They are comparing the effectiveness of SSRIs and psilocybin on depression by analyzing brain activity and the response to faces with different emotions (both agents work on serotonin receptors). Their conclusion is that while SSRIs lessens reaction to negative emotions, while psilocybin helps people think through their reactions. They conclude “Psilocybin represents a novel intervention for major depression that appears to be safe, rapid and potentially enduring in its antidepressant action.”