differences of the plus and minus pins on the Cyton board ?
lauraDurieux
France
Hello everyone,
We encountered a small issue on cyton board with 8 channels that we are not able to understand. You might have the answer which will greatly help us. If we record electrode n°1 minus the reference alone (without plugging anything more on the board) we obtained a defined signal. Now if we add a electrode n°2 minus reference (the same reference) we obtained exactly the same signal (by human eyes) between both channels (channel 1 and channel 2). However, if we do the same but reversing the + and - pins (so reference electrode minus electrode n°1 and reference minus electrode n°2) we do obtain not the same signal on channel 1 and channel 2. This new signal look more like the one when we record electrode n°1 minus reference alone on the board (and same for electrode n°2). So plugging the reference on the + pin seems more “realistic” signal to me. But we cannot understand why since: 1) everyone record with the reference on the minus; 2) the amplification of the board should logically amplify the difference between both pin + and pin -, without changing anything more (in other words, we should obtain the same signal but reversed - positive become negative and negative become positive- isn’t it ?).
If you have any idea why we are recording different signals even though it should be the same between the two configurations, let me know. It would really help me to better understand what is happening here.
I hope the question is clear enough. If you want any details to not hesitate to ask me :-)
Comments
Hi Laura,
On Cyton, in general with EEG, you ALWAYS should use the pins CLOSEST to the board surface, (sometimes called the 'bottom' pins). These are the minus polarity pins. This includes all the channels and the SRB2 reference pin and the Bias pin.
The only time the plus pins are used, are with EMG or ECG setups. Have you seen the setup tutorials?
https://docs.openbci.com/GettingStarted/Biosensing-Setups/EEGSetup/
https://docs.openbci.com/GettingStarted/Biosensing-Setups/ECGSetup/
https://docs.openbci.com/GettingStarted/Biosensing-Setups/EMGSetup/
https://docs.openbci.com/GettingStarted/Biosensing-Setups/ExGSetup/ [combined setups with EEG and ExG]
William
Yes we worked that out ^^ - Thank you a lot for your answer !
@wjcroft
If possible, could you please explain why, "with EEG, you ALWAYS should use the pins CLOSEST to the board surface" and, "The only time the plus pins are used, are with EMG or ECG setups" ?
The firmware in the Cyton has a DEFAULT configuration. And it expects you to use that wiring convention for EEG. In the default, the SRB2 pin is 'bussed' together with all the positive pins, forming a 'common reference' (SRB2) as EEG expects.
So, if I'm understanding correctly, you're advice is just general advice for people that don't want to, or don't know how to, change the default "Hardware Settings" in the GUI, right?
For 99% of users interested in EEG, the default pin mapping (with reference on SRB2) is what you want.
If you are using channels for a mixed application, with some combination of EEG, ECG, EMG, then follow the ExG tutorial:
https://docs.openbci.com/GettingStarted/Biosensing-Setups/ExGSetup/
Yup, makes sense. I'm part of the 1% doing something different, so I just wanted to double check.