Forgetting, remembering, and stress

March 9, 2021

Written by: Sara Taylor

You are running late, just trying to get out the door, but you can’t find your keys. After rushing to check all the usual places, you realize you had them in your hand the whole time. You finally get out the door and on your way. About 10 minutes down the road, you go to take a sip of your coffee only to realize that you forgot it at home on the counter. A true series of unfortunate events but also a pretty common experience! Why is that? Why do we seem to forget things when we are in a rush and stressed?

Before we dive in, it is important to put some boundaries on the types of memory we will be talking about. Memory comes in many flavors. First, we can separate out memory by time. There is short-term memory (lasting just a few seconds), working memory (on the scale of minutes), and long-term memory (everything that lasts longer). Second, we can separate long-term memory out further into two big categories – declarative / explicit and non-declarative / implicit. Declarative memory includes episodic memory, which is memory of life events and experiences, and semantic memory, or what I like to call trivia night memory, which is memory for facts. In addition to life event memory and trivia night memory, you also have implicit memory, which includes things that you know subconsciously. A classic example of implicit memory is learning how to ride a bike – you don’t necessarily know all of the things you are doing with your feet, legs, and torso to stay on the bike, but you are able to do it anyway.

This article will focus on the effects of stress on long-term memory. Most of the research referenced focuses on declarative/explicit memory and some types of nondeclarative/implicit memory. Also, the type of stress I am referring to is your run of the mill acute stress (an immediate response to a temporary stressful situation) in contrast to chronic stress (when stress persists over a long period of time) or acute stress disorder (which is an immediate stress response tied to a traumatic event). This means there is a whole vast world of details related to memory and stress that I am not covering here – something to look forward to for future reading!

Okay, let’s dive in! The first thing to know about memory and stress is that timing matters. Being stressed when you are learning something and committing it to memory (called memory encoding) has a very different effect than being stressed when you are trying to remember something (called memory retrieval). These opposite effects likely come down to the different timing of the “waves” of the stress response in your body. 

The first wave of the stress response comes within seconds of something stressful happening. This first wave involves a release of catecholamines – like noradrenaline and other molecules – in the body and in the brain. This first wave is what gets you into flight-fight-or-freeze mode. It also leads to some immediate changes in the functioning of brain areas related to memory – like the hippocampus and the amygdala. The second wave of the stress response is much slower, peaking 20 to 30 minutes later. This second wave involves the release of cortisol. Cortisol affects many brain and body systems, including some of the most important brain regions for memory. Once it gets to these areas it immediately alters neural activity, but cortisol can also have lasting effects by changing gene expression (how DNA is translated into proteins).

Because of the timing of the different waves, when the stressful event happens in relation to when you are trying to learn makes a big difference. For example, something stressful 30 minutes before you are trying to learn will make it more difficult, but something stressful right before learning will actually make it easier to remember later on! (Shown in Figure 1) Stress right as you are trying to learn something means you are getting that immediate noradrenaline and other catecholamines response. Research has shown that a catecholamine called noradrenaline gives a boost to the processes underlying memory formation.2 Connections between neurons in the hippocampus is thought to be one of the core ways that memories are formed. (See another PNK article to learn more!) Noradrenaline makes these connections stronger.

Figure 1. This figure shows the effects of stress on memory at different times in the memory process.  Stress that occurs just prior to making the memory enhances memory, while stress at least 30 minutes prior to making the memory or trying to remember something impairs memory. Memory encoding is represented on the timeline with a notebook icon. Memory retrieval is represented with a thought cloud. The green lightning bolts represent when something stressful occurred. If the lightning bolt is below the center line, it means that stress at that time would impair memory. If it is above the center line, it means that stress at that time would enhance memory. This is a modified version of Figure 2 from Vogel & Schwabe, 20161 made in BioRender.

In contrast, something stressful 30 minutes or longer before you are trying to commit something to memory means cortisol levels have been building and making changes in your brain during that time. On this timescale, stress has the effect of weakening connections between neurons in the hippocampus – making it so memories are not as well formed or don’t get formed at all.3 Stress right before you are trying to remember something (like where you put your keys) also gets in the way. (Shown in Figure 1) Research has found that a stress-inducing event anywhere between 8 to 90 minutes before trying to remember something can make it more difficult to retrieve that memory.4,5 It seems that cortisol makes it harder to remember both by weakening connections in the brain and by influencing gene expression.The changes in gene expression have the effect of making brain regions related to memory, like the hippocampus, less active.6 Part of retrieving a memory involves activating the hippocampus, so these gene expression changes impair that process.

To sum up, stress can hurt or help your memory depending on timing and what part of the memory process you are in. These patterns have to do with the different waves of the stress response and how it affects your brain. Understanding the relationship between routine acute stress and memory could be helpful for designing strategies for improving learning in school, give us insight into the relationships between other kinds of stress and memory, and at the very least serve as a reminder to give yourself a break when you forget that coffee. 

To learn more about what is happening in the brain when we forget things, see Marissa Maroni’s article What causes our brains to forget? https://pennneuroknow.com/2021/02/16/what-can-cause-our-brains-to-forget/

References:

1. Vogel, S., & Schwabe, L. (2016). Learning and memory under stress: implications for the classroom. npj Science of Learning1(1), 1-10.

2. Katsuki, H., Izumi, Y., & Zorumski, C. F. (1997). Noradrenergic regulation of synaptic plasticity in the hippocampal CA1 region. Journal of neurophysiology77(6), 3013-3020.

3. Wiegert, O., Joëls, M., & Krugers, H. (2006). Timing is essential for rapid effects of corticosterone on synaptic potentiation in the mouse hippocampus. Learning & Memory13(2), 110-113.

4. Schilling, T. M., Kölsch, M., Larra, M. F., Zech, C. M., Blumenthal, T. D., Frings, C., & Schächinger, H. (2013). For whom the bell (curve) tolls: Cortisol rapidly affects memory retrieval by an inverted U-shaped dose–response relationship. Psychoneuroendocrinology38(9), 1565-1572.

5. Schwabe, L., & Wolf, O. T. (2014). Timing matters: temporal dynamics of stress effects on memory retrieval. Cognitive, Affective, & Behavioral Neuroscience14(3), 1041-1048.

6. Wolf, O. T. (2017). Stress and memory retrieval: mechanisms and consequences. Current Opinion in Behavioral Sciences14, 40-46.

Cover Image from Pixabay

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