tim's journal

The tale of two electrodes

I got stuck thinking-writing about wet electrodes. It's time to nail things down... which has been a bit fuzzy. The question is, how does one measure changes in potential on the scalp surface which result from some kind of electrical activity beneath the scalp. In tissue - in liquids - the movement of charge occurs via ionic currents. So, the goal is to take those ionic currents and to convert them to electrical currents to do our measurements. Another related question is what exactly causes these ionic currents? If it is an electrical field perhaps that is what we should measure, rather than the ionic current.

The textbook response is that there are two ways of doing this, using a non-polarizable electrode, or using a polarizable electrode. In the case of a non-polarizable electrode, oxidation or reduction reactions occuring at the electrode-electrolyte junction create or absorb free electrons. These reactions can either occur from the electrode or from the electrolyte. This type of electrode is modeled using a resistor, since real current flows. A polarizable electrode is one in which no oxidation or reduction reactions occur. Changes in ionic concentration at the electrode-electrolyte junction cause changes in electron concentration at the junction. In this case the measurement is based on displacement current. Typically noble metals such as platinum are used for this.

Ok, so given all this, what is actually happening? I thought about this on the way to Ranch 99 and it still puzzles me. Because of the underlying electrochemical reactions in the brain, a fluctuating field is built up. This field causes ions to move around (which also affect the field in someway). Now, at the surface of the scalp, we see some of this ion movement, and if we look at two locations, there may be a net charge difference, if there is, then there is a voltage between the two. By connecting the two locations using a wire, charge will flow... in some sense. If the connection is made using non-polarizable electrodes, then oxidation or reduction take place so that the charge build-up begins to be neutralized as electrons flow across the wire to neutralize the other side. If the connection is made using polarizable electrodes then effectively electrons in the wire bunch up to neutralize the positively charged side (and the other side). This current is measured using a galvanometer (for instance).

Now here's the interesting question, if it is some sort of potential that causes the ions to be in those locations in the first place, couldn't the same potential also cause current in the wire? You could imagine, for instance, some sort of cantilever with a fixed charge on one side and the scalp on the other. In this way you could measure fluctuations without a reference. Of course, it would be much more difficult to do differential measurements, but it could still be done. This is a way of measuring EEG in a completely different way.