How can we record from the brain non-invasively?
We can read your mind! Okay, not quite, we can read the electrical activity going on in the brain and we can do this non-invasively. That’s right, you can do it from your own home if you wanted (here). It’s easy and since you don’t have to break the skin, it’s about as safe as can be. The real question here is why does this even work? For that we need to talk a bit on biology so let’s do this!
I’ve spent a lot of time trying to figure out how to best frame this, so I think what we’ll do is start at the bottom and work our way up. Normally the bottom would be the brainstem, but in this case, we’re talking about the individual neurons! A neuron is basically the smallest unit of brain cell we can talk about (oversimplification, but work with me here). We have several different types, but the one were interested in are the pyramidal cells.
So once again oversimplification, but the cortex (the outer layer of the brain) is made up of different layers (left in the image above). In those layers we have a predominance of pyramidal cells. The cell body (the triangle bit in the middle of the cell on the right) generates a current and sends it down the axon to the dendrites, which are the branch looking parts. Each time this happens one side of the cell becomes positively charged the other becomes negatively charged (see below).
We assume the body is charge neutral, so any change in charge will cause an equal and opposite change elsewhere. This is why we have dipoles. A dipole in the brain for example is exactly what it sounds like, two poles, one positive and one negative and we can imagine the current flowing around from one pole to the other just like a magnet. Using very powerful magnets we can actually measure this (MEG).
However we can also measure these electrical changes from the scalp. Now the skull is a poor conductor of electricity and that is the biggest issue with EEG (electroencephalography if you want to use the full term). The only thing less conductive than bone is fat, thankfully you don’t have a lot of fat on your head or EEG may not have ever been a thing!
Now when we say the body is charge neutral that just means that if charges (electrical activity) change, they have to change other places as well. Those dipoles in the brain, the pole facing towards the scalp creates an opposite charge on the bottom of the skull and a same pole charge on the scalp so a negative dipole facing the scalp (figure below A) will create a negative charge on the scalp. Conversely a positive pole facing towards the scalp (figure below B) will create positive charge on the scalp!
The bottom line is that this works only because the neurons are so close to the skull. We cannot record deep brain structures because the electrical activity just isn’t large enough to travel that far. In fact, I happened across a good visual to show this. The figure below shows three different ways of recording electrical activity from the brain and the amplitude of each of those methods. If we start from the bottom of the image, that is called intracellular recording, or recordings done from within the cell. We literally stick a very (VERY) tiny electrode INTO the cell. This is as close to “ground truth” as we can get, but it’s also basically as invasive as we can be! When a cell fires we get an action potential that creates a spike, so when we record them, you get lots of spikes, then nothing, then a few more and so on.
Next is the middle part and that is extracellular recording. Like that sounds it’s right outside the cell, in this case we get a local field potential. This is slightly less invasive, but still requires people cutting open your skull and shoving electrodes in. Instead of getting activity from one cell we get a lot of activity from a group of cells. The amplitude is quite a bit lower in this case and instead of spikes you get a mash of electrical activity from hundreds or thousands of cells which creates something that if you squint hard enough looks like the activity from a single cell. That’s because cells tend to fire in groups and that’s another reason why we can record the activity using EEG, we’re recording hundreds of thousands of cells firing together. Or conversely if we find less activity, they are firing separately and that tells us something important too!
Lastly we have the EEG recording. Again, that is a summation (basically) of all the activity in that area. Volume conduction, or the spread of the current, means that electrodes placed right next to each other will look virtually identical even though they are recording from a slightly different area. However, if we put electrodes all over the scalp we can use tiny differences in the electrodes to pinpoint sources of activity and we can predict one dipole per electrode with a semi-fair amount of accuracy(ish).
As I mentioned in this post, calculating dipole location relies on the inverse problem, which means we use electrical properties of the scalp, the skull, the brian, and all the tissues in between to back calculate the source of a signal. Unfortunately individual differences in how blood vessels run, tissue thickness, etc. makes this more of an educated guess than something precise. Still, if we consider that this is all calculated without breaking the skin it’s pretty amazing.
So that is a high level overview into why EEG even works! It’s a combination of luck in our evolution and curious people attaching electrodes to the scalp to see what would happen. Seriously, the invention of EEG was literally a guy named Hans Berger wondering what would happen if they put electrodes on the scalp (read more). He was more interested in “psychic energy, but that’s a whole other story not related to this! It is covered in that link I put up though if you’re interested, hint, hint!! Here’s a shot of the set up he used and some of the original recordings. Thankfully we’ve come a long way since then!