These microexpressions can alter perception and the way other people are treated or judged, said Ken Paller, co-investigator of the study and professor of psychology at Northwestern University, in Evanston, Ill.

"Even though our study subjects were not aware that they were viewing subliminal emotional expressions, their brain activity was altered within 200 milliseconds," Paller said in a statement.

Sometimes when it seems people are acting on the vagaries of instinct, the brain is responding to real information about others that bubbles just beneath consciousness, Paller said.

The study, published in the Journal of Cognitive Neuroscience, included tests to determine whether study volunteers had a tendency to experience anxiety, particularly in social situations. Those who tended to be socially anxious had the strongest brain response to subliminal expressions of fear, the study said.