“What emotion looks like in the brain depends on what your brain is preparing for you to do,” says Lisa Feldman Barrett, a distinguished professor of psychology at Northeastern University and an expert in the neuroscience of emotion. Preparing to respond to a threat triggers fight-or-flight responses in the brain. The nervous system gets ready to release hormones like adrenaline, which can make your heart race, priming your body to run fast, almost like revving your car’s engine before you speed off.