You’re sitting in an audience of a hundred or so people, watching some sort of performance. One of the performers announces that he is going to pick a random member of the audience to come up on stage and participate in a demonstration. Good thing there are a hundred other people there for him to choose from! Then he walks right down the aisle next to where you’re seated and chooses you. A hundred heads turn to stare at you, and you can sense their expectation and judgment. Your heart races, your palms get sweaty, your breath quickens. You’re experiencing an acute stress response, and your body is making you well aware of it. However, you are likely not at all aware of what this means for your brain. What is the physiological stress response, and how does it affect our most important organ? The former question has a pretty straightforward explanation, while the latter is more complicated.
The Stress Response
In response to stress, the hypothalamus initiates two parallel responses: it activates the autonomic nervous system and the hypothalamic-pituitary-adrenal axis (HPAA; see http://webspace.ship.edu/cgboer/limbicsystem.html for pretty pictures and descriptions of relevant brain regions). Activation of the autonomic nervous system results from the release of epinephrine and norepinephrine from the adrenal glands, and this yields the immediate stress response that you might experience in the above situation. HPAA activation is characterized by the release of a hormone (corticotropin-releasing factor) from the hypothalamus, which triggers the release of adrenocorticotropic hormone (ACTH) from the pituitary gland at the base of the brain (Dedovic, D’Aguiar, & Pruessner, 2009). ACTH then circulates in the blood, triggering the creation and excretion of a hormone called cortisol from the adrenal glands, which are located on top of the kidneys. Cortisol’s main function is to break down proteins, carbohydrates, and fats into glucose so that the body has energy, which in this case, is used to react to the stressful event. Areas of the brain such as the amygdala, hippocampus, and pre-frontal cortex are especially receptive to cortisol, and these areas play a role in moderating HPAA activity. For example, the hippocampus may aid in inhibiting the HPAA by helping you realize that the stressor is not actually a big deal (Dedovic et al., 2009).
Acute Stress and Your Brain
In order to determine how the stress response affects the brain, researchers have to actually stress people out in a laboratory while imaging the brain. Two techniques researchers often use to image the brain are functional magnetic resonance imaging (fMRI) or positron emission tomography (PET) scanning (see http://psychcentral.com/lib/types-of-brain-imaging-techniques/0001057 for descriptions of these techniques). Three of the most popular psychological stress-induction techniques used in imaging studies are the Trier Social Stress Test (TSST), the Stroop Color-Word Interference Task (CWT), and the Montreal Imaging Stress Task (MIST; see Dedovic et al., 2009 for a review). Let me break these down for you:
- The TSST: participants give a short speech and solve math problems while being judged.
- The CWT: color words are presented in the same or a different color font. For example, the word blue may be presented in either blue or red font. Participants’ have to name the color of the font, instead of reading the word. Go Google search “Stroop Task.” It’s brutal.
- The MIST: participants complete difficult mental arithmetic problems while an experimenter provides negative feedback.
In reviewing several studies, it is clear that the effects of acute psychological stress on the brain can differ greatly depending on the type of stress induction used. For example, consider the following two studies:
- Fechir et al. (2009) had healthy males perform the CWT. They imaged the brain via fMRI, and measured skin conductance and heart rate. Aside from brain areas that were involved in actually performing the task, fMRI revealed activation in the right insula, dorsolateral superior frontal gyrus, and the cerebellum during stress.
- Pruessner et al. (2008) had healthy participants do the MIST ( which sounds like an awkward dance involving spirit fingers). They took cortisol samples to measure the stress response, and imaged the brain using PET and fMRI. Stressed participants demonstrated increased post-stress cortisol levels, and reduced activation of the amygdala, hippocampus, hypothalamus, insula, ventral striatum, medio-orbitofrontal cortex, and posterior cingulate cortices.
You’re probably wondering how activation and deactivation of these brain regions affects cognition and behavior—I promise to address this in later posts. For now, I want to direct your attention to an important point regarding stress research: different types of psychological stress can have completely different effects on the brain. As you can see in these two studies, the CWT yielded increased activation of the insula, while the MIST lead to decreased activation of that same area. Sadly, the effects of acute stress on the brain are not (yet) uniform or easily understood, and a recent review of imaging studies that induced acute stress confirms this (Dedovic et al., 2009).
Now, inconclusiveness never leaves a reader feeling satisfied. Stay tuned, though. Future posts will provide clearer answers about the effects of stress on cognition, the nature of chronic stress, and techniques you can use to reduce feelings of stress.
Boeree, C. (n.d.). The Limbic System. Retrieved October 5, 2014, from http://webspace.ship.edu/cgboer/limbicsystem.html
Dedovic, K., D’Aguiar, C., & Pruessner, J. C. (2009). What stress does to your brain: A review of neuroimaging studies. The Canadian Journal of Psychiatry / La Revue Canadienne De Psychiatrie, 54(1), 6-15. Retrieved from http://search.proquest.com.ezproxy.library.tufts.edu/docview/621811811?accountid=14434
Demitri, M. (2007). Types of Brain Imaging Techniques. Psych Central. Retrieved on October 9, 2014, from http://psychcentral.com/lib/types-of-brain-imaging-techniques/0001057
Fechir, M., Gamer, M., Blasius, I., Bauermann, T., Breimhorst, M., Schlindwein, P., Birklein, F. (2010). Functional imaging of sympathetic activation during mental stress. NeuroImage, 50(2), 847-854. doi:http://dx.doi.org/10.1016/j.neuroimage.2009.12.004
Pruessner, J. C., Dedovic, K., Khalili-Mahani, N., Engert, V., Pruessner, M., Buss, C., Lupien, S. (2008). Deactivation of the limbic system during acute psychosocial stress: Evidence from positron emission tomography and functional magnetic resonance imaging studies. Biological Psychiatry, 63(2), 234-240. doi:http://dx.doi.org/10.1016/j.biopsych.2007.04.041