[Eeglablist] Seeking help with EEG Data Pre-processing in EEGLAB

[Anvita Gopal]

Hello,
I am new to EEG signal processing with EEGLAB and need some assistance with a few issues I’ve encountered. Despite my efforts to find solutions, I have been unsuccessful and would appreciate your help.

Here’s a brief overview of my setup and the steps I have taken, thus far:
I used a Biosemi 32-channel Active Two system with a 10/20 layout to collect the data. The setup includes CMS and DRL in the cap and five EXGs: two mastoids and three EOGs.
Initially, I did not change the default settings, resulting in a preliminary dataset with all 271 channels unedited. I corrected this after a few participants.

My questions are as follows:
Importing Data: When importing the data, should I specify the channel list as 37 channels [1:37]? The pop-up suggests taking the default option, which doesn’t seem correct. Will specifying [1:37] cause any data loss?

Referencing the Data: The "Load data using BIOSIG" pop-up states: "Reference chan(s) indices-required for BIOSEMI", However I don't know what are the indices? I want to use the mastoids but specifying these causes an issue when loading the channel location file.
If I do not insert anything, it says "WE HIGHLY RECOMMEND THAT YOU CHOOSE A REFERENCE CHANNEL IF THESE ARE BIOSEMI DATA (E.G., A MASTOID OR OTHER CHANNEL). OTHERWISE THE DATA WILL LOSE 40 DB OF SNR!" Therefore, to help this, I choose the Mastoids(2), but if I write [33 34] it says B1 B2 and causes when loading it reduces the channels to 35, thus when trying to load the channel location file (upon checking the 2D plot, the channel locations are correct); but, regardless of whether I choose to overwrite the original channels or not, or whether I save the file as a (.ced), or any other format, such as a BESA file, I receive an error message after clicking 'OK' to exit the 'Edit channel info' pop-up window. The error message states the following:
"The number of data channels (37) not including fiducials does not correspond to the initial number of channels (271), so for consistency purposes new channel information will be ignored if this function was called from EEGLAB. If you added a reference channel manually, check the "Data channel" checkbox is off."
If I choose the mastoids still the similar problem arises and the message reads ["data channels (37)" and "channels (35)"], still creating a mismatch.
I have consulted the help menu and noticed others have faced similar issues. One suggestion was to "delete extra channel(s) WITHIN THE CHANNEL EDIT WINDOW before closing it." Could you advise on how to locate and use this window?

Overflow Detection: While importing the data, should the "Enable overflow detection" be turned on? The MATLAB window states "sopen mode is "OVERFLOWDETECTION"" when loading the data.

Lastly, to rectify the use of all the channels, I later removed the extra electrodes in the Actiview for subsequent data collection. Sorry for the long email, however, I am uncertain if I am missing any steps or making errors. As a beginner with EEGLAB/MATLAB, any guidance you can provide would be greatly appreciated.
Thank you in advance for your help.
Best regards,
Anvita