EEG signal upside down?

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  • MNE version: 0.24.0
  • operating system: Windows 10

Hi,

I’ve noted from your EEG examples on ICA that blinks show up always as positive deflections, while in some data I’m testing blinks are showing as negative deflections (as in the screenshot below).

QUESTIONS

  1. Would you suggest me to invert the polarity (e.g. multiplying each sample by -1) to display the signal as you do, or does not matter at all ?
  2. In general, looking at the filtered raw signal in the screenshot (4 seconds rec), at first glance does it look a nice or bad EEG recording? Just curious about your expert opinion on this,

many thanks in advance

sample
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polarity of blink deflections does not matter, no need to invert.

At a glance it looks like a normal EEG recording, not bad (though I’m guessing a bit because there is no time scale; I’m inferring the time scale based on the length of the blinks).

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To explain a bit more, it depends on the hardware used to record the EEG. Each channel on an EEG measures the difference in potential between 2 points, the electrode and the reference. Some amplifiers return electrode - reference, while others return reference - electrode. As you can see, the polarity does not matter since it is the same for all channels of a given amplifier.

However, when working on ERP studies, some people like to have the known responses always in the same polarity. For instance, in auditory oddball, the convention is to have the N1 positive and the P3 negative (IIRC, which I am now doubting since N=Negative and P=Positive?).

And your signal looks good :slight_smile:

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Reference - electrode is not an usual G1-G2 derivation. Active electrode goes first place always, at list in routine clinical EEG, left electrodes before right ones, etc (1).
To understand the question “Why wave goes up or down?” you must remember that cornea is electropositive and retina electronegative. If the cornea is near Fp1 or Fp2, those electrode become positive and the base line goes down and vice versa. To sum, “up or down” depends if the subject is opening or closing his eyes.

(1) Seeck, Margitta, et al. “The standardized EEG electrode array of the IFCN.” Clinical neurophysiology 128.10 (2017): 2070-2077.