EOG cross talk on openBCI channels

Comments

• wjcroft Mount Shasta, CA

  January 2017

  Swap, hi.

  
  

  Where is your Ground / Bias electrode positioned? Try moving it to other locations. Similarly where is your Reference?

  
  

  Another thing you can do is look at the steps in the tutorial for how to simultaneously measure EEG, ECG and EMG. Since EMG and ECG are such a high level (millivolt vs microvolt) signals -- they will tend to swamp out the weak EEG signals. EOG is essentially the same as EMG. That is why the CHAN SET tab in the GUI is used to disconnect those channels from the EEG.

  
  

  http://docs.openbci.com/Tutorials/01-Cyton_Getting Started_Guide#cyton-getting-started-guide-v-connect-yourself-to-openbci-4-launch-the-gui-and-adjust-your-channel-settings
  

  
  

  William

  
  
• wjcroft Mount Shasta, CA

  January 2017

  Note that the scrolling on that link does not always go to the correct location. Use the left hand column to select the V.4. section link.

  
  
• swap Bekeley

  January 2017 edited January 2017

  Hi wjcroft,
  
  We are doing exactly what the tutorial showing us.
  
  Let's say we are only concerned with two channels. We have channel 1 N electrode on the outer side of the left eye and channel 1 P electrode connected to the outer side of the right eye.This channel does not contribute to the bias signal generation and SRB2 is disabled.
  
  Now channel 2 electrodes (N and P) are in front and behind the ear. Again, no contribution to bias signal generation and SRB2 pin off. Just like in the tutorial for the EMG/EKG signal.
  
  We now go one step further and have no electrodes connected to the Bias pin. Thus, we definately do no have inverted common mode rejection interfering in our measurements.
  
  What we observe is that the EOG signal picked up in channel 1 "leaks" into channel 2. In channel 2 we see a low pass filtered version of channel 1 at a lower magnitude. If we disconnect channel 1 (physically remove electrodes and unplug pins), channel 2's signal changes considerably. Thus, we believe that the different channel signals are somehow coupled together.
  
  ... thanks
• wjcroft Mount Shasta, CA

  January 2017

  You must have Bias connected somewhere to give the ADS1299 differential amps a zero reference point.
  
  If the tutorial works for simultaneous EEG, EMG and ECG, then it should (in theory) be possible for you to replicate that in your situation.
  
  It could be that your signal leads themselves are coupling by virtue of the close proximity. Conductors that travel along the same pathway induce currents in each other.
  
  OpenBCI is primarily intended for EEG, and these aux channels require care since they are of such high voltage compared to EEG.
  
  Just curious, what does in front and behind the ear give you? Is that used in EOG as well?