Stress can be defined as “any condition that seriously perturbs the physiological/psychological homeostasis of an organism” (Kim and Diamond, 2002). While acute stress is an important adaptation mechanism when we face a threat, chronic stress, on the other hand, is highly detrimental for intelligent behavior, necessary to thrive in today’s world. The effects of stress on the brain have been elucidated in a great number of complementary human and animal studies. Research shows that chronic stress reduces the capacity for higher cognitive functions while turning on a more primitive habitual emotional response. Attention, working, a long-term memory, emotional regulation, and a flexible behavior are all diminished, and an impulsive, knee-jerk fear response is strengthened.
The prefrontal cortex (PFC) is the center of our higher cognitive abilities, creating the foundation for flexible, goal-directed behavior and an abstract thought. It has a „top-down“ effect, flexibly orchestrating and integrating emotional processing, behavioral output and cognition. Uncontrollable chronic stress severely weakens the PFC and reduces its neuronal firing, while it strengthens primitive brain functions.1 In animal models, pyramidal cells in cognitive circuits lose dendrites (short branched extension of a neuron), while those in basic emotional circuits are unchanged or even stronger.2 Delay cells that are responsible for working memory are particularly vulnerable to stress exposure, witnessing a reduction in spine density (protrusions on dendrites allowing more connectivity).1 In turn, habitual functions that rely on basal ganglia circuits, and emotional conditioning operated in the amygdala, are preserved or even enhanced.3,4 In rats, performance on task that relies on sensory, motivation and motor demands are unchanged by stress, but they make more errors and have weaker attentional flexibility in tasks that require the PFC.5 In humans, structural imaging has shown that chronic stress induces a loss of PFC gray matter.6 It also reduces PFC activity in humans, and a failure to suppress amygdala activation in an attempt to suppress negative emotions.7 In healthy human subjects, the effects of psychosocial stress, that impaired functional connectivity necessary for attentional shifts, were fully reversed after a month of reducing stress.8 This effect highlighted the plastic ability of the PFC, meaning that our cognitive abilities have the capacity to recover.
In summary, stress rapidly switches the brain from a reflective state to a reflexive one via a complex biochemical cascade. Although these changes are highly detrimental they can be reversed in healthy people once the stressor is out of the way. Understanding the neurobiology of stress has important applications for designing the workplace to foster greater productivity and for strengthening our individual performance and well-being.
1. Arnsten, A. F. T. Stress weakens prefrontal networks: molecular insults to higher cognition. Nat. Neurosci. 18, 1376–1385 (2015).
2. Liston, C. et al. Stress-Induced Alterations in Prefrontal Cortical Dendritic Morphology Predict Selective Impairments in Perceptual Attentional Set-Shifting. J. Neurosci. 26, 7870–7874 (2006).
3. Elliott, A. E. & Packard, M. G. Intra-amygdala anxiogenic drug infusion prior to retrieval biases rats towards the use of habit memory. Neurobiol. Learn. Mem. 90, 616–623 (2008).
4. Rodrigues, S. M., LeDoux, J. E. & Sapolsky, R. M. The Influence of Stress Hormones on Fear Circuitry. Annu. Rev. Neurosci. 32, 289–313 (2009).
5. Murphy, B. L., Arnsten, A. F., Goldman-Rakic, P. S. & Roth, R. H. Increased dopamine turnover in the prefrontal cortex impairs spatial working memory performance in rats and monkeys. Proc. Natl. Acad. Sci. 93, 1325–1329 (1996).
6. Ansell, E. B., Rando, K., Tuit, K., Guarnaccia, J. & Sinha, R. Cumulative Adversity and Smaller Gray Matter Volume in Medial Prefrontal, Anterior Cingulate, and Insula Regions. Biol. Psychiatry 72, 57–64 (2012).
7. Kim, P. et al. Effects of childhood poverty and chronic stress on emotion regulatory brain function in adulthood. Proc. Natl. Acad. Sci. 110, 18442–18447 (2013).
8. Liston, C., McEwen, B. S. & Casey, B. J. Psychosocial stress reversibly disrupts prefrontal processing and attentional control. Proc. Natl. Acad. Sci. 106, 912–917 (2009).