Using acute stress to improve episodic memory: The critical role of contextual binding

Integrating and fragmenting memories under stress and alcohol

Stress can powerfully influence the way we form memories, particularly the extent to which they are integrated or situated within an underlying spatiotemporal and broader knowledge architecture. These different representations in turn have significant consequences for the way we use these memories to guide later behavior. Puzzlingly, although stress has historically been argued to promote fragmentation, leading to disjoint memory representations, more recent work suggests that stress can also facilitate memory binding and integration. Understanding the circumstances under which stress fosters integration will be key to resolving this discrepancy and unpacking the mechanisms by which stress can shape later behavior. Here, we examine memory integration at multiple levels: linking together the content of an individual experience, threading associations between related but distinct events, and binding an experience into a pre-existing schema or sense of causal structure. We discuss neural and cognitive mechanisms underlying each form of integration as well as findings regarding how stress, aversive learning, and negative affect can modulate each. In this analysis, we uncover that stress can indeed promote each level of integration. We also show how memory integration may apply to understanding effects of alcohol, highlighting extant clinical and preclinical findings and opportunities for further investigation. Finally, we consider the implications of integration and fragmentation for later memory-guided behavior, and the importance of understanding which type of memory representation is potentiated in order to design appropriate interventions.

Class I histone deacetylases inhibition reverses memory impairment induced by acute stress in mice

While chronic stress induces learning and memory impairments, acute stress may facilitate or prevent memory consolidation depending on whether it occurs during the learning event or before it, respectively. On the other hand, it has been shown that histone acetylation regulates long-term memory formation. This study aimed to evaluate the effect of two inhibitors of class I histone deacetylases (HDACs), 4-phenylbutyrate (PB) and IN14 (100 mg/kg/day, ip for 2 days), on memory performance in mice exposed to a single 15-min forced swimming stress session. Plasma corticosterone levels were determined 30 minutes after acute swim stress in one group of mice. In another experimental series, independent groups of mice were trained in one of three different memory tasks: Object recognition test, Elevated T maze, and Buried food location test. Subsequently, the hippocampi were removed to perform ELISA assays for histone deacetylase 2 (HDAC2) expression. Acute stress induced an increase in plasma corticosterone levels, as well as hippocampal HDAC2 content, along with an impaired performance in memory tests. Moreover, PB and IN14 treatment prevented memory loss in stressed mice. These findings suggest that HDAC2 is involved in acute stress-induced cognitive impairment. None of the drugs improved memory in non-stressed animals, indicating that HDACs inhibitors are not cognitive boosters, but rather potentially useful drugs for mitigating memory deficits.

Metabolomics reveals that chronic restraint stress alleviates carbon tetrachloride-induced hepatic fibrosis through the INSR/PI3K/AKT/AMPK pathway

Hepatic fibrosis (HF) could be developed into liver cirrhosis or even hepatocellular carcinoma. Stress has an important role in the occurrence and development of various considerable diseases. However, the effect of a certain degree stress on HF is still controversial. In our study, stress was simulated with regular chronic restraint stress (CRS) and HF model was induced with CCl4 in mice. We found that CRS was able to attenuate CCl4-induced liver injury and fibrosis in mice. Surprisingly, behavioral analysis showed that the mice in the HF group exhibited depression-like behavior. Further, the metabolomic analysis revealed that 119 metabolites and 20 metabolic pathways were altered in mice liver, especially the betaine metabolism pathway. Combined with the results of Ingenuity Pathway Analysis (IPA), the key proteins INSR, PI3K, AKT, and p-AMPK were identified and verified, and the results showed that CRS could upregulate the protein levels and mRNA expression of INSR, PI3K, AKT, and p-AMPK in liver tissues of HF mice. It suggested that CRS alleviated CCl4-induced liver fibrosis in mice through upregulation of the INSR/PI3K/AKT/AMPK pathway. Proper stress might be a potential therapeutic strategy for the treatment of chronic liver disease, which provided new insights into the treatment of HF. KEY MESSAGES: Chronic restraint stress mitigated CCl4-induced liver injury and hepatic fibrosis. CCl4-induced liver fibrosis could cause depression-like behavior. Chronic restraint stress altered metabolomic profiles in hepatic fibrosis mice, especially the betaine metabolism pathway. Chronic restraint stress increased betaine levels in liver tissue. Chronic restraint stress regulated the INSR/PI3K/AKT/AMPK signaling pathway in hepatic fibrosis mice.

When stress enhances memory encoding: The beneficial effects of changing context

The effects of acute stress on memory encoding are complex, and we do not yet know all of the conditions that can determine whether stress at encoding improves or impairs memory. Recent work has found that changing contexts between encoding and stress can abolish the effects of post-encoding stress on memory, suggesting that context may play an important role in the effects of stress on memory. However, the role of context in the effects of stress on memory encoding is not yet known. We addressed this gap by examining the effects of context on the influence of acute stress on memory encoding. In a 2 × 2 experimental design, participants (N = 103) completed either a stressor (i.e., Socially Evaluated Cold Presser Test) or control task (i.e., warm water control) before completing a memory encoding task, which occurred in either in the same room as or a different room from the stressor or control task. Memory retrieval was tested for each participant within the context that they completed the encoding task. We found that, relative to nonstressed (i.e., control) participants, stressed participants who switched contexts prior to encoding showed better memory for both negative and neutral images. In contrast, when the stressor or control task occurred in the same room as memory encoding, stress had no beneficial effect on memory. These results highlight the importance of the ongoing context as a determinant of the effects of stress on memory encoding and present a challenge to current theoretical accounts of stress and memory.

Sympathetic Vagal Balance and Cognitive Performance in Young Adults during the NIH Cognitive Test

Compromised cognitive function is associated with increased mortality and increased healthcare costs. Autonomic nervous system arousal, as measured by an electrocardiogram (ECG), has received recent attention because of its association with the blood perfusion of brain regions involved with cognitive function. The purposes of this study were to determine whether the ECG HR variation, as measured by the standard deviation of the heart rate N-to-N intervals (SDNN), and sympathetic vagal tone, as estimated by the low-frequency/high-frequency ratio (LF/HF), are increased with cognitive performance during the NIH Cognitive Test (Picture Sequence, Dimensional Change Card Sort, Flanker, and List Sorting). A total of 62 young people without cognitive impairment participated in this study. We discovered that the ECG LF/HF ratio was increased in the top 50% of participants who could: (1) inhibit information and stay attentive to a desired task during the Flanker Test; (U = 329, p = 0.03; R² = 0.76); and (2) promote cognitive function flexibility during the DCCS Test; (U = 55, p = 0.007; R² = 0.98). Taken together, these findings support that the arousal level influences performance during a cognitive test.

Stress and memory encoding: What are the roles of the stress-encoding delay and stress relevance?

The effects of acute stress on memory encoding are complex. Recent work has suggested that both the delay between stress and encoding and the relevance of the information learned to the stressor may modulate the effects of stress on memory encoding, but the relative contribution of each of these two factors is unclear. Therefore, in the present study, we manipulated (1) acute stress, (2) the delay between stress and encoding, and (3) the relevance of the information learned to the stressor. The results indicated that stress during encoding led to better memory for study materials that were related to the stressor relative to memory for study materials that were unrelated to the stressor. This effect was numerically reduced for materials that were encoded 40 min after stressor onset (23 min after the stressor had ended) compared with items encoded at the time of the stressor, but this difference was not significant. These results suggest that the relevance of the information learned to the stressor may play a particularly important role in the effects of stress on memory encoding, which has important implications for theories of stress and memory.

Survival of the salient: Aversive learning rescues otherwise forgettable memories via neural reactivation and post-encoding hippocampal connectivity

The effects of aversive events on memory are complex and go beyond the simple enhancement of threatening information. Negative experiences can also rescue related but otherwise forgettable details encoded close in time. Here, we used functional magnetic resonance imaging (fMRI) in healthy young adults to examine the brain mechanisms that support this retrograde memory effect. In a two-phase incidental encoding paradigm, participants viewed different pictures of tools and animals before and during Pavlovian fear conditioning. During Phase 1, these images were intermixed with neutral scenes, which provided a unique 'context tag' for this specific phase of encoding. A few minutes later, during Phase 2, new pictures from one category were paired with a mild shock (threat-conditioned stimulus; CS+), while pictures from the other category were not shocked. FMRI analyses revealed that, across-participants, individuals who showed aversive learning-related retroactive memory benefits for Phase 1 CS+ items were also more likely to exhibit three brain effects: first, greater spontaneous reinstatement of the Phase 1 context when participants viewed conceptually-related CS+ items in Phase 2; second, greater successful encoding-related VTA/SN and LC activation for Phase 2 CS+ items; and third, learning-dependent increases in post-encoding hippocampal functional coupling with CS+ category-selective cortex. These biases in hippocampal-cortical connectivity also mediated the relationship between VTA/SN aversive encoding effects and across-participant variability in the retroactive memory benefit. Collectively, our findings suggest that both online and offline brain mechanisms may enable threatening events to preserve memories that acquire new significance in the future.

The memory trace of a stressful episode

Stress influences episodic memory formation via noradrenaline and glucocorticoid effects on amygdala and hippocampus. A common finding is the improvement of memory for central aspects of a stressful episode. This is putatively related to changes in the neural representations of specific experiences, i.e., their memory traces. Here we show that the memory improvement for objects that were encountered in a stressful episode relates to differences in the neural representations of these objects in the amygdala. Using functional magnetic resonance imaging, we found that stress specifically altered the representations of central objects: compared to control objects, they became more similar to one another and more distinct from objects that were not part of this episode. Furthermore, higher similarity of central objects to the main stressor-the faces of the stress-inducing committee members-predicted better memory. This suggests that the central objects were closely integrated into a stressor-centered memory representation. Our findings provide mechanistic insights into how stress shapes the memory trace and have profound implications for neurocognitive models of stressful and emotional memory.

Post-Encoding Stress Does Not Enhance Memory Consolidation: The Role of Cortisol and Testosterone Reactivity

In contrast to the large body of research on the effects of stress-induced cortisol on memory consolidation in young people, far less attention has been devoted to understanding the effects of stress-induced testosterone on this memory phase. This study examined the psychobiological (i.e., anxiety, cortisol, and testosterone) response to the Maastricht Acute Stress Test and its impact on free recall and recognition for emotional and neutral material. Thirty-seven healthy young men and women were exposed to a stress (MAST) or control task post-encoding, and 24 h later, they had to recall the material previously learned. Results indicated that the MAST increased anxiety and cortisol levels, but it did not significantly change the testosterone levels. Post-encoding MAST did not affect memory consolidation for emotional and neutral pictures. Interestingly, however, cortisol reactivity was negatively related to free recall for negative low-arousal pictures, whereas testosterone reactivity was positively related to free recall for negative-high arousal and total pictures. This study provides preliminary evidence about a different reactivity of testosterone and cortisol to the MAST as well as on their effects on consolidation. Our results suggest a different pattern of relationships between these steroid hormones and the arousal of the negative images.

Stress-Induced Increase in Cortisol Negatively Affects the Consolidation of Contextual Elements of Episodic Memories

Stress can modulate episodic memory in various ways. The present study asks how post-encoding stress affects visual context memory. Participants encoded object images centrally positioned on background scenes. After encoding, they were either exposed to cold pressure stress (CPS) or a warm water control procedure. Forty-right hours later, participants were cued with object images, and for each image, they were asked to select the background scene with which it was paired during study among three highly similar options. Only male but not female participants reacted with a significant increase in salivary cortisol to CPS, and the stress and control group did not differ in recognition performance. Comparing recognition performance between stress responders and non-responders, however, revealed a significant impairment in context memory in responders. Additionally, proportional increase in cortisol was negatively correlated with the number of correctly recognized scenes in responders. Due to the small number of responders, these findings need to be interpreted with caution but provide preliminary evidence that stress-induced cortisol increase negatively affects the consolidation of contextual elements of episodic memories.

Stress and cognition: A user's guide to designing and interpreting studies

Fueling the rapid growth in our understanding of how stress influences cognition, the number of studies examining the effects of stress on various cognitive processes has grown substantially over the last two decades. Despite this growth, few published guidelines exist for designing these studies, and divergent paradigm designs can diminish typical effects of stress or even reverse them. The goal of this review, therefore, is to survey necessary considerations (e.g., validating a stress induction), important considerations (e.g., specifying the timing of the stressor and cognitive task), and best practices (e.g., using Bayesian analyses) when designing a study that aims at least in part to examine the effects of acute stress on some cognitive process or function. These guidelines will also serve to help readers of these studies interpret what may otherwise be very confusing, anomalous results. Designing and interpreting studies with these considerations and practices in mind will help to move the field of stress and cognition forward by clarifying how, exactly, stress influences performance on a given cognitive task in a population of interest.

Reactivation of the Unconditioned Stimulus Inhibits the Return of Fear Independent of Cortisol

Reconsolidation is the post-retrieval stabilization of memories, a time-limited process during which reactivated (i.e., retrieved) memories can be updated with new information, become stronger or weaker, depending on the specific treatment. We have previously shown that the stress hormone cortisol has an enhancing effect on the reconsolidation of fear memories in men. This effect was specific, i.e., limited to the conditioned stimulus (CS) that was reactivated, and did not generalize to other previously reinforced, but not reactivated CS. Based on these results, we suggested that cortisol plays a critical role in the continuous strengthening of reactivated emotional memories, contributing to their persistence and robustness. In the current study, we aimed to achieve a more generalized reconsolidation enhancement using an alternative reactivation method, i.e., by a low-intensity unconditioned stimulus (UCS) presentation instead of the more common unreinforced CS presentation. In previous studies, UCS reactivation was shown to lead to a more generalized reconsolidation effect. Therefore, we hypothesized that the combination of cortisol treatment and UCS reactivation would lead to an enhanced fear memory reconsolidation, which would generalize from previously reinforced CS to stimuli that resemble it. We tested 75 men in a 3-day fear conditioning paradigm: fear acquisition training on day 1; UCS reactivation/no reactivation and pharmacological treatment (20 mg hydrocortisone/placebo) on day 2; extinction training, reinstatement and test (of original and modified stimuli) on day 3. In contrast to our hypothesis, UCS reactivation prevented the return of fear [observed in skin conductance responses (SCR)] regardless of the pharmacological manipulation: while reinstatement to the original CS was found in the no-reactivation group, both reactivation groups (cortisol and placebo) showed no reinstatement. As the only methodological difference between our previous study and the current one was the reactivation method, we focus on UCS reactivation as the main explanation for these unexpected findings. We suggest that the robust prediction error generated by the UCS reactivation method (as opposed to CS reactivation), combined with the lower UCS intensity, has by itself weakened the emotional value of the UCS, thus preventing the return of fear to the CS that was associated with it. We call for future research to support these findings and to examine the potential of this reactivation method, or variations thereof, as a tool for therapeutic use.

Determining the biological associates of acute cold pressor post-encoding stress effects on human memory: The role of salivary interleukin-1β

Stress generally hurts many aspects of memory, but an interesting finding to emerge from the stress and memory literature is that stress that occurs shortly after learning (i.e., post-encoding stress) usually benefits memory. Although this effect is well established, the biological mechanisms underpinning this effect are not-especially in humans. We addressed this gap in the present study by collecting saliva samples from 80 participants who were randomized to a post-encoding stress (i.e., cold pressor for 3 min) or control task (i.e., warm water for 3 min) and 48 h later completed a recognition memory task. Saliva was collected both prior to and 15 min after the offset of (18 min after the onset of) the stress/control manipulation. Drawing on animal and human work, we examined how five stress-responsive biomarkers-cortisol, salivary α-amylase, progesterone, estradiol, and the proinflammatory cytokine interleukin (IL)-1β, all assessed in saliva-related to the effects of stress on memory. We found that stress enhanced recollection of negative images and that these effects were selectively related to salivary IL-1β. Moreover, we found that the beneficial effects of stress on memory were statistically mediated by salivary IL-1β. We found no robust associations-either linear or quadratic-between memory and any other biomarker, nor did we find significant interactions between biomarkers in predicting memory. These results suggest that immune system activity indexed by salivary IL-1β may play an important role in contributing to post-encoding stress effects on human memory.

Stress and the medial temporal lobe at rest: Functional connectivity is associated with both memory and cortisol

When acute stress is experienced immediately after memory encoding (i.e., post-encoding stress) it can significantly impact subsequent memory for that event. For example, recent work has suggested that post-encoding stress occurring in a different context from encoding impairs memory. However, the neural processes underlying these effects are poorly understood. We aimed to expand this understanding by conducting an analysis of resting functional connectivity in the period following post-encoding stress that occurred in a different context than encoding, using seed regions in the medial temporal lobes known for their roles in memory. In the current study of 44 males randomized to stress (n = 23) or control (n = 21) groups, we found that stress increased cortisol, impaired recollection of neutral materials, and altered functional connectivity with medial temporal lobe regions. Although stress did not significantly alter hippocampus-amygdala functional connectivity, relative to participants in the control group, participants in the post-encoding stress group showed lower functional connectivity between the hippocampus and a region with a peak in the superior temporal gyrus. Across participants in both groups, functional connectivity between these regions was related to greater increases in cortisol, and it was also inversely related to recollection of neutral materials. In contrast, the stress group showed greater parahippocampal cortex functional connectivity with a region in the left middle temporal gyrus than the control group. Moreover, greater functional connectivity between the parahippocampal cortex and the observed cluster in the middle temporal gyrus was associated with greater cortisol changes from pre- to post-manipulation, but was not related to differences in memory. The results show that post-encoding stress can alter the resting-state functional connectivity between the medial temporal lobe and neocortex, which may help explain how stress impacts memory.

No Time-Dependent Effects of Psychosocial Stress on Fear Contextualization and Generalization: A Randomized-Controlled Study With Healthy Participants

The formation of context-dependent fear memories (fear contextualization) can aid the recognition of danger in new, similar, situations. Overgeneralization of fear is often seen as hallmark of anxiety and trauma-related disorders. In this randomized-controlled study, we investigated whether exposure to a psychosocial stressor influences retention of fear contextualization and generalization in a time-dependent manner. The Trier Social Stress Test was used to induce psychosocial stress. Healthy male participants (n = 117) were randomly divided into three experimental groups that were subjected to the acquisition phase of the Fear Generalization Task: (1) without stress, (2) immediately after acute stress, or (3) 2 h after acute stress. In this task, a male with neutral facial expression (conditioned stimuli) was depicted in two different contexts that modulated the conditioned stimuli-unconditioned stimuli (=shock) association (threat, safe). Salivary alpha-amylase and cortisol levels were measured throughout the experiment. After a 24-h delay, context-dependency of fear memory was investigated with an unannounced memory test consisting of the threat and safe contexts alternated with a novel context (the generalization context). Multilevel analyses revealed that participants showed increased fear-potentiated startle responses to the conditioned stimuli in the threat compared to the safe context, at the end of the acquisition phase, indicating adequate fear contextualization. Directly after acquisition, there were no time-dependent effects of psychosocial stress on fear contextualization. Context-dependency of fear memories was retained 24 h later, as fear-potentiated startle responding was modulated by context (threat > safe or novel). At that time, the context-dependency of fear memories was also not influenced by the early or late effects of the endogenous stress response during acquisition. These results with experimental stress deviate in some aspects from those earlier obtained with exogenous hydrocortisone administration, suggesting a distinct role for stress mediators other than cortisol.