In a double-blind crossover study of 22 healthy adults, 15 mg psilocybin significantly increased global brain connectivity compared to placebo and 20 mg 2C-B, while reducing Default Mode Network integrity during peak effects. Brain imaging showed more frequent transitions between connectivity states, meaning networks became less rigid and more dynamically interconnected across the acute window. Connectivity changes closely tracked subjective intensity ratings, suggesting psilocybin temporarily loosens fixed communication patterns in mood and self-processing circuits.