I've been thinking about modeling the EEG measurement and how to write up the corresponding chapter. I think this is how it'll go. First, I will introduce the concept of impedance in a circuit element.

Impedance is the relation between voltage and current for a two terminal circuit element. In other words, if I give the element a voltage V, what is the resulting current I? The relation, impedance Z, can be expressed as a complex number. This complex notation allows us to describe responses over time. Except in ideal elements, giving an element voltage V will result in a current which monotonically and eventually, so not instantaneously, moves to a final current I.

Perfect, from there I can give a simple model of the voltage fluctuations at the scalp: a voltage source, with impedances Z1 and Z2 on each side. Of course, in real life it is more complex, the number of sources is much greater than 1, each with its own Z1 and Z2. Between the sources and the surface of the scalp, there are other sources - some of which may be more stationary, and some of which may be more dynamic. It is known, for instance, that across the skin barrier there is effectively a voltage source in the 10s of mV. This is related to a difference in ionic concentration in the outer surface and the inner surface of skin. When the subject perspires, pores are opened and filled with sweat which then changes the ionic concentration. Fortunately these changes happen rather slow, and are only relevant to recordings at very low frequencies.

We can begin our analysis with the simple model of a single source and Z1 and Z2.

**Impedance of skin.**This should be a good sized section, because it is important. First I will review different measures - in electrophysiology, for EIT, and in dermatology, to test the results of various cremes, etc. I should talk about the technique, and then look at the results.

Posted by torque at 12:25 AM
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