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A recurring theme here at the Stress on the Brain blog is that stress is not as bad as we think it is. This post continues with this theme by focusing on the sometimes-beneficial effects of stress on learning and memory.

Stress can reduce memory retrieval.

We’ve all felt the pressure of trying to remember a password in a hurry. The feelings of stress we have when trying to log in to pay our mortgage on time or to get Netflix to work on our new TV can make it seem like stress conspires to torpedo our memories. In some cases, this is true.

Trying to remember stuff that we learned days, weeks, or years ago is more difficult when we’re under stress. The mechanism for this effect has been worked out in research over the past 40 years. When we learn something, it gets stored in our brains and is accessible later through the process of memory retrieval. Memory retrieval is an active process that relies on the functioning of the prefrontal cortex. Under stress, the prefrontal cortex does not function optimally, and processes such as memory retrieval suffer.

Stress improves learning.

Contrary to memory retrieval, the act of making a memory, otherwise known as learning, can benefit from stress. This was initially discovered in research on rats who’d been trained to avoid a shock. Those rats that exhibited the largest stress response to the shock showed the best memory for the situation hours later.

The same pattern has been found in people. In research conducted in my lab, we gave people a 20-milligram dose of cortisol (equivalent to a moderate stress response) and then showed them pictures to remember. Some of these pictures contained emotional material, such as a snarling dog, while some of the pictures were emotionally neutral, like a picture of a table. One week later, people were asked to recall these pictures. Those who’d received the dose of cortisol remembered more of the pictures than those who’d received a placebo. Memories of the emotional pictures received the greatest memory boost.

Why would the stress hormone cortisol improve learning? And why would it specifically improve the learning of emotional experiences? Stressful events are often emotional events. Car accidents. Childbirth. Winning a championship. Getting married. All these events are stressful, emotional, and meaningful. Our brain’s memory system is very good at remembering what is meaningful. Our brain’s stress system works with the memory system to signal what is meaningful for us to remember.

"Meaningful" in this context represents what is useful for our well-being. It’s good to avoid car accidents and to have a successful childbirth. Remembering these situations is important to our survival. Stress serves as a marker for what we should remember clearly to avoid future dangers and enhance our well-being.

Stress, memory, and post-traumatic stress disorder.

The problem with this scenario is that sometimes having a heightened memory for stressful events can lead to disorders like post-traumatic stress disorder (PTSD). In this case, our memory system gets hijacked, leading to intrusive memories of a highly stressful event. This pattern—normal brain mechanism being hijacked and resulting in pathological states—represents a real challenge in treating psychological disorders. How do we protect against the pathological while leaving the underlying system intact? More research is needed to better understand the complexities of these systems.

It’s useful to understand the complexities of how stress interacts with memory under normal conditions. This makes it possible to better understand how stress and memory can go awry in PTSD and other psychiatric illnesses. This understanding can help with the goal of developing therapies to reduce the impact of these disorders.