A boost of activity at the back of the brain while processing emotional information predicted depressed patients' responses to an experimental rapid-acting antidepressant. The potential neuroimaging biomarker may eventually help to personalize treatment selection by revealing brain-based differences between patients. Scopolamine can lift depression in many, but not all, patients within a few days. The study found that the more dysfunctional a patient's acetylcholine chemical messenger system, the better they responded to the drug.