Marijuana can alter the way one processes their emotions, according to a new study. Colorado State University researchers found that pot can affect users’ ability to recognize, process and “empathize with human emotions.”

The study, published Monday in the journal PLOS ONE, looked at the brain activities of 70 participants over the course of two years. Researchers classified participants based on whether they considered themselves chronic, moderate or nonusers of marijuana. 

“We tried to see if our simple emotion-processing paradigm could be applied to people who use cannabis, because we wanted to see if there was a difference,” said Lucy Troup, lead researcher and assistant professor of psychology at Colorado State University in Fort Collins. “That’s how it all started.”

Troup and her team analyzed participants’ brains using an electroencephalogram (EEG), which shows the electrical activity of the brain. While each participant was hooked up to an EEG, they were shown photos depicting various facial expressions. Cannabis users demonstrated a greater response to faces showing negative expressions, such as anger, when compared to participants who did not use cannabis. The study also revealed that cannabis users were less responsive to happy faces.

“We’re not taking a pro or anti stance; but we just want to know, what does it do? It’s really about making sense of it,” Troup said in a statement.

Recreational marijuana sales began in Colorado the same year the study was launched. Colorado is among the four states, in addition to Washington, D.C., that have legalized marijuana entirely.