Today, I am going to introduce EEG, which you may hear people talking about very often. There is also a lot more research using EEG in the academic field than other BCI methods(That’s just what I think, I don’t have actual data to prove it.)
The reason why it is so popular may be because it was discovered at an early time. After knowing that nerves use electrical signals to transmit information, the most intuitive idea is to use metal to capture those electrical signals, and that’s exactly what EEG is doing.
The full name of EEG is Electroencephalography. You may have heard of it very often, but you don’t seem to know about it very well, so let’s look at what EEG is doing today.
How does EEG work?
Nerve cells use electrical signals to communicate, and the bioelectricity signal is created by ionic differences between the inside and outside of nerve cells. For more details, you can review the high school biology textbooks or online resources.
By putting some metal electrodes on multiple sites over your head, the brain signal is able to be captured, so that we know which part of the brain is active. Yes, it is that simple.
You may ask, well, how do you measure the EEG signal since the metal is not in direct contact with the nerve?
Here, we have to talk about Maxwell’s equations, which describe the relationship between electric and magnetic fields. We know that as long as the electricity is flowing in the direction of the metal electrodes, there will be electrical signals generated in the electrodes.
Does this mean that if the electrical signal is not going straight (yellow arrow in the picture below) but going in a 90-degree vertical direction (blue arrow in the picture below), the electrode would not be able to pick up the signal? Smart, that’s what happens exactly! This is a huge limitation of EEG. The signal in the blue direction will become very weak to the electrodes above it, but it is not completely gone if it is not perfectly 90 degrees. It will then be mixed with the strong signals, causing the recorded signal to be chaotic.
Source
Methods
The electrodes were put all over the subject’s head, in some designated positions, to record electrical signals. There are companies that make EEG caps so that when you wear the cap correctly, the electrodes will be around the correct locations. You only have to adjust them slightly. Of course, when doing an actual experiment, people would measure the distance more precisely. The number of electrodes can be adjusted according to the experiment!
It looks like this:
For the types of electrodes, there are wet electrodes and dry electrodes. The wet electrodes will add some gels or saline between the scalp and the electrodes, which can make them more conducive and more sensitive to the electrical signal created by brain nerves.
Of course, you can place the electrodes only in the designated positions where you want to recode. Later, when we talk about some products on the market now, we will explain the different brain areas later. You can also see where the electrodes are placed to speculate what they are measuring. After the electrodes are all connected properly, the user would be asked to finish some tasks accordingly. It depends on what kind of signal you want to record for the BCI.
Advantage of EEG
1. Low price
Compared to a huge machine like an MRI machine, this is a fairly cheap way of detecting the brain signal.
2. Real-time
EEG detects electrical signals. Basically, the machine can pick them up once the signals are presented in the brain. Unlike fNIRS, you would have to wait for the blood to flow.
3. Very intuitive signal
The signal represents what the brain is emitting, no further translation is needed. This is actually a huge benefit when it comes to decoding brain signals
Disadvantage of EEG
I don’t know why but I always think about more disadvantages than advantages, but I guess that’s how human works
1. Poor resolution
The spatial resolution can be increased by adding more electrodes, but it can only reach a certain level. The brain signals have to pass through the cerebrospinal fluid, skull, and scalp in order to reach the electrode which means the signal itself is weak already when picked up by the electrodes. In addition, these body tissues also have the effects of filtering the signal, so the signals that are recorded may only have a specific frequency range. But I think it is not very fair to blame EEG so hard. It is like asking if you can hear what the person inside the wall is talking from outside the wall, and when you can’t hear clearly we blame you for having a problem with your ears.
2. Noisy signal
Brain nerves are the only cells using electrical signals to communicate, all nerve cells in the body use electrical signals to communicate, including muscles. Muscles create even stronger action potentials, so the brain signals will be overshadowed by the muscle signal easily. Basically, when you blink your eyes, the electrodes on the head will detect a super large electrical signal. All nerve signals in the brain are blocked, so users are suggested not to move during the recording.
3. Takes a long time to implement
Dry electrodes are easier to implement, but the quality of the signals is really poor. Therefore, people use wet electrodes for recording. There will be around 20-50 electrodes to be set up, you have to put gel in each of them and attach them to the scalp which will take around 30 to 60 minutes.
4. Only detect the signals on the surface
This is the disadvantage for almost all noninvasive BCI. The signal detected is always from the surface of the brain, deeper regions cannot be detected.
5. Requires a large number of neurons to be triggered together
In fact, this can also be listed as poor resolution, because there is no way to achieve the single-nerve resolution, so a large group of nerves must be excited together to be recorded by the electrodes. Some small-scale neural excitations may contain useful information but will be ignored if recorded with EEG.
Join the experiment myself
I don’t have this section in other BCI introductions, but I was lucky enough to participate in an experiment with EEG during my time in master’s degree. I’ll share a little bit about the procedures.
First of all, sit down and chat with them, and then they will come up with a form, in which they will talk about some privacy, psychological, and physiological considerations. If you agree to join the experiment, you’ll have to sign them.
They will then take out an EEG cap that fits your head size and put it on for you. The measurement is taken to make sure all the electrodes are in the designated positions. What they put in was called the 10-20 system. (as in the picture below) There are other position methods like the 10-10 system.
Source
The point is, it doesn’t end here. They will take out some wooden sticks like chopsticks, and then poke them in at various positions and rotate them on your scalp. The purpose of this is to push your hair and some skin away so that the electrodes can be in contact with the scalp to record electrical signals more accurately.
In fact, it is not as painful as it sounds. It is a bit like scalp massage, but it still causes a certain degree of damage to the scalp. How do I know? Because the next step is to inject saline gel. The gel is conductive because there is salt in it. When the gel is injected, it’s a bit like a wound touching salt water. Ouch!
They had a screen in front of us to show the conductivities of each electrode, that is to see if every electrode is placed perfectly. It took about half an hour to be fully implemented.
Once it’s done, they will ask you not to blink your eyes too often since all the muscle signals will be recorded. You can’t stretch your legs either, that will make your body an antenna that receives other electrical signals from the environment. Yes, it is that sensitive.
I’m not going to talk about the experiment in detail since it might be confidential. Mostly, I was asked to perform tasks like focusing on some things or not focusing on some things. To be honest, I didn’t feel like I was controlling anything. He said I’d have to join the experiment more times in order to start getting the feeling of it.
I feel relieved after the experiment since I was not allowed to move during the experiment. The whole experiment was about 45 mins to an hour. You can wash your hair after the experiment.
My Point of View
Now when talking about non-invasive BCI, many people will intuitively think of EEG. Yes, it has its advantages, but as mentioned above, because it will be affected by many other signals, It is very sensitive, so it is not suitable for people to use it when they are performing other tasks. If you are in the office, you may be more suitable since you can sit and use it, but you will not be able to move after it is implemented on your head, and it is going to take a long time to implement, and the accuracy is not that high. After all, I am not very optimistic about EEG. I feel that this technology still has a long, long way to go.
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