STANFORD — One of the characteristics of humans that sets us apart from most other animals in the animal kingdom is our ability to plan for the future. That we are able to effectively think ahead and adapt plans in response to unexpected events is crucial for achieving goals, both big and small. Now, a new study out of Stanford University shows that unwanted stress can interfere with people’s planning abilities by preventing them from remembering past experiences.
“We draw on memory not just to project ourselves backward into the past but to project ourselves forward, to plan,” says senior author Anthony Wagner, in a release. “Stress can rob you of the ability to draw on cognitive systems underlying memory and goal-directed behavior that enable you to solve problems more quickly, more efficiently and more effectively.”
People “replay” their past experiences to make a plan for similar situations that they encounter in the future. This method of planning ensures that they consider the mistakes they made and recall the aspects of their plans that worked well. If people cannot remember their past experiences because they are stressed, they risk repeating their mistakes over and over again.
“It’s a form of neurocognitive privilege that people who are not stressed can draw on their memory systems to behave more optimally,” says Wagner. “And we may fail to actually appreciate that some individuals might not be behaving as effectively or efficiently because they are dealing with something, like a health or economic stressor, that reduces that privilege.”
To test their hypothesis, researchers recruited participants to learn to navigate through virtual towns they created. Once the participants became familiar with all of the roads and routes they could take in each town, researchers placed the participants on one of the roads in a town and instructed them to walk to a specific location in the town. They used functional Magnetic Resonance Imaging (fMRI) to record the brain activity of participants as they planned and navigated through the virtual maze.
In order to add stress to their subjects, researchers told some participants that they might get a mild electric shock while making their way through the maze. The stressed group had a more difficult time planning their journey through the maze and often wandered around the virtual town until they came across the end goal. Many of the participants in the control group were able to rely on their past experiences and plan shortcuts to take them from start to finish.
Furthermore, the fMRI analysis shows that throughout the task the brains of control participants are a lot more active than the brains of stressed participants. Researchers note that the hippocampus, a part of the brain that plays a key role in memory replay, is generally “offline” in stressed individuals as they plan the route they will take through the maze. This means they are not replaying their previous journeys through town as they plan out this new trek.
Additionally, the scans of stressed participants show reduced activity in the frontal-parietal lobe networks as they make their way through the maze. These brain regions are involved in action-planning and attention, and they keep brain rhythms in-sync as people accomplish their goals.
These results indicate that reduced activity between these two brain regions seems to be strongly affected by stress. “It’s kind of like our brain is pushed into a more low-level thought-process state, and that corresponds with this reduced planning behavior,” says lead author Thackery Brown.
Researchers report that this might be the first study to show how stress brings memory replay offline by disrupting the activity of the hippocampal-frontal lobe brain network.
Looking ahead, the authors plan on continuing their virtual navigation studies in older adults to try and understand how stress affects memory in older populations. Older people are at a higher risk for reduced memory function, especially since they face the stress associated with health and economic burdens more frequently.
“It’s a powerful thing to think about how stressful events might affect planning in your grandparents,” says Brown, who is now an assistant professor at Georgia Institute of Technology. “It affects us in our youth and as we interact with and care for older members of our family, and then it becomes relevant to us in a different way when we are, ourselves, older adults.”
The study is published in Current Biology.