Effects of Stress on Memory, Learning & Cognition
Each person is a microcosm, you might say. We are endowed with a basic genetic scaffolding but what happens at the cellular level is entirely context dependent. What's more, most biological events that occur within us remain below the level of conscious awareness—that is, until we experience abrupt or acute manifestations of disease, discomfort, and distress. But it is equally important to acknowledge that normalizing such consequences comes naturally to us as well. That is to say, if outcomes are not of an immediate mortal concern, we can simply stuff them back into the foreground of our consciousness. This is where we enter the realm of chronic damage.
“We live well enough to have the luxury to get ourselves sick with purely social, psychological stress.” - Robert Sapolsky Neuroendocrinologist & Standford Professor
One thing we can count on is our ability to appraise our experiences and choices that ultimately affect the outcome of our physical and mental health. The first step to making better choices is awareness. Part of this process involves understanding the nuts and bolts of our physiology.
The Nuts & Bolts
As seen in the figure above, the amygdalae and hippocampus are located deep in the brain and are vital components of the limbic system. The limbic system is the primary mechanism in which emotions, drives, and memory are governed 1. The locality of these mechanisms also illustrates a point in our evolutionary history that predates the iconic folded structure of the outer cortex 2. It is ancient, sophisticated and much of what it does cannot be voluntarily managed. Its primary function is to process information from the environment while autonomously calibrating the rest of the body's reflexes, hormone secretion, and learning.
This is often regarded as the "fear center" of the brain as it plays a large role in emotional memory but this blanket description has since been refuted by researchers because the truth is, there is no single location for something as complex as fear to be situated 3. Instead, we must regard this organ as merely part of a larger relay system involved with fear conditioning in addition to other functions such as pheromone processing, appetite, and memory consolidation.
One thing to note is that the extent to which the amygdala is not functioning properly clues us into the nature of abnormal behavior. Functional magnetic resonance imaging (fMRI) scans reveal decreased activity in the amygdala of psychopaths and increased activity in those who exhibit higher than normal levels of aggression 4.
During a stress response, sensory information from visual, tactile, and audio pathways are filtered through the amygdala 5. If the amygdala perceives a threat, it relays a distress signal to the hypothalamus and pituitary gland which are responsible for secreting hormones such as glucocorticoids and insulin.
For more general information about the stress response, click here
More importantly, the amygdala plays a large role in remembering threats from previous experiences. Over repeated exposure to sensory cues associated with stress, cues will become more salient to the amygdala, therefore, more susceptible to triggering the stress response in the future 6.
Among its many functions, the hippocampus is best known for its role in memory but more specifically its role in episodic memory which refers to events and names of people, places, and things.
While there is still much we do not know about the neural properties of memory itself, we now know that old theories concerning the finitude of neurons is false. Much of what we believe about memory is rooted in observable potentiation (impulses between nerve cells) and neurogenesis (growth of new nerve cells) 7,8.
It's important to point out that memories are not perfect snapshots of details and events. Our memories are quite fallible and subject to distortion over time. Memory, especially in children, can be effortlessly altered through suggestible coercion. It is because of this fact that eyewitness testimony in the court of law is becoming regarded with increasing distrust 9.
Synergy and Executive Control
From this, you might begin to see a tight relationship in how we perceive, react to and recall events across time. If we consider the single-minded, impulsive nature of the amygdala and the lack of reliability on part of memory in general, we can also imagine a recipe for the perfect storm as it relates to psychophysiological stress and knee-jerk responses.
Fortunately, this primordial mechanism of the brain no longer works alone. What separates us from the animal kingdom is the evolved portion of the brain that allows us to occasionally override these systems, namely the prefrontal cortex (PFC). This is the part ourselves that can carry out more in-depth calculations and generate inhibitions which keep us from acting on our impulses. It is essentially the little angel on our shoulder or our "conscience", so to speak. The only drawback is that we have to wait about 25 years or so before the PFC is fully developed. It's influence on behavior, or lack thereof, can be observed in the adolescent stage of life 10.
Stress & Learning
Consider the classic platitude "practice makes perfect". This isn't at all a new concept for humans. We apply it when we first learn to walk and when we learn our multiplication tables in grade school. It is this behavioral imperative as a species that produces great works of art or amazing technology that lets us gaze deeper into the cosmos. But we're just now beginning to scratch the surface of what happens in the brain every time we take another step forward.
Let's imagine for a moment that you're a defensive midfielder on a soccer team but your coach decides to try you out as a goalkeeper. You're standing between an 8 foot wide net with big gloves and your job is to neutralize a round projectile from slipping through that space. The coach orders the entire team to launch a pile of soccer balls at you in rapid succession. You're intimidated. You feel foolish. You might say you're even a bit stressed....
Adrenaline is coursing through your blood, digestion is slowed, sugar is quickly metabolized for energy, and your sensory acuity is heightened beyond normal levels. Your body is actually drawing in twice as much information from your environment as you would in a restful state. Such levels of stimulation are actually ramping up the frequency of communication between neurons, therefore "strengthening" their connections over time. This is what's referred to as long-term potentiation (LTP) 11. The more neurons work together, the more they branch out and create tighter bonds. As a result, the branches facilitate quicker and more detailed information to be passed through them. We can actually see new connections in the hippocampus after just a day or two of soccer practice, however, the converse is also true. It's likely that you would lose some of those connections over the weekend.
In short, the end result of so many round projectiles hurled at you after several weeks to months time is an adaptive, learned ability to perform with increasing skill and effectiveness.
Now we begin to see a clearer relationship between stress and memory. But there is a distinction to be made between good stress and bad stress...
Otherwise known as eustress, can be identified by its motivational factors. It can help propel us toward goals and even induce states of excitement and happiness. Examples include:
- Roller Coaster Rides
- Enjoying a scary movie with a loved one
- Sky Diving
- Watching your best friend get married
- Studying for a test
- Scoring a touchdown
The physiological markers of eustress can also be indicated by the immediate recovery time after exposure to a stressor. For example, after hopping off a roller coaster you might feel a little weak in the knees several minutes after but it is unlikely that you will be symptomatic in the hours following.
Unlike the former, chronic stress can impede or hinder function and motivation in life. Some of us are luckier than others but no one is capable of avoiding it altogether. Examples include:
- Death of a loved one
- Car accident
- Relationship strain
- Financial strain
As you can see, feeling stressed, depressed or anxious would be the appropriate response to these types of events. Where it becomes a problem for millions of people on a daily basis is our automatic interpretations of threats. We can sometimes feel the same effects worrying about what President Trump tweeted this morning as we would having recently lost a loved one. We understand intellectually the difference but our bodies do not.
Moreover, we suffer the physiological effects long after exposure to a stressor. Say you've spent an hour in traffic and you're late for work. Someone cuts you off and you just spent that morning arguing with your spouse. For the rest of the day, your blood pressure is increased while hormones wreak havoc inside you. Imagine sustaining this for days, weeks, months and years. You might be surprised at how many people do. It is reported that 44% of Americans suffer from chronic stress and that number grows each year 12. That's roughly 142 million Americans each day suffering the effects of long-term stress making them more vulnerable to diabetes, cancer, inflammation, heart disease and various mental disorders 13.
The important message here is that we train ourselves to respond this way. It becomes habitual and conditioned. It is learned through the same process hearkening back to the soccer example.
Stress in the Brain
Now let's apply the same principles of strengthened connections (LTP) in certain parts of the brain that we covered earlier. We can think about them in these simple terms...
The hippocampus, among its many functions, remembers details of neutral information, i.e. your phone number or your mother's birthday.
The amygdala, among its many functions, remembers emotionally charged information i.e. bombs exploding in Iraq or that guy who cut you off in traffic.
The more transient or positive cues of stress actually promote LTP in the hippocampus. It has been shown that varying levels of intense exercise can enhance spatial pattern recognition in addition to significant increases in cell proliferation in the hippocampus 14.
In a similar study, mice were divided into age-related groups and encouraged to run voluntarily in an exercise wheel. The aim of the study was to compare age-related cognitive decline to younger subjects and the effects of exercise on hippocampal regeneration. The first was an elderly mouse group (19 months) and the second a young group at 3 months of age. Over the course of 45 days, each group was introduced to a maze to test memory and cognition after intermittent trials of physical exercise. Researchers discovered an astounding 50% reversal in hippocampal cell death in the aged mice which correlated closely with the increase of efficiency when completing mazes over the course of the study 15.
A little too much stimulation or stress over time can actually turn this process on its head. Prolonged secretion of glucocorticoids from stress increases LTP in the amygdala while reducing LTP in the hippocampus 16. Additionally, the more excitable activity in the amygdala, the less involved the PFC becomes in decision making which accounts for impulsivity and irrational behavior. To understand this, we have to remember that the amygdala, by nature, is designed to bypass these higher cortical functions. Another important factoid about the PFC is its involvement in working memory or short-term memory which is essentially concerned with conscious perceptual and linguistic processing. Imagine running into a celebrity at an airport. You can't help but feel nervous and scared when you approach them and inevitably stumble over your words like a babbling fool.
The point is, overwhelming stress causes important details or cues from the environment to be overlooked. We arrive at a state of mind that can't help but interpret the world as a perpetual threat. There is also well-documented evidence of this in those who suffer from post-traumatic stress disorder. Brain scans reveal subjects with PTSD have lower hippocampal volume and enlarged amygdala 17. This also might explain the severe flashbacks of emotionally charged memories that never seem to go away. The same is also true for those who suffer from chronic depression although shrinkage of the hippocampus can be mediated through various therapies and pharmaceutical intervention. 18.
The evidence is clear, although most people hate to accept it, exercise is beneficial on almost every single level of our existence. It strengthens the heart, it keeps us agile, stabilizes our metabolism and contributes to a healthy functioning brain. Imagine where would be as a species if we hadn't risen out of bed in the morning and spent the next 12 hours traversing the land in search of food. Today, most necessities are well within reach. We don't even have to ask where our food comes from. We simply walk to the refrigerator and scarf down a ready-made meal. We don't have saddle up a horse or walk to work, we simply sit down and let a complex machine take us. The necessity for movement has declined enormously over the last century and its no wonder we don't see much of the benefits of exercise in our daily life. I shouldn't have to tell you how to exercise or what types of exercise are out there. You know what it is because if you didn't, you wouldn't be so afraid of it. It's good for you, do it. It's that simple.
Remember my little tidbit on the PFC and executive control? This is what the Hindus and Buddhists have been raving on about for centuries. Science has finally caught up to this wisdom. According to the National Center for Integrative and Complementary Health 19 :
- Researchers compared brain images in 2012 from 50 adults who meditate and 50 adults who don’t meditate. Results suggested that people who practiced meditation for many years have more folds in the outer layer of the brain. This process (called gyrification) may increase the brain’s ability to process information.
- A 2013 review of three studies indicated that meditation may decrease or reverse changes that take place in the brain due to normal aging.
- Another study form 2012 suggest that meditation can positively affect activity in the amygdala and that different types of meditation can affect the amygdala differently even when the person is not meditating. This eludes to the idea meditation is also a learned response with long-term benefits.
“All of humanity's problems stem from man's inability to sit quietly in a room alone.” -Blaise Pascal, 1654
Despite what you may believe about the implicit metaphysical jargon associated with meditation, it's really just a matter of taking a break for at least 10 minutes by yourself. Easy enough, right? Think again. A disturbing poll was taken in 2014 where people were given the choice to either sit alone in a room for 12 minutes or receive an electric shock. Results indicate that 70 percent of the men and 25 percent of the women chose to shock themselves during that twelve minutes 20.
The confines of our mind, emotions and our racing thoughts can be intimidating but meditation can help us dissociate ourselves from them. This is critical when we approach stressful situations that would otherwise overwhelm and make us sick. There's nothing magical about it. In the video below, neuroscientist Sam Harris does an excellent job speaking to its practical application in this guided meditation.
Sam Harris - 30 min Guided Meditation
- Rajmohan, V & E. Mohandas (2007) The Limbic System. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2917081/
- Biven, L & Panksepp, J (2012) The Archaeology of Mind. Neuroevolutionary Origins of Human Emotions. W.W. Norton & Company. New York, NY.
- LeDeaux, J. Ph.D (2015) The Amygdala is NOT the fear center of the brain. Retrieved from https://www.psychologytoday.com/blog/i-got-mind-tell-you/201508/the-amygdala-is-not-the-brains-fear-center
- White et al. (2012) Reduced Amygdala Response in Youths With Disruptive Behavior Disorders and Psychopathic Traits: Decreased Emotional Response Versus Increased Top-Down Attention to Nonemotional Features. Retrieved from http://ajp.psychiatryonline.org/doi/abs/10.1176/appi.ajp.2012.11081270
- Herman et al. (2012) Neural regulation of the stress response: glucocorticoid feedback mechanisms. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3854162/
- McGaugh, J (2004) The amygdala modulates the consolidation of memories of emotionally arousing experiences. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/15217324
- Sapolsky, R (2017) Behave. The Biology of our Best and Borst Selves. Chapter 5. Pp 137-153. Penguin Press. New York, New York.
- Deng, W et al (2010) New neurons and new memories: how does adult hippocampal neurogenesis affect learning and memory? Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2886712/#R1
- Howe, M & Knott, L (2015) The fallibility of memory in judicial processes: Lessons from the past and their modern consequences. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4409058/
- Casey, B et al (2008) The Adolescent Brain. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2475802/
- Purves, D. (2001) Long-Term Synaptic Potentiation. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK10878/
- APA (2011) Stressed in America. Retrieved from http://www.apa.org/monitor/2011/01/stressed-america.aspx
- Salleh, M (2008) Life Event, Stress & Illness. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3341916/
- So, J. et al (2017) Intense Exercise Promotes Adult Hippocampal Neurogenesis But Not Spatial Discrimination. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5281566/
- Praag, H. et al (2006) Exercise Enhances Learning and Hippocampal Neurogenesis in Aged Mice. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1360197/
- Rodrigues, S. et al (2009) The influence of Stress Hormones on Fear Circuitry. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/19400714
- Bremner, J. (2006) Stress and Brain Atrophy. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3269810/
- Sapolsky, R. (2001) Depression, Anti-depressants, and the shrinking hippocampus. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC60045/
- NIH (2016) Meditation In-Depth. Retrieved from https://nccih.nih.gov/health/meditation/overview.htm
- Wilson, T. et al (2014) Just Think: The challenges of a disengaged mind. Retrieved from http://science.sciencemag.org/content/345/6192/75