Delusions—beliefs that can seem bizarre and appear unsupported by evidence—are a hallmark of the most severe mental illnesses.

Yale investigators used brain imaging to probe the persistence of delusions, which often endure in the face of contradicting evidence.The study suggests that being in a delusional state can strengthen the formation of both positive and negative memories.

In the current experiment, healthy subjects were shown pictures that predicted either a negative or a positive memory—an unpleasant loud noise or a sip of sweet fruit juice, respectively.

The next day, researchers induced a brief and reversible delusional state among a subset of those individuals. While in this state, individuals were shown the picture again.

The following day, the scientists measured the individual's response to the picture. Compared to memories that were not reactivated and memories that were reminded under a placebo infusion (salty water with no delusional effects), the scientists found that the delusional state made the reactivated memory stronger.