I left off yesterday wondering about sources of EEG - actually, I read a few papers. I'll try to summarize what I found. Essentially, the accepted description is that the EEG is a result of extracellular currents, i.e., the ions which flow following neuronal discharge.
To be precise, let me quote from Ebersole and Pedley's "Current Practice of Clinical Electroencephalography". In the first chapter, Buzsáki, Traub and Pedley write:
Membrane currents generated by neurons pass through the extracellular space. These currents can be measured by electrodes placed outside the neurons. The field potential (i.e., local mean field), recorded at any given site, reflects the linear sum of numerous overlapping fields generated by current sources (current from the intracellular space to the extracellular space) and sinks (current from the extracellular space to the intracellular space) distributed along multiple cells. This macroscopic state variable can be recorded with electrodes as a field potential or EEG or with magnetosensors (superconducting interference devices [SQUIDs] ) as a MEG. These local field patterns therefore provide experimental access to the spatiotemporal activity of afferent, associational, and local operations in a given neural structure. To date, field potential measurements provide the best experimental and clinical tool for assessing cooperative neuronal activity at high temporal resolution. However, without a mechanistic description of the underlying neuronal processes, scalp or depth EEG is simply a gross correlate of brain activity rather than a predictive descriptor of the specific funcional and anatomical events. The essential experimental tools for the exploration of EEG generation have yet to be developed. [1, p. 1]
A straightforward approach to decomose the surface (scalp) recorded event is to study electrical activity simultaneously on the surface and at the sites of the extraccellular current generation. Electrical recording from deep brain structures by means of wire electrodes is one of the oldest recording methods in neuroscience. Local field potential measurements, or "micro-EEG", combined with recording of neuronal discharges is the best experimental tool available for studying the influence of cytoarchitectural properties, such as cortical lamination, distribution, size, and netowkr connectivity of neural elements on electrogenesis. However, a large number of observation points combined with decreased distance between the recording sites are required for high spaital resolution and for enabling interpretation of the underlying cellular events. Progress in this field should be accelerated by the availability of micromachine silicon-based probes with numerous recording sites. Information obtained from the depths of the brain will then help clinicians interpret the surface-recorded events.Buzsáki et al. go on to describe various sources of extracellular current flow. What seems to be missing is some discussion on how the fields then affect all these other things. They list the following sources:
...
In principle, every event associated with membrane potential changes of individual cells (neurons and glia) should contribute to the perpetual voltage variability of the extracellular space. Until recently, synaptic activity was viewed as the exclusive source of extracellular current flow or EEG potential. Progress during the 1990s revealed numerous sources of relatively slow membrane potential fluctuations, not directly associated with synaptic activity. Such non-synaptic events may also contribute significantly to the generation of local field potentials. These events include calcium spikes, voltage-dependent oscillations, and spike afterpotentials observed in various neurons. [1]
References
[1] G. Buzáki, R.D. Traub and T.A. Pedley, "The Cellular Basis of EEG Activity," in Current Practice of Clinical Electroencephalography, 3rd ed., J.S. Ebersole and T.A. Pedley Eds. Philadelphia: Lippincott Williams & Wilkins, 2003, pp. 1-11.