magazine issue 9 2018 / Issue 38

The Effect of Acute Stress on Memory: How It Helps and How It Hurts

• written by Carey Marr, Melanie Sauerland, Henry Otgaar, Conny Quaedflieg & Lorraine Hope
• edited by Lucas Keefer

Although we often perceive the effect of stress negatively, research suggests that stress effects are not always detrimental. This review paper examines the impact that acute stress has on memory formation and retrieval by highlighting the ways that stress can help improve memory, but also the ways in which it typically harms memory. This paper focuses on the neurobiology behind the stress response, the ways in which researchers typically examine stress in laboratory experiments, and how such research applies to the real world. This paper also includes suggestions for how to best utilize stress effects to assist our memory ability. 

Think of a time when you have given an important presentation in front of a large group of people. Recall the way your body responded: your heart accelerated, your palms got sweaty, and your entire body became tense. This response could occur in many different situations: receiving a phone call with bad news, fighting with a friend, taking an important test, being interviewed for a new job, or even being attacked. From everyday activities to more severe events, we frequently experience acute stress in our lives. How does our reaction to stress affect our memory? This question lies at the core of the current paper. 

Stress and the Brain

Stress can have contrasting effects on memory depending on the stage of memory in which it occurs. The first two stages, encoding and consolidation, occur during memory formation. In encoding, information is processed and categorized, before being stabilized from short-term memory into long-term memory in the consolidation stage. The third stage of memory is retrieval, in which previously encoded and stored events or information are accessed. Research suggests that short-lived stress from a specific situation, called acute stress, typically benefits memory formation, while acute stress during retrieval typically impairs memory (1). To understand why acute stress can play this differential role in memory, it helps to know what happens in our body and brain when we experience stress. Imagine, for example, that a dog attacks you and tries to bite you. Within seconds, your autonomic nervous system is activated. This system releases the stress hormone adrenaline that leads to typical symptoms of acute stress, such as increased heart rate, blood pressure, and sweating while enhancing arousal and alertness (2). This adrenaline release also allows stress-associated brain areas such as the amygdala to exert influence directly on memory-associated brain areas such as the hippocampus (3). Acute stress reactions lead to the well-known fight-flight-or-freeze response, as you could use this stress response to deal with the dog attack in different ways: you could run away, you could fight the dog off, or you could just freeze. The acute stress caused by the dog attack will also result in a second slower release of the stress hormone cortisol via activation of the hypothalamic-pituitary-adrenal axis in the brain. The effect of cortisol on the brain ensues two phases: (a) a rapid phase in which the effects start about 15-20 minutes after the stressor and continue for around one hour and (b) a delayed phase in which the effects start 60-90 minutes after the stressor (2 4 5).

These two cortisol phases have different effects on different memory stages. The rapid autonomic nervous system and cortisol hormones induce a memory formation mode, which eases the creation of new memories. This mode facilitates the formation of stressor-related memories (i.e., during encoding) due to an increased excitability in brain structures related to memory (2 4). However, the shift towards this memory-formation mode in the brain during the rapid phase also causes reduced ability to retrieve stressor-unrelated information (2). Thus, when acute stress occurs during or prior to retrieval, memory will likely be impaired (1, 6). Past research on memory retrieval also suggests that memory for negative information will be more impaired than memory for neutral information (7 8). On the other hand, the delayed cortisol hormones have the opposite effect: during this memory storage mode, the consolidation of stressor-related material is facilitated while formation of stressor-unrelated memories is impaired (4). Additionally, memory retrieval will continue to be impaired due to the delayed cortisol hormones (1).

Acute stress can also have an impact on the quality of our memories. In a stressful situation, for example, we may shift from thinking in a flexible way to thinking in a more habitual way (2, 9). For example, imagine you are not hungry, yet you get an urge to eat when you pass by the fridge. This habitual way of thinking is amplified by stress. Therefore, if you experience acute stress, you are more likely to to open the fridge and eat when you pass by it. In contrast with the flexible thinking, this habitual way of thinking is more rigid and focuses less on relationships and complexities between stimuli or tasks. This shift is often adaptive in that it is less cognitively demanding than flexible thinking, and such a state can save mental resources and helps avoid hesitation in situations where fast action is required (10 11). Such habitual processing does not necessarily reduce learning performance (e.g., 12), but may make it more difficult to apply such learning to novel stimuli or contexts (11).

In sum, the timing of the acute stressor plays a key role in its effect on memory and this timing is crucial to determine whether it helps or hurts our memory. In practice, this means that the same acute stress response that may be detrimental during retrieval can be beneficial during encoding. Therefore, stress cannot be classified solely as beneficial or harmful. Rather, it can have contrasting effects on our memories depending on the circumstances.

Stress in the Lab

How do researchers actually test the effects of stress on memory? To experimentally examine stress in humans in the laboratory, several different methods have been applied. For example, some researchers have administered cortisol to participants, simulating the physiological stress reaction (13). Other studies have shown participants negatively arousing materials such as photographs or videos to enhance arousal (e.g., 14 15). Some researchers have also developed use standardized laboratory stressors that reliably increase physiological and subjective stress. These include the Cold Pressor Test (16), in which participants immerse their hands in ice water, the Trier Social Stress Test (17), in which participants are forced to present a speech and perform difficult mental arithmetic in front of judges, and the Maastricht Acute Stress Test (18), which combines methods from these two other tests using alternating blocks of cold water hand immersion and socially-evaluated mental arithmetic. These procedures reliably induce mild to moderate levels of stress, but it is important to remember that levels of stress experienced in laboratory settings are likely not as high as those experienced in extremely stressful real world settings.

During experiments, these stressors are induced before or after memory tasks. When examining stress at encoding, participants are exposed to stimuli either during or after these stressors. Then, following a retrieval interval of at least 24 hours, they are tested on these items in various memory tests (e.g., free recall, cued recall, recognition, etc.). Thus far, experimenters have examined this phenomenon for memory of different stimuli, including words, pictures, slideshows, videos, and real events, and found support that a mild to moderate level of stress at encoding typically improves participants’ memory for the previously learned stimuli, particularly for emotional stimuli (19 20 21 22 23). When examining stress at retrieval, participants are exposed to the stimuli first. After an interval, participants are then stressed and tested on their memory either during or following the stressor. As mentioned previously, much support has been found suggesting that stress at retrieval impairs memory for the previously presented stimuli (e.g. 7 8 24).

How Stress Helps: Examples

It is possible to find examples in real life of how stress can help memory formation. One example of how acute stress during encoding may benefit memory includes the situation of becoming an eyewitness to a crime. Neurobiology work theoretically suggests that experiencing a mild to moderate level of stress, and particularly emotional stress—as crimes often are—should improve eyewitness memory (1 4), although work from the eyewitness field has not yet demonstrated this idea robustly (e.g., for mixed evidence see 25). The role one plays within a stressful situation could also contribute to how stressful the incident is and may matter for one’s memory of the situation (e.g., 26). Research also suggests that lower levels of stress can benefit memory at encoding (1 6), and another example of this benefit of acute stress that of students studying for exams. Experiencing mild to moderate stress during this learning phase could help them perform better on subsequent tests (9). Overall, understanding the positive side of stress allows us to consider how to best use such information to benefit our memory. We will return to this issue in more depth in the final section of this paper.

How Stress Hurts: Examples

We can use these same examples to highlight the negative impact of stress on retrieval in everyday life. For example, after viewing a crime, eyewitnesses are often called into the police station to be interviewed about what they experienced. Some eyewitnesses may become stressed during such an interview, for instance, if they have experienced negative encounters with police in the past, have social anxiety, or are scared of the perpetrators about whom they are reporting. If eyewitness do experience stress, it is likely that their memory will be impaired. Because similar impairments for retrieval stress have been found in children as well as adults (27), another example setting is in school, where students are frequently faced with stressful situations where they need to retrieve memories, such as class presentations or exams. That means that heightened levels of stress during an assessment can negatively affect student memory, and thus their performance and grades (9). Additionally, stress can impair one’s ability to incorporate new information into what one already knows, a process that often is required for learning in academic settings. Teachers could also face negative impacts of stress in classroom settings. For example, if teachers experience acute stress, they may switch from a flexible to a habitual teaching style, which could hinder students’ ability to effectively learn (9). Such examples demonstrate how acute stress before retrieval can impair memory skills in the real world, but we can try to overcome such impairments.

What Now: Applied Suggestions

The question now is how we can use the knowledge of the different effects of stress to help improve our memory performance? More specifically, how can we use the positive benefits of mild to moderate levels of stress during encoding and mitigate the negative effects of retrieval stress? Although inducing stress during learning (e.g., encoding) is not recommended, presenting academic material in a more emotional way could be employed in academic settings to help improve memory formation. For example, teachers could show video clips that relate what is being learned to a familiar setting or place the learning into an emotional context. Such techniques could produce an aroused state in students that can help them memorize and later remember this material (9).

Addressing the ways in which stress can hurt memory, how material is initially learned and studied could help protect memories against the detrimental effects of acute stress before or during retrieval. We know that viewing to-be-remembered stimuli multiple times (i.e., restudying) is less effective than being tested on to-be-remembered stimuli multiple times before the real test (i.e., retrieval practice; 28 29). Interestingly, when engaging in retrieval practice, negative effects of stress can be mitigated (30). This is not the case when simply engaging in restudying (30). That means that students can benefit from engaging in retrieval practice while learning material to help mitigate the negative stress effects they will likely experience during the final test. Such recent findings indicate that when memories are strongly represented, stress will not always impair memory (30).

Another way to mitigate memory impairments caused by stress could be to teach students coping mechanisms for dealing with the stress they may experience during tests (9). This includes developing skills such as problem-focused and emotion-focused strategies, with the former involving changing or altering the source of stress directly and the latter focusing on regulating negative emotions associated with the stressful event (32). Utilizing such coping mechanisms can significantly reduce students’ anxiety during testing situations (33), which could damper arousal as well as protect mental health. Not only are such coping skills helpful for school and university exams, but they also assist students in dealing with acute stress experienced outside the classroom (e.g., stress at home, peer stress). This is important because stress effects can have an impact for up to a few hours after the stressor is encountered. As a result, events experienced before school at home or during lunchtime might later affect how well students do in class (9). Teaching students how to manage stress is an easy first step that teachers can take when attempting to alleviate negative academic outcomes stemming from memory impairment.

In some professions, stressors are inherent to the job, for example, surgeons, pilots, and police officers. Because experiencing acute stress can cause a shift from flexible thinking to more rigid and habit-based thinking, one good way to prepare for such stress is to have frequent practice training (34). Practicing skills to make them more automatic allows individuals to respond rapidly and accurately to high levels of stress, even when flexible thinking is reduced. Therefore, practice training could improve habitual responses to help shield stressed individuals from negative effects of stress on retrieval. Other coping mechanisms likely useful for improving mental health include increasing emphasis and training on non-technical skill (e.g., improving communication, leadership and decision making abilities) and establishing training for crisis management in addition to traditional supervision (34).

This review focuses on experiences of acute stress and how these affect memory. Under conditions of chronic stress, on the other hand, typically alters the cognitive memory system (35). Specifically, chronic stress has shown opposite effects on brain structures involved in memory, as compared to acute stress (35). Additionally, there are stress-related disorders that are characterized by memory disturbances such as involuntary flashbacks (i.e., intrusions) in post-traumatic stress disorder ( PTSD) and bias to remembering negative memories (i.e., rumination) in depression. Thus, chronic stress has been associated with both increased and decreased HPA axis activity.

Conclusion

Without a doubt, the relationship between acute stress and memory is more complex than often assumed. On the one hand, stress can benefit memory as mild to moderate levels of stress during encoding can facilitate memory formation. On the other hand, acute stress can be harmful to memory and memory-related performance at both encoding and retrieval. For instance, if stress is present during retrieval, we are typically worse at remembering events than when we are not stressed. In summary, not all stress is bad for our memories, and research can help us understand the nuances of these effects in order to both utilize the positive and combat the negative effects of acute stress.

 

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