Short answer
When you have a blackout, your brain cannot access the information you have learned before. This happens because the messenger-molecules that your brain and body release during a stressful event incite your brain to focus on the current situation and store that in mind. This, in turn, impairs access to earlier knowledge. This was likely very useful in our evolutionary history, but can be frustrating in modern society, for instance during exams.
Longer answer
An exam can be a stressful event which triggers a complex stress response, resulting in increased release of messenger-molecules such as hormones (e.g., cortisol), neurotransmitters (e.g. noradrenaline), and peptides. Many researchers believe that this stress response has evolved to allow us to optimally cope with stress. For instance, it fuels our muscles with energy to fight or flight and it reduces unnecessary energy expenditure (e.g. for digestion). But it also affects memory processes, and this can be annoying during an exam.
Stress effects on memory: Stress induces a brain state focused on storing the stressful event in your memory, the exam in this case. Cortisol and noradrenalin interact to form a detailed and long-lasting memory trace of this event. However, at the same time, they impair access to prior knowledge. Researchers think that this system evolved so that we can memorize potentially dangerous situations; how they occurred, what happened before, and how we got out of them, so that we are better prepared for the next time. However, in these stressful situations in evolutionary history, it was likely less necessary (for survival) to retrieve facts learned before – which would be essential for passing exams in the modern world. The memory traces are thus not gone, they are just temporarily not accessible. This leads to the paradox that we can have a blackout during an exam where we cannot remember anything, but we will later have very vivid memories of this unpleasant experience.
How to prevent a blackout: The best strategy to prevent a blackout seems to be extensive practice of memory retrieval. While students often learn by studying the material over and over again, research has shown that thoroughly practicing retrieval of the material seems to make memories immune to the influence of stress. An alternative may also be to try to prevent the bodily stress response from overshooting, for instance with relaxation training.
Further reading:
– Wikipedia: Effects of stress on memory https://en.wikipedia.org/wiki/Effects_of_stress_on_memory
– A Guide for Young People on Coping with Exam Stress https://www.youngminds.org.uk/young-person/coping-with-life/exam-stress/
References / literature sources
Schwabe, L., Hermans, E. J., Joëls, M., & Roozendaal, B. (2022). Mechanisms of memory under stress. Neuron, 110(9), 1450–1467. https://doi.org/10.1016/j.neuron.2022.02.020
Shields, G. S., Sazma, M. A., McCullough, A. M., & Yonelinas, A. P. (2017). The effects of acute stress on episodic memory: A meta-analysis and integrative review. Psychol Bull, 143(6), 636-675. https://doi.org/10.1037/bul0000100
Smith, A. M., Floerke, V. A., & Thomas, A. K. (2016). Retrieval practice protects memory against acute stress. Science, 354(6315), 1046-1048. https://doi.org/10.1126/science.aah5067
Vogel, S., & Schwabe, L. (2016). Learning and memory under stress: implications for the classroom. npj Science of Learning, 1(1), 16011. https://doi.org/10.1038/npjscilearn.2016.11