Pii: S1388-2457(98)00008-x

As opposed to slow waves, spontaneous and stimulus-induced oscillations in the gamma-band show no polarity reversal in cortical depth, which cannot be explained by the classical equivalent current dipole model usually proposed as a model of pyramidal cell synaptic activity. Here we propose a ring-shaped distribution of dipoles as a source model for these fast oscillations. This distribution generates a field potential that does not reverse through cortical depth. Such a geometry could correspond to horizontally oriented dendritic fields. Moreover, this distribution generates a potential field, but no, or weak, magnetic field on the scalp surface, which corresponds to the observation that visually-induced gamma-band oscillations are detectable in EEG data, but not in simultaneously recorded MEG data. q 1998 Elsevier Science Ireland Ltd. All rights reserved.