The study is a randomized, double-blind, placebo-controlled, expectancy-controlled, mechanistic trial. This study looks to explore whether a single dose of psilocybin (25mg) reduces amygdala reactivity to negative socioemotional stimuli during the Facial Emotion Recognition Task (FERT) compared to a placebo in adults who feel lonely or those with unresolved grief symptoms.

The main purpose of this study is to determine whether psilocybin engages validated neural and behavioral targets associated with social information processing (SIP), specifically, amygdala threat reactivity, dorsal anterior cingulate, and insula activity during social rejection, and whether this early neural "target engagement" predicts downstream improvements in loneliness and social connectedness. By establishing these mechanistic links, this research aims to provide a data driven, experimental medicine foundation for developing effective, mechanism-based interventions to reduce loneliness.

Inclusion Criteria:

• Medically healthy
• Ability to provide consent
• High loneliness defined by the UCLA-3 Loneliness Scale
• Willingness to comply with all study procedures and availability for the duration of the study

Exclusion Criteria:

• Any unstable medical condition, psychiatric disorder, or circumstance that, in the opinion of the investigator, could compromise participant safety, interfere with study participation, or hinder the participant's ability to comply with study procedures and requirements
• Positive pregnancy test or women who are breastfeeding
• Any contraindication to MRI such as a pacemaker or metal implants