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Add info on reversed polarity in EEG Setups #251

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Mar 18, 2025
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Add info on reversed polarity in EEG Setups #251

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10 changes: 9 additions & 1 deletion website/docs/GettingStarted/Biosensing-Setups/01-EEG-Setup.md
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Expand Up @@ -69,7 +69,7 @@ a) We're going to start with the electrodes on your head. Begin by scooping Ten2

![SRB2](../../assets/GettingStartedImages/earlobe.JPG)

b) Now apply this electrode to either one of your earlobes (either A1 or A2 as seen on the 10-20 system image below). You can use some medical tape (or electric tape!) to give this electrode some extra stability, ensuring that it does not fall off. This electrode is the reference that all of the EEG electrodes on your head will be measured in comparison to. The uV reading that will appear in the GUI's EEG DATA montage is a measure of the potential difference between each electrode and this reference electrode (SRB2). SRB1 (the top SRB pin) can also be used as a reference pin, but we won't discuss that here. Check out the other docs on how to maximize the usage of the other pins!
b) Now apply this electrode to either one of your earlobes (either A1 or A2 as seen on the 10-20 system image below). You can use some medical tape (or electric tape!) to give this electrode some extra stability, ensuring that it does not fall off. This electrode is the reference that all of the EEG electrodes on your head will be measured in comparison to. The uV reading that will appear in the GUI's EEG DATA montage is a measure of the potential difference between each electrode and this reference electrode (SRB2). Check out the other docs on how to maximize the usage of the other pins!

![Fp2](../../assets/GettingStartedImages/Fp2.JPG)

Expand All @@ -91,6 +91,14 @@ e) Now follow the same procedure as step 2 above to apply the black electrode to

You're now done connecting electrodes to your noggin! I like to use a cheap cotton hairband to add extra stability to all of the electrodes connected to my head, by placing it gently on top of all of the electrodes.

:::info

Due to the fact that the electrodes collecting EEG data are connected to the Negative inputs of the ADS1299 and the common reference is connected to the Positive Input through SRB2, the saved raw data from the OpenBCI GUI will appear inverted when processed in another application.

If you would like to reverse this polarity, it is recommended to do so through software and not change the hardware setup of OpenBCI. This is because SRB2 offers much more flexibility as compared to SRB1 when used as a common reference connection. To reverse the polarity of the data through software, you will have to pass the raw data through a band pass filter between 0.5Hz and 45Hz (to remove the DC offset) and then multiply by -1.

:::

## 3. Launch the GUI and adjust your channel settings

a) If your OpenBCI GUI is not already running, relaunch it and configure the DATA SOURCE mode to LIVE (from Cyton) and Serial (from Dongle). Select your Cyton board from the list of devices, set the Channel Count to 8, and click START SYSTEM. Refer to **section IV** of this guide for more information on this process.
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