I have several recordings from a Grass Technologies Comet EEG system similar to http://www.mfimedical.com/comet-portable-eeg.html. I would like to import them into EEGLAB and for that I am looking for accurate coordinate values in Cartesian and spherical systems. Any help is greatly appreciated.
Hi Mastaneh!
If you are using standard electrode locations you can use some of the ready made electrode location files http://sccn.ucsd.edu/eeglab/channellocation.html
I you want to create your own coordinate values, you can do it the way I did.
Use one of 10-20 head models you can find online, in literature or manuals.
1. Import that model into the drawing/imaging software you are familiar with.
2. On a second layer create a square. Centre both to the position of Cz.
The height of the square is the maximum length on the head model.
3. Then create a grid into the square. For my purpose, I used 96x96. But if you desire more accuracy, you can increase the number of grids.
4. At this point you can read the x,y values of the centre of each electrode position.
Thank you both!
Dear Atte, I indeed have the .ced and .loc fil from the eeglab website, but after registering the channel locations, I found out that some electrodes were placed slightly distant from where they were inseted during the actual experiment. Although I was told that electrode positions do not need to be very accurate for low number of channels, I still want to make sure I get a roughly fair mapping.
Dear Daniel, it was a very interesting solution; thanks for sharing it! I will use it to find the accurate values from my setup.
I new system using radians and concentric rings out from Cz would be better than the silly 10-20 system.
Just moaning.
Isn't this all a post hoc chase for unachievable precision- after the recordings have been made and the electrodes detached? If the collection notes or staff say the placements were non-standard, see if it is possible to determine if there is a consistent offset recoverable from whatever logic was used to locate electrode sites. If not, you probably just need to accept the (reasonable in any case, given the approximations of landmark referenced proportions) that the topographic analysis is regional at best. Beware the hazards of false precision and specificity. First, find a strong effect and use that to justify some followup recordings to verify the finding and (possibly) improve localization.
Thomas is right, EEG spatial localization is anyway poor. Plus the necessity of precisely localize electrodes depends a lot on what you intend to achieve.
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