“Scalpel!” calls the surgeon. You’d like to yell, too, since you are a patient who has just come to in the middle of an operation-the knockout drops that the anesthesiologist gave you have worn off. But you can’t move a muscle, because the paralyzing drug you were also given is still working just fine.

Nightmare scenarios like this one, dubbed “awareness” episodes, affect one in 500 surgery patients (or about 40,000 each year in the U.S.). Existing methods to detect awakening, which include measuring heart activity and brain waves, are far from perfect. To improve on them, experimental psychologist Henry Bennett invented the FACE monitor. This device detects micro-expressions-say, barely perceptible grimaces of pain-with electrodes that measure voltages in four groups of facial muscles. Bennett says the software he’s written can identify the emotion behind the signals, which originate deep in the brain. The FACE monitor is being developed by his Chatham, N.J., company, Patient Comfort.