Stress Alters the Neural Context for Building New Memories

How emotional contexts modulate item memory in individuals with high and low negative affect and worry

Emotional stimuli are usually remembered with high confidence. Yet, it remains unknown whether-in addition to memory for the emotional stimulus itself-memory for a neutral stimulus encountered just after an emotional one can be enhanced. Further, little is known about the interplay between emotion elicited by a stimulus and emotion relating to affective dispositions. To address these questions, we examined (1) how emotional valence and arousal of a context image preceding a neutral item image affect memory of the item, and (2) how such memory modulation is affected by two hallmark features of emotional disorders: trait negative affect and tendency to worry. In two experiments, participants encoded a series of trials in which an emotional (negative, neutral, or positive) context image was followed by a neutral item image. In experiment 1 (n = 42), items presented seconds after negative context images were remembered better and with greater confidence compared to those presented after neutral and positive ones. Arousal ratings of negative context images were higher compared to neutral and positive ones and the likelihood of correctly recognizing an item image was related to higher arousal of the context image. In experiment 2 (n = 59), better item memory was related to lower trait negative affect. Participants with lower trait negative affect or tendency to worry displayed higher confidence compared to those with high negative affect or tendency to worry. Our findings describe an emotional "carry-over" effect elicited by a context image that enhances subsequent item memory on a trial-by-trial basis, however, not in individuals with high trait negative affect who seem to have a general memory disadvantage.

Hippocampal mechanisms support cortisol-induced memory enhancements

Stress can powerfully influence episodic memory, often enhancing memory encoding for emotionally salient information. These stress-induced memory enhancements stand at odds with demonstrations that stress and the stress-related hormone cortisol can negatively affect the hippocampus, a brain region important for episodic memory encoding. To resolve this apparent conflict and determine whether and how the hippocampus supports memory encoding under cortisol, we combined behavioral assays of associative memory, high-resolution functional magnetic resonance imaging (fMRI), and pharmacological manipulation of cortisol in a within-participant, double-blinded procedure. Hydrocortisone led to enhanced functional connectivity between hippocampal subregions, which predicted subsequent memory enhancements for emotional information. Cortisol also modified the relationship between hippocampal representations and memory: whereas hippocampal signatures of distinctiveness predicted memory under placebo, relative integration predicted memory under cortisol. Together, these data provide novel evidence that the human hippocampus contains the necessary machinery to support emotional memory enhancements under stress.

Distinct stress-related changes in intrinsic amygdala connectivity predict subsequent positive and negative memory performance

Experiencing stress before an event can influence how that event is later remembered. In the current study, we examine how individual differences in one's physiological response to a stressor are related to changes to underlying brain states and memory performance. Specifically, we examined how changes in intrinsic amygdala connectivity relate to positive and negative memory performance as a function of stress response, defined as a change in cortisol. Twenty-five participants underwent a social stressor before an incidental emotional memory encoding task. Cortisol samples were obtained before and after the stressor to measure individual differences in stress response. Three resting state scans (pre-stressor, post-stressor/pre-encoding and post-encoding) were conducted to evaluate pre- to post-stressor and pre- to post-encoding changes to intrinsic amygdala connectivity. Analyses examined relations between greater cortisol changes and connectivity changes. Greater cortisol increases were associated with a greater decrease in prefrontal-amygdala connectivity following the stressor and a reversal in the relation between prefrontal-amygdala connectivity and negative vs. positive memory performance. Greater cortisol increases were also associated with a greater increase in amygdala connectivity with a number of posterior sensory regions following encoding. Consistent with prior findings in non-stressed individuals, pre- to post-encoding increases in amygdala-posterior connectivity were associated with greater negative relative to positive memory performance, although this was specific to lateral rather than medial posterior regions and to participants with the greatest cortisol changes. These findings suggest that stress response is associated with changes in intrinsic connectivity that have downstream effects on the valence of remembered emotional content.

Stress enhances emotional memory-related theta oscillations in the medial temporal lobe

Stressful events impact memory formation, in particular for emotionally arousing stimuli. Although these stress effects on emotional memory formation have potentially far-reaching implications, the underlying neural mechanisms are not fully understood. Specifically, the temporal processing dimension of the mechanisms involved in emotional memory formation under stress remains elusive. Here, we used magnetoencephalography (MEG) to examine the neural processes underlying stress effects on emotional memory formation with high temporal and spatial resolution and a particular focus on theta oscillations previously implicated in mnemonic binding. Healthy participants (n = 53) underwent a stress or control procedure before encoding emotionally neutral and negative pictures, while MEG was recorded. Memory for the pictures was probed in a recognition test 24 h after encoding. In this recognition test, stress did not modulate the emotional memory enhancement but led to significantly higher confidence in memory for negative compared to neutral stimuli. Our neural data revealed that stress increased memory-related theta oscillations specifically in medial temporal and occipito-parietal regions. Further, this stress-related increase in theta power emerged during memory formation for emotionally negative but not for neutral stimuli. These findings indicate that acute stress can enhance, in the medial temporal lobe, oscillations at a frequency that is ideally suited to bind the elements of an ongoing emotional episode, which may represent a mechanism to facilitate the storage of emotionally salient events that occurred in the context of a stressful encounter.