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Effects of Stress on Short-Term Memory

By: Sai Srihaas Potu

A stressful situation can trigger a cascade of stress hormones that produce well-orchestrated physiological changes. A stressful incident can make the heart pound faster and breathing quicken. This combination of reactions to stress is also known as the fight-or-flight response because it evolved as a survival mechanism, enabling people and other mammals to react quickly to life-threatening situations. The carefully orchestrated yet near-instantaneous sequence of hormonal changes and physiological responses helps someone to fight a threat off or flee to safety. Unfortunately, the body can also overreact to stressors that are not life-threatening, such as traffic jams, work pressure, and family difficulties.


Over the years, researchers have learned not only how and why these reactions occur but have also gained insight into the long-term effects that chronic stress has on physical and psychological health. Over time, repeated activation of the stress response takes a toll on the body. Research suggests that chronic stress contributes to high blood pressure, promotes the formation of artery-clogging deposits, and causes brain changes that may contribute to anxiety, depression, and addiction. More preliminary research suggests that chronic stress may also contribute to obesity, both through direct and indirect mechanisms.


Stress has long been pegged as the enemy of attention, disrupting focus and doing substantial damage to working memory — the short-term juggling of information that allows us to do all the little things that make us productive. By watching individual neurons at work, a group of psychologists at the University of Wisconsin-Madison has revealed just how stress can addle the mind, as well as how neurons in the brain’s prefrontal cortex help remember information in the first place.


Working memory is a form of short-term memory, allowing the brain to hold a large amount of information close at hand to perform complex tasks. Without it, you would have forgotten the first half of this sentence while reading the second half. The prefrontal cortex is vital to working memory. But without your prefrontal cortex, you would be unable to stay on task or modulate your emotions well. People without a prefrontal cortex are very distractible. The neurons of the prefrontal cortex help store information for short periods. Like a chalkboard, these neurons can be written with information, erased when that information is no longer needed and rewritten with something new.


It is how the neurons maintain access to that short-term information that leaves them vulnerable to stress. David Devilbiss, a scientist working with Berridge and lead author on this study, applied a new statistical modeling approach to show that prefrontal neurons were firing and re-firing to keep recently stored information fresh.


Even though these neurons communicate on a scale of every thousandth of a second, they know what they did one second to one-and-a-half seconds ago. But if the neuron does not stimulate itself again within a little more than a second, it will forget that information. Apply some stress — in the researchers’ case, a loud blast of white noise in the presence of rats working on a maze designed to test working memory — and many neurons will stop stimulating signals. The researchers were simultaneously watching dozens of individual neurons firing in the rats’ brains, and under stress, those neurons got even more active.


Without the roar of white noise, which has been shown to impair rats in the same way it does to monkeys and humans, the maze-runners were reaching their goal about 90 percent of the time. Under stress, the animals completed the test at a 65 percent clip, with many struggling enough to fall to blind chance.


Recordings of the electrical activity of prefrontal cortex neurons in the maze-running rats showed these neurons were unable to hold information key to finding the next chocolate chip reward. Instead, the neurons were frenetic, reacting to distractions such as noises and smells in the room.


The effects of stress-related distractions are well-known and dangerous. Previous studies and meta-analyses have shown us that stress plays a role in more than half of all workplace accidents, and a lot of people have to work under what we would consider a great deal of stress. People in the military have to carry out these thought processes in conditions that would be very distracting and now we know that this distraction is happening at a cellular level in the brain.


The researchers’ work may suggest new directions for the treatment of prefrontal cortex dysfunction. Based on previous drug studies, it had been believed stress simply suppressed prefrontal cortex activity. These studies demonstrate that rather than suppressing activity, stress modifies the nature of that activity. Treatments that keep neurons on their self-stimulating task while shutting out distractions may help protect working memory.


Many people are unable to find a way to put the brakes on stress. Chronic low-level stress keeps the HPA axis activated, much like a motor that is idling too high for too long. After a while, this affects the body and leads to different health problems.


Persistent stress surges can damage blood vessels and arteries, increasing blood pressure and raising the risk of heart attacks or strokes. Elevated cortisol levels create physiological changes that help to replenish the body's energy stores that are depleted during the stress response. But they inadvertently contribute to the buildup of fat tissue and weight gain. For example, cortisol increases appetite, so that people will want to eat more to obtain extra energy. It also increases the storage of unused nutrients as fat.


Over the last couple of decades, stress has become a normal part of our lives and is wreaking havoc on our health and productivity. Fortunately, researchers continue to conduct more studies in order to better understand the mechanism behind stress and the psychological, emotional, and physical impacts it can have on our body.


References:

1. David M. Devilbiss, Rick L. Jenison, Craig W. Berridge. Stress-Induced Impairment of a Working Memory Task: Role of Spiking Rate and Spiking History Predicted Discharge. PLOS Computational Biology. 2012.

2. Koolhaas JM, Bartolomucci A, Buwalda B, De Boer SF, Flugge G, Korte SM. Stress revisited: a critical evaluation of the stress concept. Neuroscience and Biobehavioral Reviews. 2011.

3. Sandi C. Stress, cognitive impairment, and cell adhesion molecules. Nature Reviews Neuroscience. 2004.

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