Channel location problem with reference electrodes

Hello,

We are using the latest version of Matlab and EEGLAB (2022). For some reason, we can't get EEGLAB to recognize that EXG1 and EXG2 are reference electrodes.

We have a 64 channels system. During recordings we had 2 mastoids (EXG1 EXG2) and 1 vertical electrode on the eye (EXG3).

This is what we are doing in the EEGLAB:

File -> Import data -> Use eeglab functions and plugin -> From Biosemi BDF file(biosig toolbox).

Channels list (default all) -> 1:67

Reference indices -> 65 66

At this point, GUI shows us correctly that the Reference electrodes are EXG1 and EXG2, but channels are unspecified.

So we then go to

Edit -> Channel location -> (here we use default channel location file for electrode position – as recommended in the EEG Tutorial, we use the MNI file)

Once we click ok accepting the default channel location, the GUI gives us the info that Channel locations are known (Yes); however, we lose the reference.

It says reference -> Unknown

If we plot the channels (plot 2-D), we can only see the EXG3 (eye electrode), but none of the reference ones.

We also tried not defaulting to the MNI file and loading the 64biosemi_loc file with electrode coordinates, but then MATLAB tells us the file is incorrect and has more columns than allowed (even if the format of the loc file is as specified and the file itself obtained through this site- see attached). We tried to modify the 64biosemi_loc file by adding the coordinates for the EXG1 and EXG2 as obtained from the Cap_coords_all, excel sheet (see attached the modfiied 64bioseminew_loc file with added coordinates for the references channels), but we keep getting the same error message saying the format is incorrect as we have too many columns in the file.

Could you please advise on how we can proceed?

Thank you!

Anna

[Kaitlin Lewin]

Hi Anna! I'm experiencing this same problem, were you able to find the solution?

[Umar]

Hi Anna,

Please see my response to your mentioned problems.

1. Verify that the custom channel location file (64biosemi_loc) is formatted correctly according to EEGLAB's requirements. Ensure that each row corresponds to a specific channel with appropriate coordinate information.

2. Double-check the number of columns in the custom location file to match EEGLAB's expected format. You may need to adjust the file structure to align with the software's specifications.

3. If modifying the existing file proves challenging, consider creating a new channel location file specifically tailored for your setup, including coordinates for all relevant electrodes.

4. After updating the channel location file, reload it in EEGLAB and ensure that the software recognizes both channel locations and reference electrodes correctly.

If you follow these steps and making sure that your custom channel location file adheres to EEGLAB's formatting requirements, you should be able to successfully resolve the issue of EEGLAB not recognizing EXG1 and EXG2 as reference electrodes.

If you encounter any further difficulties or require additional assistance, feel free to provide more details for further guidance.