An article in the Guardian, "Magic mushrooms may help with depression, say leading scientists" turned me on to two pieces of research.
One, "Neural correlates of the psychedelic state as determined by fMRI studies with psilocybin," shows some evidence that "the subjective effects of psychedelic drugs are caused by decreased activity and connectivity in the brain's key connector hubs, enabling a state of unconstrained cognition." This might add somewhat to the general thrust of a post several days ago, "Reduced Coherence in Meditation." So both meditation and psilocybin might share this characteristic of decreased connectivity.
The second study, "Implications for psychedelic-assisted psychotherapy: a functional magnetic resonance imaging study with psilocybin," provides "evidence that psilocybin enhances autobiographical recollection, implying that it may be useful in psychotherapy either as a tool to facilitate the recall of salient memories or to reverse negative cognitive biases."
EDIT: Time Magazine gives a few more tidbits from the first piece of research above, i.e. "Two regions that showed the greatest decline in activity were the medial
prefrontal cortex (mPFC) and the posterior cingulate cortex (PCC)."