Polysomnography Step 2: Electrode Placement

Understanding biodata measuring:

I am still very far from having mastered the details in this area, but here’s my non-specialist sense of what is involved in getting data from a subject’s head. The key point to understand is that all of these measurements are COMPARATIVE. Electrodes are not like a thermometers, which measure the temperature of whatever they is touch. Here we are measuring the “electrical potential”, which means the difference between TWO points of interest. So in an EEG for example, an electrode on the scalp is compared with another electrode, typically placed in a more “neutral” location like an earlobe, mastoid bone (the bone behind the earlobe), or central location. This means that for each measurement, I technically need TWO electrodes, one at the location I want to measure, and one at some sort of reference point.

Three kinds of measurement:

To do a decent PSG, you minimally need to measure electrical activity on the scalp (EEG or “brainwaves”), eye movements (EOG) and muscle tone (EMG). Professional labs will also measure breathing, heart rate and all kinds of things, but if you’re mainly interested in figuring out sleep stages (rather than curing your snoring problem), these are the three main measurements.

Electrode Placement:

EEG placement is described in terms of a 10-20 international system that allows scientists to map the head without resorting to saying things like “a little bit up from the ear, but then back to where the bump starts” (see illustration). You can see it makes a certain amount of sense (odd numbers on the left, even on the right, F = Frontal, C = Central, P = Parietal, O = Occipital…). People getting diagnostic EEGS, or people really into figuring out what their brainwaves are doing, can connect a LOT of electrodes, but two or three are sufficient for polysomnography. In fact, many expensive professional PSG systems use only a couple on the forehead, as those are more liable to stay in place during the tosses and turns of sleeping.

However, exact positioning is a matter of at least some debate. The system developed by Rechtschaffen and Kales in 1968 involves a certain setup (and this is the one used in the SpiSOP system). The more modern (but not necessarily better?!) setup recommended by the American Academy of Sleep Medicine involves three measurements covering frontal, central and occipital regions. This system is described at length in this helpful paper I found from the “Annals of Indian Academy of Neurology” (which is in fact an entire manual on polysomnography!). It requires electrodes placed at F4, C4 and O2, (or, if one prefers, the same placements on the left), and each of which is measured against the same mastoid reference.

I use my new, supercool headband kit to provide the electrodes on the forehead (F4) and “backhead” (O2). Technically this puts me around Fp2 rather than F4, but I think it’s close enough for me. I then use the “wet” electrode for the C4 position, and I can easily add one at T3, or add backups on the opposite side (i.e Fp1, C3, and O1). Likely an expert can tell me what I’m missing by using Fp2 rather than F4, or what would be gained by adding T3, which is really on the side of the head?

EOG and EMG placements are quite straightforward, but different than the EEG readings. EEGs all measure the activity of the scalp relative to the same “neutral” point – an earlobe. But in EOG, we measure the activity between the front and back of they eyes themselves (if I understand it correctly – something about retinas and corneas having opposite polarity?). Now, the helpful paper from The Indian Academy of Neurology suggests that I’m not doing it according to the AASM standard! It requests measurements from each eye corner relative to a neutral mastoid or central forehead! This I did not know until just now. Up until now I’ve been placing one just outside the corner of the eye (careful not to get electrode paste in anyone’s eye!), and measuring it against another electrode placed a centimetre or so above the centre of the opposite eye, and I get seemingly pretty good results! This also conforms to the setup recommended by the SpiSOP system.

The same holds for the EMG setup. The AASM suggests three electrodes – two underneath the chin being compared with a reference placed on the front of the chin. However, so far I’ve just placed two electrodes under the chin about 3-4 cm apart, using one as a measurement and one as a reference. This accords with the SpiSOP setup, and seems to be just fine. However, it isn’t very easy to keep those chin electrodes on, particularly during a long night! Sports tape to the rescue!

Observant readers will notice that I have not yet mentioned anything about a reference electrode for the EEG readings. For this I use one of the handy, pasteless earlobe clips that came with my headband kit. This will be plugged into the “SRB” pin on the Cyton board, and be used as a reference for all the EEG points (or even the EOG points, according to the AASM!). Finally, I attach the other earlobe clip to the other ear, which will be connected to the “BIAS” pin and (to my understanding) helps eliminate unwanted background noise.

Et voila. I’m now ready to connect all those cables to the Cyton board, turn the whole thing on, and start sleeping…