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Stress comes in different forms — pain, hunger, , psychological upset. When we’re under stress, it’s often assumed that we are on heightened alert. If something hurts us, we imagine that an internal security system clicks on, scanning for other signs of danger. But a new study by New York University neuroscientists reveals that being under actually diminishes our ability to pay to changes around us that might be threatening, like a swerving car or an unexpected shift in a person’s behavior. That delay in recognition may put us at risk of not responding appropriately to new dangers.

To understand the new study, it helps to understand stress. The body’s resting state is known as homeostasis. In that condition, heart rate, breathing, and other systems tick along normally and relatively peacefully. Stress represents a physiological change that knocks you out of homeostasis. A part of the nervous system known as the HPA axis — a chain reaction involving the hypothalamus, pituitary and adrenal glands — kicks into action, releasing cortisol and other to try to calm the body down and return it to homeostasis.

This means that when your body is under stress, it is aroused. That is not always a bad thing. Think of your stress response as an upside-down U-shaped curve. Too little stress and you are under-aroused; too much and you are over-aroused. But there’s a point at the curve U where just the right amount of arousal can enhance performance. That fact contributes to the general sense that stress makes us more alert and responsive, and explains why some people think they do best under pressure.

Previous research, however, has already identified some of the ways that stress disrupts our ability to think and learn — our cognitive function. The new study, recently published in the Proceedings of the National Academies of Sciences, is part of a body of work investigating more deeply how acute stress affects our emotional control and flexibility. In this context, flexibility means our ability to learn from our environment and update our responses according to what we encounter.

To test this, cognitive Candace Raio, the study’s lead author, and her colleagues at New York University and Peking University brought participants into the laboratory on two consecutive days. On the first day, everyone viewed images on a computer screen. Some of the images were coupled with a mild electric wrist-shock (a “threat cue”). Other images were never paired with a shock (a “safe cue”). This experiment is a much-used form of threat conditioning. Once conditioned, the body responds to threat cues by releasing stress hormones, and it doesn’t show any physiological changes in response to safe cues.

On the second day of the study, everyone returned to the lab. Half of the participants had to plunge one arm into an ice-water bath for three minutes — a standard procedure that elevates two stress hormones, cortisol and alpha-amylase. The other half of the group was spared the ice bath. Then everyone repeated the threat conditioning in which they viewed computer images and had their wrists shocked. This time, however, the threat and safe cues were reversed. “The thing that was previously safe is now threatening, and the thing that was previously threatening is now safe,” explains Raio, who did the study in the NYU laboratory of Elizabeth Phelps, but is now a post-doctoral fellow at NYU’s Center of Neural Science. To respond accurately to threats, the people in the study had to update what they’d learned about sources of danger and sources of safety.

When the scientists measured each participant’s physiological responses to the differing cues, including changes in cortisol and alpha-amylase, they found that the stress group, who had suffered the ice-water bath, were much slower to recognize that the cues had shifted. “Everyone needs a little time to learn the new [cues],” says Raio, even those who hadn’t been . But the stressed group took longer to learn that what had been safe was now threatening. They did respond appropriately in the end, but needed to be exposed to the new threat many more times than the group that hadn’t been stressed. “It takes you longer to get there,” Raio says.

Although laboratory studies like this one don’t always generalize to daily life, these results suggest that under stress, “we need many more encounters with a threat that was previously safe in order to learn that it is actually dangerous,” Raio says.

The study is a reminder of a basic point that is worth repeating: While there are plenty of sources of stress that we cannot prevent — a skateboarder careening toward us on the sidewalk, for example — it’s important to reduce stress where we can by getting enough sleep, eating right, and taking breaks from work. Such mental and physical better prepares the nervous system to meet the unexpected and changing challenges of daily life. In other words, control what you can, so you can respond better to what you can’t.