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 propose
d as a model of pyramidal cell synaptic activity. Here we propose a ring-sh
aped distribution of dipoles as a source model for these fast oscillations.
This distribution generates a field potential that does not reverse throug
h 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 detectab
le in EEG data, but not in simultaneously recorded MEG data. (C) 1998 Elsev
ier Science Ireland Ltd. All rights reserved.