BioSemi 64 Coordinates

Discussions
1

Hello,

I’m having some trouble figuring out the built-in BioSemi 64 10/10 coordinates in Brainstorm. We use a BioSemi 64 system with a few additional electrodes. To build an appropriate forward model, I need to know how Brainstorm tracks electrode locations so I can make a custom montage. My understanding is that Brainstorm uses CTF coordinates, and that this system is in cm with and axes through the fiducials forming the basis; however, when I look at the BioSemi coordinates in the file that comes with Brainstorm most of the numbers are on the order of 10^-1 or 10^-2. For a human brain, these cannot be coordinates in cm. I must be missing something and I’d appreciate any advice. Thanks!

-Alexander

2

Hi Alexander,

If you talk about the files in Brainstorm format (channel_*.mat) everything is in international units (meters), and in CTF/SCS coordinates:
http://neuroimage.usc.edu/brainstorm/CoordinateSystems

It would probably be easier for you to use the graphic tools to create your channel file, rather than editing it manually.

  1. Load a piece of EEG recordings in the database, it will create a channel file with the electrodes listed in the correct order
  2. Add the default locations of an existing electrode cap in Brainstorm: right-click on the channel file > Add EEG positions > Colin27 > … (or Import from file, if you have a set of digitized positions for your cap)
  3. Right-click on the channel file > Edit the channel file: locate the electrodes for which the position was not set. Change their type back to “EEG”, then set their location to something close their neighbors
  4. Right-click on the channel file > MRI registration > Edit: Move the electrodes on the subject’s head to design your cap (read the tooltips of the buttons in the toolbar). This interface is very flexible, you can place the electrodes where you want.

Cheers,
Francois

3

Thanks, Francois. That is very helpful.

Alexander