A study by researchers from Harvard, Yale and the University of Toronto published in this month’s Psychological Science reveals the existence of unconscious race bias as measured by brain activity.

The study, conducted at Yale University, suggests that racial bias can be found in the amygdala, a region of the brain involved in emotional response, before it is censored by the frontal cortex and the conscious brain.

“The study originated out of an earlier project where we didn’t find amygdala activation in comparisons of black faces and white faces,” said William A. Cunningham, assistant professor of psychology at the University of Toronto and an author of this project. “We figured that what was happening was that the subjects had an automatic response but were then trying to control it.”

The study included 13 white subjects between the ages of 18 and 30 who were shown pictures of the faces of black people while their brain activity was being monitored. The subjects were presented with these images for either 30 milliseconds—too short for the subjects to be aware of seeing an image—or 525 milliseconds, which is long enough to recognize faces.

While these images were viewed, the researchers used functional magnetic resonance imaging (fMRI) to monitor brain activity. The researchers paid special attention to activation of the amygdala and the fusiform gyrus, the region of the brain known to be involved in processing human faces.

The white subjects, who were proven before the experiment to be consciously unprejudiced, experienced greater activation in these regions when shown black faces subliminally than white faces. In contrast, when the images were displayed for 525 milliseconds, the frontal cortex—the area of the brain that deals with inhibition and control—experienced greater activation and the amygdala experienced less activation than in the subliminal condition.

The researchers have not yet studied the brain activity of black subjects viewing white faces.

“It’s a study that we really wanted to run, but it’s trickier. If we had run the study at Yale with African-American students, would that be representative of the whole culture? We want to do the study in Toronto, where we have a more multicultural sample,” said Cunningham.

The researchers are optimistic about the implications of their study for human nature.

“There’s a lot of research that suggests that prejudice is inevitable, or that because people have these automatic responses, they’re not responsible for those responses,” said Cunningham. “But this shows that although the responses exist, people do have the ability to control them. Perhaps our conscious attitudes can limit our personal responses.”