The increase or the decrease in power is always in comparison to a baseline situation. For instance, when we say that alpha power decreases when someone is awake, this would mean that power is lower than compared to when that person is asleep–asleep would be the baseline. Another example: if alpha power decreases when we see someone in pain, this would mean that alpha power is lower compared with when we see someone who is not in pain. In this example, the brain's response to the person who is not in pain would be the baseline. Previous studies showed that the decrease and the increase in alpha power both have different functions. However, scientists have been working on figuring out the exact meaning of these changes in alpha power. So, looking at the alpha rhythm is informative for better understanding how the brain operates in different contexts. When seeing someone in pain, the brain can respond by alpha power increase (red circle) or by alpha power decrease (blue circle). But when the other is not in pain, the brain does not respond with either of the two activation types.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00082