Psychologists Showed People a Traumatic Film Then Gave Them Laughing Gas

A paper published today in Psychological Medicine reveals findings from a study in which researchers showed participants notoriously traumatic scenes from the French film Irréversible before giving them nitrous oxide (laughing gas) to see if it affected their memory of the distressing scenes.

Researchers from University College London led by Ravi Das set out to investigate how nitrous oxide could affect the brain’s processing of “intrusive” memories such as those experienced by people with post-traumatic stress disorder.

“Our interest in nitrous oxide is that we know it blocks a certain receptor that’s important in how memories are stabilised—that’s called the NMDA receptor,” explained Das. NMDA receptors have been shown to be involved with the consolidation of memory, so the team suggested that “blocking” them with nitrous oxide after a traumatic event “may prevent the consolidation of long-term maladaptive memory traces, reducing PTSD symptomatology.”

They were particularly interested in nitrous oxide because it’s often given as a painkiller by emergency services, so the researchers wondered if it could also be having unintended effects when given to people after an accident.

The study found that the group who had been given nitrous oxide showed a “markedly faster drop-off in intrusion frequency than the Air group.”

Fifty-two volunteers watched two rather unforgettable clips from Gaspar Noé’s controversial film: a scene where the protagonist brutally bashes a man’s face in with a fire extinguisher (which the British Board of Film Classification calls “unusually explicit”) and a graphic rape scene known for its length (around 10 horrific minutes) and filmed from just one angle and as if the viewer is watching in “real-time.”

In the study, watching the film is an analogue for experiencing a real trauma. “We can’t obviously epically give people post-traumatic stress disorder,” said Das, explaining that previous work has shown that unpleasant film clips can result in a similar kind of “intrusive memories”—involuntary, spontaneous thoughts—as occur in PTSD (though obviously the effect is much weaker).

The researchers selected Irréversible because they found in pilot studies that the scenes produce robust intrusive memories. “They seemed better than the previous kind of grainy footage of German autobahn accidents and stuff that we’d had before,” said Das.

After watching the clips, half the participants were given a mixture of nitrous oxide and oxygen (“gas and air”) for 30 minutes while the other half just got air. The participants kept a diary of when they had intrusive memories linked to the film over the following days and completed a recall task.

The study found that the group who had been given nitrous oxide showed a “markedly faster drop-off in intrusion frequency than the Air group.” While they had about the same number of intrusive memories, these reduced in frequency at a quicker pace (for the nitrous oxide group, there’s a particularly marked drop-off in the frequency of intrusions from the day after they saw the film).

The graph shows the number of reported “intrusive memories” related to the film for each group after viewing it on Day 1. The dotted line represents the first night of sleep; the Day 2 difference is of particular interest.

“It seemed like—as we hypothesised from the way nitrous oxide works—it had blocked the stabilising of the memories of the film into the long term,” said Das. “We see that after sleep, because we know that sleep is critical for that process.”

While a lot more work would need to be done to suggest clinical applications, Das said that ultimately nitrous oxide could potentially be used with people who have experienced trauma, such as in the military.

In this case the gas was administered immediately after the traumatic event but he’s currently looking at whether it’s possible to “destabilise” an existing memory and then try it. “Our next step is to look at whether we can get the same effects by destabilising people’s preexisting memory of the unpleasant film and then giving them nitrous oxide,” Das said. “And if we can, then that means there’s essentially no time limit on when it could be useful, so that’s really exciting for us.”