The Effect of Stress on Memory
Does stress affect memory?
The effects of stress on memory are well-studied and well-recorded. At the most basic level, the main effect that stress has on memory impacts the brain's ability to both encode information and retrieve information. When a person experiences stress, their body releases stress hormones into the blood streams; it is these hormones which have an impact on the brain and can influence a person's ability to remember information through encoding and retrieval disruption. The most well-known stress hormone which negatively affects memory is Glucocorticoids--more commonly known as cortisol. Cortisol disrupts the ability of the hippocampus to encode new information and retrieve existing information by diverting the glucose levels in the hippocampus to surrounding muscles, which deprives the hippocampus of necessary energy it needs to perform regular memory functions. The areas of the brain which are usually the most affected by stress are the hippocampus, the prefrontal cortex, and the amygdala.
Chronic stress vs. acute stress and memory
Studies have shown that different types of stress will have different effects on memory.
Chronic stress refers to stress which is ongoing over a long period of time. This affects the body by keeping it in a state of physiological activity or arousal which is not considered normal--in a sense, the body is in a constant state of "flight-or-fight" response. Normally, when the body experiences stress and this response is triggered, the body returns to its regular state, or a state of homeostasis, after the stress or perceived stress is over. With chronic stress, however, the body continues to remain in a state of flight-or-fight and never returns back to its normal state. This type of constant stress is known to negatively affect both memory and learning processes.
A study done on rats to examine the effects of chronic stress on memory involved exposing rats to a cat for five weeks and testing the rat's ability to remember how to conduct themselves through a radial arm water maze, which involves teaching rats the location of a platform below the surface of water which will rise when they step on it, taking them out of water. The rats that were exposed to the cat for five weeks and exhibited signs of chronic stress showed significantproblems remembering where the platform was in the radial arm water maze, when compared to the rats that were not shown that cat and did not exhibit signs of chronic stress.
Because chronic stress exposes the body to higher levels of cortisol for lengthy periods of time, it has been suggested that it can actually cause memory conditions such as earlier dementia in the elderly and a general decrease in cognitive function. However, some studies have shown that chronic stress with higher levels of cortisol, when compared to chronic stress with lower levels of cortisol, may result in a slower cognitive decline.
Acute stress refers to stress which is, unlike chronic stress, not ongoing but merely a reaction an immediate perceived threat or other stress trigger. Because acute stress is comparatively short when compared to chronic stress, the impact that acute stress has on memory has resulted in mixed findings by researchers. Some studies have concluded that acute stress impairs memory, especially short term memory; other findings have concluded that acute stress actually enhances memory and makes it easier for the brain to encode and retrieve information.
It is important to note that many of these studies have found that there are differences in the actual type of memory or information which is either enhanced for forgotten when exposed to acute stress. For example, it has been found that neutral stimuli is more likely to be remembered during an acute stress event while emotional stimuli is more likely to be forgotten—for example, many people who undergo the trauma of finding a family member injured and in need of medical assistance often report remembering neutral stimuli, such as the flashing lights of the ambulance outside of their window at night—rather than how they were feeling at the time.
However, even this finding on neutral vs. emotional information is debated, as other studies have shown that the opposite is evident in the brain. Some studies have found that it is actually the timing of the acute stress response that affects what type if information is negatively or positively affected by acute stress. Emotional information, for example, is typically enhanced when the perceived stress trigger is induced before the information is encoded--and that retrieval of the emotional information must occur shortly afterwards. And for emotional information to be negatively impacted by acute stress the acute stress trigger must occur after the encoding process and memory retrieval must occur after a delay.
Effects of stress on different types of memory
Just as different types of distress may affect memory differently, stress may affect different types of memory differently as well.
Stress has been often shown to significantly impact short term memory, or the ability to store a relatively small amount of information for a short amount of time. An example of short term memory would be remembering where someone has set down their car keys a minute or two after they have done so. Stress often affects short term memory because short term memory relies on quick encoding and quick retrieval, which are done in the hippocampus which is negatively affected by cortisol.
Stress has also been shown to affect working memory, or the ability to temporarily store information that will then be used to perform a complex task, such as reasoning. An example of working memory would be remembering information given during a lecture and using that information to ask questions or argue during the second portion of a university class. Stress has been shown to impair working memory but, in some studies, to also improve it. Some studies have shown that stress affects working memory by actually increasing the reaction time in information processing during working memory, which may actually assist someone who is in a life-threatening situation. For example, someone may remember the license plate number of a car that they witness in a drive-by shooting.