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Digitization

Take a lot of care about this step because this one is crucial for head position measurement and for further data processing. So, check after digitization that everything is OK (no aberrant point)

Determination of the position of the head position indicator coils and head shape/EEG electrodes digitization is performed with the help of Polhemus 3-D digitizer. Digitization is performed outside of the magnetically shielded room, in the consultation room next to the MEG suite.

 * Bring the wooden chair away from the desk and from any metal.
 * Seat the subject on the wooden chair.
 * Put the Isotrak googles on the subject and secure its position by tightening the ropes - it is crucial that the googles remain in place.
 * check that the Isotrak unit is powered on (there is a switch in the rear panel)
 * log in the laptop (same credentials as on the MEG acquisition workstation)
 * start the Prepare Acquisition software on the laptop by clicking the corresponding icon on the desktop
 * select your project, select/enter the subject info, load your settings, and hit "Change" right next to HPI.
 * on the digitization window, click on "Coordinate frame alignment" to start the single-operator digitization
 * point the stylus and touch the left pre-auricular point (LPA) and click on the stylus once. Stylus has to be maintained tangentially to scalp surface as shown below

 * repeat for the Nasion
 * repeat for the right pre-auricular point (RPA)
 Check that the difference in X axis between LPA an RPA is less than 5 mm. if it is more, redo your digitization.
 * repeat for all four HPI coils in any order

Then, if you only record MEG:

 * digitize a few more anatomical landmarks (>20) starting with three points on the side of the nose and, for instance, following three lines along the scalp (2 lateral and 1 median).

If scalp EEG channels are selected for acquisition, the program moves on to digitize the EEG electrode locations:

 * digitize all EEG electrodes in the ascendant channel number order (see EEG digitization scheme), starting from the reference electrode

Once all points have been digitized, click on the stylus away from the subject's head to abort the digitization process. Don't forget to click OK and to save your preparation (file/save normal preparation) !!!