 Quote by ryan_m_b
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If you read the wiki article you linked to on EEG's you'll see that it asserts that what an EEG picks up is not "electrochemical action potentials traveling down neurons in a neural network" but a phenomenon called "volume conduction":
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Neurons are electrically charged (or "polarized") by membrane transport proteins that pump ions across their membranes. Neurons are constantly exchanging ions with the extracellular milieu, for example to maintain resting potential and to propagate action potentials. Ions of like charge repel each other, and when many ions are pushed out of many neurons at the same time, they can push their neighbours, who push their neighbours, and so on, in a wave. This process is known as volume conduction. When the wave of ions reaches the electrodes on the scalp, they can push or pull electrons on the metal on the electrodes. Since metal conducts the push and pull of electrons easily, the difference in push or voltage between any two electrodes can be measured by a voltmeter. Recording these voltages over time gives us the EEG.[4]
The electric potentials generated by single neurons are far too small to be picked by EEG or MEG.[5] EEG activity therefore always reflects the summation of the synchronous activity of thousands or millions of neurons that have similar spatial orientation. If the cells do not have similar spatial orientation, their ions do not line up and create waves to be detected. Pyramidal neurons of the cortex are thought to produce most EEG signal because they are well-aligned and fire together. Because voltage fields fall off with the square of the distance, activity from deep sources is more difficult to detect than currents near the skull.[6]
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So, rather than the "travel" of signals (i.e. as along an axon), the EEG picks up the EMF of the mass "reloading" and "firing" as it were, of large populations of neurons, according to this article. "Exchanging ions with the extracellular millieu" refers to the pumping of ions from inside the neuron to the outside of the neuron, and, conversely, the sudden migration of those ions back into the inside of the neuron when it "fires". That's not the traveling signal from one neuron to the next via neurotransmitters that we think of as the operative procedure of a neural net. The EMF of volume conduction does not travel along a network but is dependent on the spatial orientation of the neurons, instead (and, interestingly, independent of direct connection between the neurons involved), and on the number of neurons taking part synchronously ("thousands or millions"). Apparently, according to the article, the neurons have to be oriented in a specific way with respect to each other for a volume conduction to reach the surface and be detected.
The only "travel" involved, if you want to speak of travel here, is the travel of positive ions from outside the cell to the inside, and visa versa. The EEG is not picking up the travel of signals from, for example, thalamus to cortex.
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