Kutas, M. and Dale, A. Electrical and magnetic readings of mental functions, in Cognitive Neuroscience, M.D. Rugg (Ed.), University College Press, 1997, pp. 197-237.
Ever since Berger's (1929) discovery that brain electrical activity (electroencephalogram (EEG)) can be measured at the human scalp, it has been assumed that in these voltage fluctuations are hidden the mysteries of the workings of the human mind. While classical neurophysiologists questioned the likelihood that such "simple" fluctuations could be the key to the complexities of understanding, talking, reasoning, imagining and supposing, the past 70 years have proven otherwise. A large body of evidence has shown that electrical and magnetic activity (human or otherwise) encode information about brain states and brain processes and, by inference, about mental states and mental processes. (p. 197)
This is a nice point. At first glance, it is surprising that signals in the 10's of Hertz or less can provide meaningful information about brain processes. Individual neurons fire at a rates orders of magnitude higher, from 250-2,000 Hz [1].
The net flow of current across the neural membrane generates an electric potential in the conductive media both inside and outside the cells. It is this electric potential that forms the basis for the electrophysiological recordings made both invasively, by lowering elecrodes into the brani, and non-invasively, by placing electrodes on the scalp for EEG/ERP (Nicholson & Freeman 1975, Nunez, 1981). The same transmembrane current flows are also responsible for the magnetic fields recorded outside the head for MEG (the magnetoencephalograph). (p. 199)
This doesn't feel completely accurate. I would, instead, paint the following picture. Whenever a neuron discharges, the net electric field in the brain fluctuates because the distribution of shielded versus unshielded charges will change. Information about the new field is propagated to all charges (as well as the scalp) at nearly the speed of light. To minimize the energy of the system, free ions, i.e., those unconstrained by cell walls, then redistribute in response to the field. Kutas and Dale's statement makes it seem like this redistribution, the "flow of current", generates the electric potential at the scalp. My conjecture is that this larger flow of current will be very very slow, much slower than changes associated with billions of neurons charging and discharging. Assuming this to be true, and accepting that the local electric field can influence neuron firing rate, it is not to far fetched to consider that it is in fact the fluctuation in the electric field can encode information.
References
[1] V. Gerasimov, "Information Processing in Human Body," [Online document], 1998, [cited 10 Sept 2001].