Psychiatrists diagnose people with schizophrenia, ADHD, bipolar disorder and other mental illnesses by spending time with them, looking for the particular behavior symptoms of each. What follows can be a hit-or-miss series of medications and dosages until disruptive behaviors go away. By deciphering the circuitry of the medial frontal cortex—an area beneath the top of the head—those diagnoses could become much more efficient and precise by allowing physicians to diagnose based on how neurons respond to a simple series of behavior tests.