Problems with Channel location EasyCap 128

1

Dear everyone, I have a problem that I'm not solving with a channel location. It is a long story,unfortunately. I recorded EEG signal with micromed system, Easycap 128 channels. Unfortunately, the default channel location in brainstorm cannot be used in my case. I fact when I plot the 2D disc, I have a bad representation on the scalp of the activity. In fact when I imported for the first time the raw signal (.TRC format), I noticed that brainstorm was able to read a chanloc. Even if the values of the channels positions were empty, braistorm was able to read the channel label. I noticed that the channel order in the raw file was different from the one of the default EasyCap 128 chanloc, and moreover some channels were not the same.
I followed two strategies:

  1. Using the order and the labels of the channels from the raw file, I created a personal chanloc using the xyz position available on internet. Now I have a good 2D Disc representation. Unfortunately some channels in this case are not in the right position, and when I plot the 2D sensor cap the cap has not the right shape, even if the plot on the 2D Disc is correct.
  2. I tried to modify the default Easycap 128 chanloc of brainstorm, trying to follow the order of the channels in the rawfile. Unfortunately, some channels do not match, therefore I cannot complete the chanloc.
    Could you help me to solve this problem?
    If you need I could upload 3/4 sec of my signal, so that you can try by yourself to build a chanloc. I can also upload the chanloc I created, so that you can better understand what is my issue.
    These data come from an epilepsy population, and I have to reconstruct the source activity of the spikes. Therefore, I cannot approximate the position, as I did, but I need to be as precise as possible, and for this reason I need an additional support from you.
    Thanks for your help
2

Hi Gian Marco,

Just to make sure: what menu are you using to add your positions to your recordings?

In the TRC files, the order of the channels can vary depending on the acquisition setup. You cannot guess that the order will be the same in two different files.
Therefore you need to use the menu "Add EEG positions" and not "Import channel file", so that it matches the data channel with the template positions using the channel labels. You have red warnings trying to discourage you to use the latter, unless you are using an older version of Brainstorm (which you should update immediately).
https://neuroimage.usc.edu/brainstorm/Tutorials/Epilepsy#Prepare_the_channel_file

If there is some issues with the current EasyCap128 template, and it doesn't match positions you observe in your local setup: can you please post a figure illustrating the cap montage?
Please include any documentation possible that could help me create a new entry for your new (or old, or modified?) cap.

Cheers,
Francois

3

Hi Gian Marco,

Thanks for the example files.
It appears that the channel names in you recorded data file (converted to .edf) don't match the naming conventions of the EasyCap M15, as described on the easy cap website (https://www.easycap.de/wordpress/wp-content/uploads/2018/02/Easycap-10-based-electrode-layouts.pdf) and in the corresponding Brainstorm template (ICBM152 > BrainProducts > EasyCap 128).

For example, your EDF file, there is a channel named "PO6", while its name is supposed to be PPO6h. Because of this, its 3D position is not found in the Brainstorm template when using the menu "Add EEG positions". Do you know why your acquisition is configured this way?

image
image1215×552 115 KB

The simplest is probably for you to rename the channels to their correct convention immediately after creating your link to the file in the Brainstorm database (menu "Review raw file"). Right-click on the channel file > Edit channel file. Rename the following channels:

  • PO6 => PPO6h
  • PO5 => PPO5h
  • PPO2 => PPO2h
  • PPO1 => PPO1h
  • AFF2 => AFF2h
  • AFF1 => AFF1h
  • TPP10 => TPP10h
  • POO10 => POO10h
  • POO4h => Not in EasyCap doc
  • POO3h => Not in EasyCap doc
  • PPO10 => PPO10h
  • FTT10 => In the doc, but not in Brainstorm ??
  • FTT9h => In the doc, but not in Brainstorm ??
  • AFP4h => Not in EasyCap doc
  • AFP3h => Not in EasyCap doc
  • M1 and M2: mastoids, not EEG ?

The missing electrodes can be added manually by editing the channel file in 3D (right-click on channel file > MRI registration > Edit), like you did already I guess.

I attached also the .xyz chanloc that I created (file named GianMarco_Chanloc). If you take a look of it, you will notice that it is quite approximate, because the shape of the 2D sensor cap is not correct, even if the 2 Disc is pretty nice.

What you've done looks good, except for two things: you should probably not include the mastoids in the EEG anyway, and I'm not sure how you ended up having this rotation around the vertical axis (the "z" electrodes are not aligned vertically anymore).

Your positions look like this:

image
image

While it should look more like this I guess:

image

If your cap corresponds to a new standard EasyCap model that is not documented yet on the EasyCap website or the Brainstorm templates, please let me know what are the exact references of the cap, and send me update positions (the channel.mat file) so I can add it to the list of Brainstorm default EEG caps.

Cheers
Francois

4

Hi Francois, I'm sorry for my delay. First of all, as usual thanks for your precious help. I sent an e-mail to ask if the channel loc I sent you was specific for a commercial cap, but I'm still waiting for a reply. I'll let you know as soon as I have a reply from them, so that if other brainstorm users have a problem with the chan loc of the Easycap 128 channels, they may have a solution. In any case if any other user shows the same problem I had, you can give them the last channel location I sent you, which should be the correct one, or they can write me and I'll be happy to share it.
best
Gian Marco