Adjusting the balance either retains or eliminates fear, said the study by researchers from the University of Basel in Switzerland.
The researchers, led by Andreas Luethi, found two distinct groups of nerve cells in a part of the brain called the amygdala, which processes memory of emotional reactions.
One group of nerve cells eliminates "established fear responses" and the other refreshes those memories, the researchers said in the scientific journal Nature.
The authors suggested people normally activate one group of nerve cells or the other based on sensory and contextual information they receive from their surroundings.
A separate study also published in Nature suggested a different population of neurons within another part of the amygdala, called intercalated amygdala neurons, also helps us to become "not scared" of previously scary stimuli.
A team led by Denis Pare of Rutgers State University in New Jersey found rats that had these neurons destroyed remained afraid of things of which they had learned to be afraid.
Both studies could provide therapeutic clues for fear and anxiety disorders, such as post-traumatic stress disorder, Nature said.