Sometimes a quick look at a person’s facial expression can be telling of their emotion, but many times, it’s often misread. In an effort to understand why judgements about ambiguous or intense emotions are made, researchers at Caltech studied a specific region of the brain, called the amygdala.

“We have long known that the amygdala is important in processing emotion from faces,” said study author Ralph Adolphs, in a news release. “But now we are starting to understand that it incorporates a lot of complex information to make fairly sophisticated decisions that culminate in our judgements.”

Read: Sleep Loss Hurts Your Ability To Recognize Facial Expressions Such As Happiness Or Sadness​

In the study, published in Nature Communications, researchers analyzed how a person’s brain cells in the amygdala reacted while they looked at photos of faces expressing varying degrees of happiness, fear, and ambiguous or neutral emotions. The participants had to indicate whether the face appeared fearful or happy.

The scientists found that two specific groups of neurons in the amygdala respond to facial expressions. The first group of neurons detected how intense a single emotion was, and the second group identified the ambiguity of the perceived emotion.

When the participants were shown black-and-white images of ambiguous faces, they often judged the same image to be fearful on some occasions and happy at other times.

“Many people are familiar with feeling that a face just looks too ambiguous to really decide what emotion the person is having,” said study author Shuo Wang. Furthermore, he says the findings indicate “the amygdala is involved in making decisions rather than simply representing sensory input.”

In another study, the researchers used brain scans to study the emotion judgements of three rare individuals with damaged amygdala. The results revealed the participants had a very low threshold for determining when a face showed fear. A study, unrelated to this research, found a similar effect. Researchers either damaged or removed the amygdala in rats’ brains. After the procedure, the rats were no longer fearful.

Wang's findings provide insight into the brain mechanisms behind autism, anxiety, and posttraumatic stress disorder.

“Patients with severe PTSD are thought to have a hyperactive amygdala, which electrical stimulation might be able to inhibit,” said study author Ueli Rutishauser.

Clinical trials are currently being conducted that involve stiumalting the amygdala through deep-brain stimulation among people with autism and PTSD.

See also: Introducing The 'Not Face,' A Universal Facial Expression That Indicates Negative Emotions

Treatment-Resistant Depression: Deep Brain Stimulation May Provide Relief For Those With Severe Symptoms