[Eeglablist] Dipoles and Volume Conduction

[Ahmed Almurshedi]

DearExperts,
Ihave noticed in some ERP experiments that the response is flipped in theposterior part than anterior part of scalp. For example in checkerboard patternVEP, P100 response on mid occipital electrode and N100 response on the frontalelectrode.Accordingto the discussion with colleagues they said that ERP response are usually fromthe generation of a dipole, the field of which is picked up at differentelectrode locations.  Thus, some willpick up the positive end, and others will pick up the negative end. This isoften called phase reversal, as the phase of waveform is 180 degrees reversed.
Myquestion is, why the dipoles distribution is symmetric over the scalp?According to my understanding there is an infinite number of dipoles maycontribute in a specific source.  Isthere any justification (Please correct me if I’m wrong)?
Alsowhy the volume conduction effect (transmission of electric current from primarysource electrode to other electrodes) can be seen in some experiments onlywhile it’s much less in some other experiment? What's the experimental factorsmay effect on volume conduction?
Thank you,
AHMED              
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