Mnemonic Discrimination Task. A. The task had two phases, an incidental encoding phase, where subjects were asked to indicate with a key press whether the picture could be classified as an ‘indoor’ or an ‘outdoor’ item and a recognition phase where subjects were asked to indicate with a key press if they considered the picture to be ‘old’, ‘similar’, or ‘new’. B. Task performance in a group of first episode psychosis (FEP) and healthy volunteers (HV); pattern separation scores were significantly lower in FEP compared to HV (p= .04). Dots represent individual measurements, and bars represent the overall group’s performance. C. Task performance in HV at baseline and during a ketamine challenge; pattern separation scores were significantly lower during the ketamine challenge compared to baseline (p= .003). Dots represent individual measurements, and bars represent the overall group’s performance. BPS: Bias corrected pattern separation score; BPR: Bias corrected pattern recognition score.