New research published in the journal Neuropsychopharmacology shows LSD alters the processing of emotional facial information while increasing prosocial behavior.

LSD, one of the best known psychedelic drugs, quickly sparked the interest of scientists after its psychoactive properties were discovered in the 1940s. But research on the powerful hallucinogen was severely restricted in the 1960s after it became associated with the hippie counterculture and antiwar movement.

LSD is currently being investigated as a tool for psychotherapists to use in the treatment of various mental ailments, including anxiety, addiction and post-traumatic stress disorder. But the research is still in the preliminary stages.

This new double-blind, placebo-controlled study found that LSD tends to reduce a person’s ability to recognize negative emotions while enhancing a person’s empathy and prosociality. “The present study also showed that LSD was well tolerated in a controlled setting in healthy subjects,” lead researcher Patrick C. Dolder and his colleagues wrote in their study.

The researchers recruited 40 adult participants from the University of Basel in Switzerland. Eleven participants had previous experience with LSD, but the remaining 29 had never used the psychedelic drug before.

Each participant was given a single oral dose of either 100 μg or 200 μg of LSD, or an inactive placebo in a quiet hospital patient room. About 5 to 7 hours later (which is approximately 3 hours after the peak effects of LSD), each participant completed a range of psychological tests designed to measure their mood and the processing of emotional information. Specifically, the participants completed the Face Emotion Recognition Task, Multifaceted Empathy Test, Social Value Orientation Test, Visual Analog Scales, and the Adjective Mood Rating Scale.

Participants under the influence of LSD were less likely to recognize fearful and sad facial expressions. But the drug had no effect on the recognition of neutral, happy or angry facial expressions.

LSD decreased cognitive empathy, but increased emotional empathy. Participants under the influence of LSD had trouble correctly inferring the mental state of a person in a photograph, but were more likely to feel concern for the person’s well-being.

In addition, the drug increased prosocial behavior as measured by the Social Value Orientation Test. The test asked participants to choose how to distribute a small sum of money between themselves and other participants. Those under the influence of LSD tended to choose a more equal distribution, rather than seeking more for themselves.

The psychedelic drug also impacted the participants’ subjective mood. LSD was associated with dreaminess, feelings of closeness to others, wanting to be with others, happiness, openness, trust, and introversion.

The researchers observed physiological effects, but these were expected based on previous research. Participants who ingested LSD had increased blood pressure, heart rate, and body temperature as well as enlarged pupils.

Dolder and his colleagues said their findings could have clinical significance. “These effects of LSD in healthy participants likely have translational relevance to LSD-assisted psychotherapy in patients and can be expected to reduce the perception of negative emotions and facilitate the therapeutic alliance,” the researchers explained.