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I am into analyzing EEG signals with the help of this guide. So, I choose to inspect power in a frequency region before and after an event of interest (at t=o) and obtain the following plot:

Power in MEG 1142 electrode

How to make sense of the blue shade corresponding to negative values? Power can only be positive!

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  • $\begingroup$ Is there not a baseline correction? Meaning you see the difference relative to the baseline time window? $\endgroup$
    – Irreducible
    Commented Dec 1, 2022 at 8:21
  • $\begingroup$ @Irreducible Ah, this is indeed the case; sorry for a stupid query. Can you write this as an answer, so that I can accept it? $\endgroup$
    – AvadaMouse
    Commented Dec 1, 2022 at 8:43

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Neurophysiological data is often analyzed relative to a time window (baseline). This is also described in the guide, which was used to generate the figure.

So what are you seeing is the change relative to the defined baseline, which indeed can be positive or negative.

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