What's the science?
Stress can impair episodic memory, meaning memory for specific events. In the brain, the hippocampus and frontoparietal network (a network consisting of the frontal and parietal lobes of the brain) are responsible for this type of memory. When remembering, medial temporal lobe structures are responsible for a sense of familiarity ('I've seen that before'), and the hippocampus is responsible for recalling specific details of events ('I saw that yesterday at the park'). Frontoparietal networks are recruited when recalling the memory is difficult - they guide the recollection by directing attention - such as toward cues to help someone recall. Stress can impair the hippocampus, which can then contribute to poor memory retrieval. But how does stress impair these memory circuits in the brain exactly? This week in Cerebral Cortex, Gagnon and colleagues used functional magnetic resonance imaging (fMRI) to understand memory circuits in the human brain during acute stress.
How did they do it?
47 young healthy male (18-35 years) participants completed the experiment (44 completed fMRI scans). Half of the participants were randomly assigned to a stress group, and half to a control group. On the first day, all participants completed a study session in which they memorized the association between random words and random scenes (192 word-scene pairs). On the next day, participants were assigned to either the stress group or the control group, and shown words they had seen before as part of the pairs, in addition to new words, while completing fMRI scanning. Participants were asked to report whether the word was old (seen before) or new, and if it was recalled as old, how sure they were about whether the scene paired with the word was indoor or outdoor. At the beginning of each run (6 runs totalling 252 trials), participants in the stress group were told the run would be a shock run (an orange border appeared on the screen during the run) or a safety run (a teal border appeared on the screen during the run). During the shock runs, participants in the stress group received an electric shock (previously calibrated for each individual to be moderately painful) via electrodes on their ankle (1-2 shocks at some point during the run). Participants in the control group were informed that the run would be an 'orange run' or a 'teal run', however no shocks were received. The authors hypothesized that stress hormones (released during stress) would act on the brain throughout the entire experiment for the stress group, and that the stress group would experience the additional difficulty of being distracted by the stressful prospect of a shock during the shock runs only. Both groups also rated whether they felt positive (happy, safe) or negative (anxious, stressed) during each run of the experiment. [The study also included cortisol testing and weak and strong levels of memory coding - see paper for details].
What did they find?
Stressed participants reported feeling more negative than control participants during shock runs, and more negative than during their own safe runs. Associative memory (recalling scene as indoor versus outdoor) performance was found to be reduced by stress (in the stress group). However, performance when simply discriminating old words from new words was only marginally affected by stress. Surprisingly, stress due to shock runs (versus safe runs) in the stressed group did not impact associative memory (recalling scene indoor vs. outdoor) performance. These results suggest that recall is influenced generally by exposure to stress (stress group vs. control group), but not by the immediate presence of a distracting threat (shock runs vs. safe runs). The authors examined neural activation across trials to study hippocampal activity. A larger increase in the hippocampal BOLD response (fMRI measure of brain activity) was seen for trials on which recall was correct. The relationship between hippocampal activity and associative memory was also found to be disrupted during stress. In the stress group (but not the control group), reduced activation in the hippocampus was found during correct recollection of old words versus correct rejection of new words. During trials in which participants reported they were certain about whether the scene was indoor or outdoor, there was a relationship between frontoparietal engagement and reaction time in the control group, however, this relationship was disrupted in the stressed group. This result indicates that the control group but not the stressed group is able to flexibly recruit these networks.
What's the impact?
This study examines the neural mechanisms underlying changes in memory recollection during acute stress using fMRI. Stress can impair episodic memory recollection, and likely does so by disrupting the relationship between hippocampal activity and recollection. The results have important implications for how stress can impact memory, especially in clinical populations in which neural mechanisms may be altered - for example, in post-traumatic stress disorder.
Gagnon et al., Stress Impairs Episodic Retrieval by Disrupting Hippocampal and Cortical Mechanisms of Remembering. Cerebral Cortex (2018). Access the original scientific publication here.