In a review article published in the journal Current Opinions in Neurobiology, researchers from The Rockefeller and Stanford Universities summarized the results of multiple scientific studies describing how stress affects cognition.
While short-term stress has been shown to enhance memory, long-term or severe stress has negative effects on cognition. During long-term stress, data from the scientific literature suggest that the structure of neurons in the hippocampus changes, and that these changes are reversible. However, for very long-term stress, neurons in the hippocampus become damaged in a way that is irreversible. These irreversible changes may be related to cognitive impairments seen in older age.
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The authors discuss the different chemical signals that are thought to mediate the body’s response to stress. Following a physical or psychological stressor, a first set of chemical signals are secreted within seconds by the sympathetic nervous system in a “fight-or-flight” response. A familiar chemical signal among these is adrenaline. A second set of chemical signals begins to be secreted within minutes of a stressor; however, the effects can take hours to be seen. These steroid hormones are secreted by the adrenal gland and a familiar steroid hormone among this type of hormone is cortisol.
EVIDENCE FROM THE SCIENTIFIC LITERATURE
Short-term emotional arousal and memory enhancement
In this study, researchers devised a set of experiments to test whether the heightened emotional content of a short story would affect the listener’s memory of the story, relative to a more emotionally neutral story. Study participants were read two stories that were the same length, and that had the same beginning and the same ending. In one version of the story, a boy and his mother travel through town to visit the father, who is in the hospital. When they arrive, both hospital employees and the father demonstrate a variety of medical procedures for the boy.
In a second version of the story, the boy and his mother are traveling through town when the boy is gravely injured by a car, taken to hospital, and has different medical procedures performed on him. The researchers found that memory was enhanced for this more emotionally arousing version of the story. Participants were administered a beta blocker, which inhibited the effects of the fast-acting, adrenaline-like chemical signals described above. After this drug was administered, researchers noted that the previously observed enhanced memory of the emotionally arousing story was eliminated. Memory for the more emotionally neutral story was unchanged. This study provides evidence that fast-acting adrenaline and adrenaline-like chemical signals are involved in the memory enhancement observed in emotionally arousing experiences.
The effects of stress and cortisol-like steroids on learning and memory
Several studies have indicated that there are detrimental effects on cognition following high-doses of a cortisol-like steroid administered over the course of several days. These studies report an association between impairments in learning and memory and the hippocampus.
In one of these studies, participants were treated with dexamethasone, a cortisol-like steroid, for four days. The researchers then probed the participants’ declarative memory. This type of memory concerns facts and events that can be explicitly recalled, compared with procedural memory, which involves knowing implicitly how to do something, such as walk or ride a bike. To probe declarative memory, participants were read a paragraph and asked to recall information from the paragraph both immediately after hearing it and following a short delay.
While short-term stress has been shown to enhance memory, long-term or severe stress has negative effects on cognition.
In these otherwise healthy individuals, dexamethasone was associated with an increased rate of making mistakes in recalling information from the paragraph. Other tasks that were less declarative and more procedural or arousal in nature were unaffected by the presence of dexamethasone. This study provides evidence that a cortisol-like steroid can selectively impair declarative memory.
The effects of prolonged stress on cognitive function
A number of studies have shown that prolonged exposure to cortisol and cortisol-like steroids result in impairments to cognitive functions that involve the hippocampus. Moreover, research has indicated that there are individual differences in the level of cortisol and cortisol-like hormones in the body and that there are individual differences in cognitive impairment.
In a study of healthy older individuals (average age of about 70 years), researchers sought to examine the nature of the cognitive deficits observed following prolonged exposure to cortisol. Researchers measured participants’ baseline level of cortisol for four years. They then placed participants into one of three groups, based on patterns observed in their baseline cortisol levels. The people in the first group had baseline cortisol levels that increased over the four years, such that their cortisol levels were now considered high. The people in the second group also had baseline levels of cortisol that increased over the four years, but their cortisol levels were still considered to be normal. Finally, participants in the third group had baseline levels of cortisol that either stayed the same or decreased over the four years.
Participants were administered a variety of psychological tests of memory. The study authors report that people in the first group, whose baseline cortisol levels had increased and were considered high, performed worse on explicit memory tests than those people in group two (increasing but still normal baseline cortisol levels) and group three (stable or declining baseline cortisol levels). No impairments were observed in implicit memory in any of the groups.
RELEVANCE OF THIS SCIENTIFIC REVIEW
The purpose of this article was to review and summarize the evidence reported in various scientific studies on the effects of stress on aspects of cognition, including memory. The article provides an overview of some of the discoveries reported in the scientific literature on this topic and it compares the results from a number of different laboratories. By synthesizing the data from various studies, the authors are able to suggest some possible ways by which the observed experimental results could occur in the human body. These hypotheses can then be tested in future studies.