Acute stress makes it difficult to link memories of past events with fresh information, a study suggests. The results help to explain why people struggle to show insight under pressure.
The study, published in Science Advances, combined brain imaging and psychological testing to show how stress disrupts people's ability to tap into records of previous experiences and make deductions.
The combination of behavioural testing and neural imaging "to actually see what's going awry is really compelling," says Brice Kuhl, a neuroscientist at the University of Oregon in Eugene, who was not involved in the study.
Only connect
The brain connects new and old information to make inferences through a cognitive process called integration. For example, if you have a memory of your friend wearing a bright green jacket, and you see a bright green jacket on a park bench, you might integrate your memory and the visual input to infer that your friend is at the park. This ability can be impaired in individuals with some mental-health conditions, such as anxiety disorders and psychosis.
The brain area called the hippocampus is essential for integration. Since it is also particularly vulnerable to stress, Lars Schwabe, a cognitive psychologist at the University of Hamburg in Germany, and his colleagues decided to test how acute stress would affect the brain's ability to integrate information and make inferences.
Memory task
On the experiment's first day, 121 participants were asked to memorize a series of paired images, each containing one image of an animal and one image of either a face or a scene.
The next day, roughly half of the participants underwent a mock job interview that required them to defend their suitability for a hypothetical role and perform complex mental mathematics. Participants in the control group, meanwhile, were asked to give a speech about a topic of their choice and complete a simple mental maths task.
Afterwards, participants were presented with another series of paired images, with each pair containing a picture of an animal and of a 3D shape.
Then, the participants were shown, one by one, the 3D shapes that they'd seen previously alongside a collection of various faces and scenes. They were asked to select the face or scene most likely to be associated with each 3D shape.
For example, an individual might hypothetically have memorized a pair containing a cat and a forest scene on the first day and a pair with a cat and a blue cube on the second. If the person's brain had successfully integrated the memories from the two separate days, then they should associate a blue cube with a forest scene.
Missing in action
To understand what was happening in participants' brains, Schwabe and his colleagues used a method called functional magnetic resonance imaging (fMRI), which provides a real-time view of brain activity.
As a baseline, the researchers used fMRI to pin down which parts of the hippocampus "lit up" when participants viewed images from the various categories. Animals lit up one location and 3D shapes another. Faces and scenes activated a third region.
When participants in the high-stress group looked at the 3D shape, their brains did not show as much activity in the area that corresponded to faces and scenes. This suggests that their brain had not inferred the connection between the shape and the face or scene as strongly as had the people in the control group.
"Normally when you encode something new, there's this little flicker of the past experience coming to mind, and we think that's what supports the integration," says Kuhl. "That flicker is basically missing" in the stressed participants.
Even so, the accuracy of the stressed participants when inferring which face or scene was linked to a specific 3D shape was the same as that of the control-group participants. The authors say this might be because the fMRI method of detecting memory linkage is more sensitive than the behavioural test.
The next step for Schwabe and his colleagues involves experiments on rodents to understand the mechanisms behind this phenomenon — and to find ways to reduce the effects of stress.