Under the Hood

Marijuana Affects How People Interpret Facial Expressions, Process Emotions

Marijuana significantly affected participants’ abilities to recognize, process, and empathize with expressions of emotion. ashton, CC by 2.0

Films from the 1970s stereotypically depict the experience of smoking pot as a dull haze of blurred faces, an effect created when the camera’s focus slowly melts. A Colorado State University research experiment suggests this cinematic representation is fitting. Marijuana significantly affected study participants’ abilities to recognize, process, and empathize with expressions of emotion.

Over the past two years, Dr. Lucy Troup, an assistant professor of psychology, and her colleagues have been conducting electroencephalogram (EEG) experiments on 73 student volunteers who identify as chronic, moderate, or nonusers of cannabis. An EEG uses electrodes attached to the skull to measure brain activity. Once connected to an EEG, the participants were asked to view faces depicting four separate expressions: neutral, happy, fearful, and angry. Cannabis users were more responsive to negative expressions (particularly angry faces) than nonusers and less responsive to happy faces compared to nonusers, the study results indicated.

When asked to pay attention and explicitly identify emotions, the users and nonusers of marijuana became virtually indistinguishable. However, when asked to focus on the sex of the face and then identify the feeling expressed, cannabis users scored much lower than nonusers on this test of "implicit" identification. The marijuana users also appeared less able to empathize with the emotions, the researchers discovered.

For the current study, Troup and her colleagues take these EEG experiments one step further by focusing on event-related potentials.

What is an ERP?

Event-related potentials (ERPs) provide information about how we process emotion by measuring activity taking place in the cerebral cortex following a specific event. If an EEG tracks and records brain wave patterns, an ERP is the direct response to a specific event (whether sensory, cognitive, or motor). For example, if during the course of an EEG you are shown a photograph of your mother, the ERP would be the voltage response occurring in your brain as you look at the snapshot.

One ERP known as P3 is particularly important as it elicits a positive change in voltage within milliseconds of a stimulus. Scientists often use P3 during experiments because it is ubiquitous and reproducible — for instance, it can be recorded when we make decisions. In past research, P3 has been consistently linked to emotion processing, Troup and her colleagues explained, with emotional stimuli eliciting greater P3 amplitude than neutral stimuli during passive viewing and specific tasks. 

For the current study, then, the researchers measured participants’ P3 during the same facial expression experiments they used in the past. What they discovered was a significant difference in the P3 amplitude among the cannabis users compared to the abstinent study participants. Specifically, the marijuana users showed a decreased P3 to happy faces and an increased P3 to angry faces when compared to non-users.

“These effects appear to increase with those participants that self-reported the highest levels of cannabis consumption,” Troup and her co-authors wrote. Apparently, as 70s movies suggest, the faces do blur within the haze of marijuana smoke.

Source: Troup L, Bastidas S, Nguyen MT, et al. An Event-Related Potential Study on the Effects of Cannabis on Emotion Processing. PLOS ONE. 2016.