In everyday speech, freezing is often conflated with tonic immobility, but they are not the same — tonic immobility is more extreme. Collapsed immobility, another extreme response, involves a precipitous drop in heart rate and blood pressure, causing limp muscles, unlike the rigid muscles in tonic immobility. Victims usually faint or collapse and take a while to recover because their brain hasn’t had enough oxygen. Hopper once worked on a case in which a man tried to force a victim to perform oral sex, but she couldn’t hold her head up. “She reported that her neck muscles were totally limp, and her head literally flopped around,” he said. Victims might describe the experience with phrases like “I felt dizzy,” “I felt faint” or “I felt sleepy.” Some victims describe this as “blacking out,” which can lead insufficiently trained investigators to think the victim drank too much alcohol.

Freezing tends to come early in an attack, and extreme responses tend to come later, but they can happen in any order. Shifts between behaviors can occur within milliseconds. And some people threatened with rape will be able to make decisions, such as to acquiesce, because they think it will help them avoid death or severe physical injury. Some will fight or flee, and some won’t experience a trauma response at all. But all these responses can have profoundly different effects on people’s consciousness and memory.

Neuroscientists often talk about the brain in terms of circuitries, collections of connected areas responsible for certain functions. The defense circuitry is one of the best studied, and it works in the same basic way in all mammals: If a threat is detected, the defense circuitry can rapidly dominate brain functioning, with major consequences for thinking, behavior and memory. It takes up to three seconds for the defense circuitry to hit the prefrontal cortex with sufficiently elevated levels of stress chemicals to seriously impair it, and once the prefrontal cortex goes quiet, so does our ability to reason. Our language centers are impaired. Our attention changes, and so does the way we encode memories.

Amy Arnsten, a neuroscientist at Yale University, is one of the leading researchers on the way stress impairs the prefrontal cortex. In a study from last year, her team found that exposure to even mild but uncontrollable stress quickly impaired the prefrontal cortex in humans and animals. “Under stress, your brain disconnects from its more recently evolved circuits and strengthens many of the primitive circuits, and then these unconscious reflexes that are very ancient kick in,” she told me over the phone.

Arnsten described walking through the woods in Vermont some years ago when a bear dropped out of a tree. Without thinking, she froze. The bear looked at her but didn’t see her. “It just is a reflex,” she said. “Most animals see movement and not detail, so freezing — especially if you’re in a position where you can’t escape — has had survival value across the eons.” But freezing and tonic immobility evolved to keep ourselves safe from animal predators, not human ones. Human predators don’t always lose interest if their human prey looks dead.