Approximating dipoles from human EEG activity: The effect of dipole sourceconfiguration on dipolarity using single dipole models

Dipolarity is the goodness-of-fit of the observed potential distribution wi th one calculated using specific assumptions about the source of the electr ical potential distribution. We used computer simulations to examine the ef fect of different distributions of sources on their resulting dipolarity va lues, Electric dipoles were placed in a head-shaped model with uniform cond uctivity using four different dipole configurations (randomly oriented dipo les, a uniform dipole disk layer, a dipole disk layer with uniformly distri buted holes, or one with randomly oriented dipoles), The best-fitting singl e dipole for each configuration was calculated and the dipolarity was compu ted as the mean squared error of the electrical potential distributions gen erated by the actual dipole configuration and by the best-fitting single di pole, The simulations show that: 1) a smooth dipole layer with or without h oles gives dipolarities above 99.5% even when extended over areas as large as 1256 mm(2); 2) randomly oriented dipoles under a smooth dipole layer als o give dipolarities above 99.5%; and 3) randomly oriented and distributed d ipoles, even if contained in a small portion of the total area, give dipola rities below 93.0%. These simulations show that inhomogeneity (holes) withi n a dipole disk layer per se do not lower dipolarity; rather, it is the ran dom orientation and distribution of these dipoles which lowers dipolarity, Furthermore, dipolarity is not lowered by such randomly oriented and distri buted dipoles when they are beneath a dipole disk layer.