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2 - Creating a file with the electrode coordinates

Creating an electrode coordinates file has to be done only once for a given electrode configuration, for example when setting up a new recording machine.

Once the file is created, you can use it to display the maps of potentials, interpolating bad channels, etc...


Actually getting the electrode coordinates

First of all, you have to get these coordinates from somewhere:

  • You're lucky, the manufacturer of your recording machine are nice guys, and on their web site (or on request) you can download a file for a typical average head template. That's usually the best, because it has been controlled for the accuracy of the electrode positions.
  • You're less lucky, you don't have this file from the manufacturer. But your setup is based on the 10-10 system. So get this file, this is a spherical model template for 105 standard electrode positions. Remove the electrodes you don't have in your system, check the sequence of the remaining ones, and step to the next paragraph.
  • This is not your day, you don't have the file from the manufacturer, or you don't trust it. Or you need a real per subject measure of your net. You therefore need a device to record the actual data from somebody wearing that net (beyond this guide).
    In case you want a general template, not a subject's one, just repeat these measures on a set of subjects, then average the positions (in polar coordinates).


Checking these coordinates 

There are quite a few points to check before proceeding to creating the file for Cartool: 

  • Make sure the sequence of the electrodes is exactly the same as the sequence of the electrodes of your recordings / EEG files. This is of utmost importance! Cartool does not know about the sequence of your electrode set, you have to triple check it yourself.
  • Check you don't have duplicate locations, that is, electrodes with the same coordinates! This of course has no physical meaning, but it can happen f.ex. if you edited an existing file.
  • Finally a good news, Cartool does not care in what orientation and size the electrode coordinates are. They can be in any orientation you like, as long as it reflects accurately the geometry of your net.


Generating a file suitable for Cartool

At this stage, you should have a file with a correct sequence of electrode coordinates.

All you need now is to reformat this file to match either the els format (best) or the xyz format (obsolete). There are examples on these two pages.

The els format is a bit more versatile, in that it allows you to create complex set of electrodes. Like intra-cranial stripes, with a grid and some spare electrodes here and there within the same file.

Cartool Software FBMLab,
Oct 29, 2010, 7:26 AM
Cartool Software FBMLab,
Oct 29, 2010, 7:26 AM