Stress effects on cognitive function in healthy adults

Resumo

In our lives, we are constantly exposed to different sources of stress, specifically psychological or social. Our reaction to stress is an adaptive response due to its role in facilitating survival. However, stress can also have immediate and delayed damaging consequences for health, and it is considered one of the most significant health problems of the 21st century, according to the World Health Organization (2001). The impact of stress extends to most of the physiological systems (i.e. cardiovascular, digestive, immune, neuroendocrine or nervous), resulting in numerous diseases. Cognitive problems stand out among the stress effects related to the nervous system. Given the large impact that these problems can have on society in general, and on individuals in particular, the need to understand more about this link is clear. This is one of the reasons for the growing interest in investigating the main mechanisms underlying the stress impact on different cognitive processes, such as memory, attention or executive functions. Several factors related to the characteristics of the stressor, the individual and the cognitive process assessed seem to play an important role in determining the direction of these stress effects. Thus, this thesis focuses on the way stress affects cognition, specifically memory performance, in healthy adults, analyzing the role of some of these factors. The first section of the first chapter discusses the evolution of the stress concept and explains what the stress response is. In the second part, the link between stress and memory is explained, detailing which brain structures are related to the control of the stress response and the cognitive processes. Then, a brief summary of the studies about the effect of acute stress on memory performance is presented. Moreover, the Hypothalamus-Pituitary-Adrenal axis (HPA-axis) in basal conditions (non-stress) is addressed, again summarizing the studies that have investigated the relationship between HPA-axis functioning and cognitive performance. Finally, the chapter ends with the main goals and hypothesis of this thesis and a general description of the material and methods used in the empirical chapters. In the second chapter, the first study is presented. In this study, we examined the effects of stress-prior learning on two types of memory (i.e. non-declarative and declarative memory) in young adults. Here, the material to be remembered is neutral, and the role of sex is considered. Next, in the third chapter of the thesis, following a similar design, the second study carries out a direct comparison of older and young adults. The fourth chapter describes the third study, which investigates the stress effects on memory retrieval. Now, the stressor is applied before the retrieval tasks, and the material to be remembered is neutral and emotional. Again, older and young adults of both sexes are compared. In the last study, the fifth chapter analyzes the relationships between the cortisol awakening response (CAR) and the diurnal cortisol slope (DCS), two different components of the diurnal cortisol cycle, and different memory tasks. The sixth chapter contains a general discussion and the main findings of the aforementioned studies; the strengths and limitations of this thesis and the direction of the next steps in the research on this topic are discussed here. Finally, the seventh chapter presents the main conclusions of the studies included in this thesis.