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Dear Pegah, If you used standard electrode locations available in SPM (like the 10-20 system) perhaps it'll be better to use the SPM defaults rather than your locations on a sphere as transforming them will introduce more error. If this is some kind of very special cap you can try getting it to XYZ to more or less reasonable size and then use some electrodes that you can locate on the template head as fiducials and do coregistration. SPM will fit the electrodes to the head and distort the sphere if necessary. Spherical head can be used but we don't support it. There is a possibility to use a spherical model but the electrodes need to be in MNI coordinates to use that. Best, Vladimir On Thu, Feb 9, 2012 at 4:01 PM, Tayaranian Hosseini P. <[log in to unmask]> wrote: > Dear SPMers, > > > > I have some 68-channel EEG data (66 channels of EEG and 2 channels of EOG), > in my recordings, there are 2 more channels for reference (nose tip) and > ground electrodes. I have the polar locations of the electrodes (phi and > theta) and I can find the xyz locations given a specific head radius > (assuming head to be sphere). > > > > 1. According to a standard average head size, what head radius do you > suggest I use for all my subjects? > > 2. Can a spherical head with a fixed radius be a good model for source > reconstruction and DCM? > > 3. What should I do about fiducials when I do not have exact > measurements? Can I for example use the positions of two of the electrodes > which are closest to pre-auricular points as lpa and rpa and the one closest > to nose bridge as nsa? > > > > Many thanks, > > Pegah