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The traditional six basic human emotions are happy, sad, fearful, angry, surprised and disgusted. For years, researchers have focused on these six categories, which are often depicted via specific facial muscles, when they assess people's moods. Now, according to a new study headed by associate professor Aleix Martinez from Ohio State University, the researchers have identified 15 more facial expressions, which they called "compound emotions."

"When I started to research on that, it seemed like six was a weird number and was really small," Martinez said reported by ABC News. "We started to divide this theory into compound facial expression. You can feel happily surprised or angrily surprised."

For this study, Martinez and colleagues examined the six basic emotions in 230 volunteers. After observing their facial expressions, the team broke each of the emotions down into compound emotions. They found that all of the facial contortions for each compound emotion were different from one another. For example, the facial muscles involved with depicting "disgustingly surprised" and "happily surprised" varied greatly. However, when the volunteers depicted one particular compound emotion, the facial muscles were similar to one another.

"Compound emotions are those that can be constructed by combining basic component categories to create new ones. For instance, happily surprised and angrily surprised are two distinct compound emotion categories," the authors explained.

The team had utilized a Facial Action Coding System to determine that all 21 facial categories were indeed different from one another. The researchers believe that the study's findings could help improve research on mental illnesses ranging from schizophrenia and autism. In order to effectively treat humans, being able to read their facial expressions to help assess moods is important. The new research could also be used in robotic systems created to interact with humans.

"Understanding the different categories of facial expressions of emotion regularly used by us is essential to gain insights into human cognition," the authors wrote.

The study, "Compound facial expressions of emotion," was published in PNAS.