Foundation and the National Institute of Mental Health.