How Stress Impacts Mouse Memories (and How You Should Be Studying)
Photo by Tim Gouw
Why is it harder to remember things when you’re stressed? As a student, I've found it easier to remember tasks and assignments when there's not much on my plate, but the busier (and more stressed) I am, the harder it is to keep track of things. Recently, researchers at the Federal University of Rio Grande do Norte, Uppsala University, the Edmond and Lily Safra International Institute of Neuroscience, and the Universidade Federal de São Paulo shed new light on this phenomenon.
Their experiment involved 128 male rats, all of which were housed under the same conditions and received three meals per day for 7 days. On the first day, the rats were placed in a maze with two arms. One arm, the "non-aversive arm", was perfectly normal in every way. The other arm, the "aversive arm", was designed to bombard the rats with 100 decibels of sound and 80 watts of light, to irritate them. The rats were given a 10-minute “learning period” to explore the maze and determine which arms were normal and which were irritating. Then, they were split into two equal groups. One group was exposed to some simple, low-stress handling. Meanwhile, the second group of rats was placed in a separate acrylic tube with air holes. Due to their animal instincts, rats are innately averse to restrictions on movement and were immediately stressed in the confining acrylic tube. Both groups of rats were returned to the maze, and their behavior was monitored and recorded over the next several days. The goal was to determine whether the stressed rats in the acrylic tube had learned to avoid the irritating arm and how their memory compared with that of the stress-free rats.
At the end of the experiment, the results were clear. Over the course of the experiment, an average of 25% of the rats went back to the aversive (AV) arm, indicating memory loss. However, by the fifth day, the percentage of stressed rats that returned to the AV arm increased to above 50%, whereas the behavior of low-stress rats did not change significantly (there was still a slight increase, as the subjects were rats and would inevitably return to the AV arm regardless, but a clear trend was evident). Stress did not necessarily affect memory formation, as all the rats remembered to avoid the AV arm for the first couple of days. Rather, it affected memory retention: the stressed group quickly forgot the memory of irritation, whereas the low-stress group did not.
To further test this, researchers euthanized a select few rats after either gently handling them (low-stress) or stressing them and examined their brains. The researchers aimed to investigate how two genetic markers, Zif268 and C-fos, changed in the rats following stress. Zif268 and C-fos are relevant because they are considered "memory-supporting" genes, and their activation helps the brain retain memory after learning.
The researchers performed surgery on euthanized rats and used immunohistochemistry, a technique that uses antibodies to stain for specific proteins. Proteins associated with Zif268 and C-fos are expressed at higher levels when their respective genes are active, so immunohistochemistry allows researchers to effectively determine gene activity based on protein levels. The researchers examined the rat’s hippocampus (the memory center) and counted the number of stained cells in the rat brains. Overall, the stressed rats showed reduced activation of both Zif268 and C-fos compared to the typical post-learning activation observed in low-stress rats.
These experiments suggest that stress biologically impairs our ability to retain information, a finding relevant not only to rats but also to humans. Perhaps this is a sign to place more emphasis on relaxation in our study routines. Intense studying and constant stress could actually reduce performance, contrary to common belief. Personally, I've always been told stress is positive (to an extent) because it means you care about something—but what if stress actually holds us back from truly realizing our full potential? The key to building a research-informed study schedule is probably not the excision of stressful activities altogether, but rather minimizing stress as much as possible. TL;DR: You still need to study, but high-stress, last-minute studying is biologically less effective. Try consistency, and a cup of tea?
