Psychedelic drugs like LSD reduce the recognition of negative facial emotions by altering activity the amygdala, a brain region known to be important for responses to threatening situations, according to a systematic review published in Therapeutic Advances in Psychopharmacology.

“We have been investigating the effects of ayahuasca on humans in the last 15 years. It seems that its effects — and the effects of other serotonergic hallucinogens such as LSD and psilocybin — involve modifications of emotional processing, which could contribute to its therapeutic effects on depression and anxiety,” said study author Rafael Guimarães dos Santos, a postdoctoral fellow at the University of Sao Paulo.

The researchers examined two previous studies on LSD and five previous studies on psilocybin. All seven experimental studies evaluated the recognition of facial expressions.

“Our most consistent finding was that these drugs reduced the recognition of negative emotions and modulated amygdala activity to these stimuli. This effect was correlated with antidepressive effects in depressed patients, but since this was observed only in a single study with an open-label design, it is difficult to conclude if this result has any clinical relevance,” the researchers explained in their review.

Previous research has also indicated that individuals suffering from anxiety disorders and depression tend to have an bias in favor of the recognition of negative facial emotions. Reducing the recognition of negative facial emotions could in theory help treat those conditions.

“Altering emotional processing by modifying facial emotion recognition could be one of the mechanisms involved in the therapeutic potentials of ayahuasca and other serotonergic hallucinogens.” Santos told PsyPost.

“Indeed, the studies reviewed showed that a single dose or a few doses of LSD or psilocybin was associated with a modified pattern of recognition of negative emotions that could be interpreted as beneficial, since several of these studies showed that these modifications were correlated with increases in positive mood and/or anxiolytic and antidepressant effects,” the researchers wrote.

Santos noted, however, that his team of researchers were only able to find a relatively small number of studies on the topic — and those studies had small sample sizes.

The article, “Serotonergic hallucinogens and recognition of facial emotion expressions: a systematic review of the literature“, was authored by Juliana M. Rocha, Flávia L. Osório, José Alexandre S. Crippa, José Carlos Bouso, Giordano N. Rossi, Jaime E. C. Hallak, and Rafael G. dos Santos.