Neurocomputing 38}40 (2001) 1209}1216 The spatial resolution of scalp EEG T.C. Ferree*, M.T. Clay, D.M. Tucker Electrical Geodesics, Inc., University of Oregon, Riverfront Research Park, Eugene, OR 97403, USA Computational Science Institute, University of Oregon, OR, USA Department of Mathematics, University of Oregon, OR, USA Department of Psychology, University of Oregon, OR, USA Abstract The scalp electroencephalogram (EEG) exhibits spatiotemporal dynamics re#ecting syn- chronousdendriticactivityofcorticalpyramidalneurons.RecentadvancesinEEGacquisition and electric head modeling are improving the spatial resolution of scalp EEG, but the skull remains an obstacle. We use lead "eld theory to quantify the spatial resolution of scalp EEG, contrastingtwoelectrodespacingsandtwovaluesofskullconductivity.Weshowthat,without corticalconstraints,19-electrodeEEGsystemshaveoptimalspatialresolutionnear22}37cm, while 129-electrode systems have 6}8cm. These results emphasize the bene"ts of more electrodes, but also the need for methods of measuring local skull conductivity.  2001 Elsevier Science B.V. All rights reserved. Keywords: Electroencephalography; Sensitivity; Spatial resolution; Skull conductivity 1. Introduction Scalpelectroencephalography(EEG)ismadepossibleinlargepartbythecellular- level anatomy of mammalian cortical tissue. The apical dendrites of most cortical pyramidal cells are aligned along the local normal to the cortical surface. Associated transmembranecurrentsgenerateextracellularelectric "eldswhicharemeasurableat adistance.Becausethe "eldduetoasingleneuronistiny,manynearbyneuronsmust exhibit synchronous dendritic activity for the net "eld to be detectable at the scalp. * Corresponding author. Electrical Geodesics, Inc., University of Oregon, Riverfront Research Park, 1850 Millrace Drive, Eugene, OR 97403, USA. Tel.: #1-541-687-7962; fax: #1-541-687-7963. E-mail address: tom@cortex.egi.rrp.net (T.C. Ferree). 0925-2312/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0925-2312(01)00568-9