Smoking cannabis alters a person's ability to perceive and judge emotions, a study has found.

The drug interferes with user's capacity to recognise, process and empathise with human emotions, including happiness, sadness and anger.

But the results also suggest that the brain may be able to counteract these effects depending on whether the emotions are directly, or indirectly detected.

The complex biochemistry of marijuana and how it affects the brain is only beginning to be understood.

Dr Lucy Troup, assistant professor of psychology at Colorado State University, has set out to answer specifically how, if at all, cannabis use affects a person's ability to process emotions.

She has long been fascinated by the psychology of drugs and addiction.

“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,” she said.

For almost 20 years Dr Troup and her graduate students have been conducting experiments to measure the brain activity of about 70 volunteers. They all identified themselves as chronic, moderate or non-users of cannabis.

They were all vetted as legal users of cannabis, and were either medical marijuana users aged 18 or older, or recreational users aged 21 or older.

The experiments involved the participants looking at faces depicting four separate expressions: neutral, happy, fearful and angry, while they were hooked up to an electroencephalogram (EEG), which shows the electrical activity of the brain.

Cannabis users showed a greater response to faces showing negative expressions, especially angry faces, when compared with a control group of non-cannabis users.

In contrast, those who used the drug showed a smaller response to positive expressions - happy faces - compared with the control group.

Those taking part in the study were also asked to pay attention to the emotion and identify it.

Researchers noted in those cases, users and non-users of cannabis could not be told apart.

But, when they were asked to focus on the sex of the face, and then identify the emotion, cannabis users scored much lower than non-users.

This signified a depressed ability to ‘implicitly’ identify emotions.

Cannabis users were also less able to empathise with the emotions, the scientists found.

They said their findings seem to suggest the brain's ability to process emotion is affected by cannabis, but there may be some compensation that counteracts those differences.

The study is published in the journal PLOS ONE.