This is the last introduction to the non-invasive BCI. Although there are still some methods that I missed, the ones I introduced are relatively mainstream. I will introduce others later if you are interested.
The full name of MEG is Magnetoencephalography.
Its function is a bit similar to the EEG I introduced in the previous article, but it is not exactly the same. Let’s take a look.
How does MEG work?
As mentioned in the previous article, bioelectricity is used for neural signal transmission. EEG detects electrical signals to see what nerves in the brain are active.
What comes with electricity is magnetics. MEG uses the magnetics that comes along with electricity emitted by neurons to monitor brain activities.
It’s just that the magnetic force is so much smaller than electricity, so it must be detected with a more accurate measuring instrument, and the subject must enter a space that is not disturbed by other electromagnetic waves from the environment. And of course, it is very expensive. Hahaha.
SQUID
There are two main methods of MEG, one of them is called SQUID（superconducting quantum interference device）How it works is a little tedious. I will just put its Wikipedia page
In short, the machine is composed of many small devices that are very sensitive to magnetic force, and then the devices are submerged in liquid helium, and then put on the head to detect the magnetic changes inside the brain, the device looks like this
By Unknown NIMH author – NIMH Image library, http://infocenter.nimh.nih.gov/il/public_il/image_details.cfm?id=80, Public Domain, https://commons.wikimedia.org/w/index.php?curid=11909237
Yes, unlike the EEG cap, the device is HUGE!
OPM
Then there is the OPM (Optically pumped magnetometers). I read its related articles for a long time and couldn’t understand it. It uses the principle called the Zeeman effect of quantum mechanics. Go read it if you are interested. The whole device looks like this
Source
It has more mobility compared to SQUID. The sticks in the back can be taken out individually. It’s easier to set up and take out.
MEG detects the signal opposite to EEG. Remember that EEG detects the yellow ones and ignores the blue ones? MEG detects the blue ones and records the yellow ones.
Source
Methods
The subject would be brought into a room that is isolated from electromagnetic waves, and then put on the device. There are no wet or dry electrodes. The subjects were then asked to perform tasks for the brain activities to be detected
Advantages of MEG
Represent real signal
Magnetic signals will not be filtered by the skulls and other body tissues, so it is more accurate than EEG
Real-time
Just like the EEG signal, the MEG signal is also recorded in real-time
No reference needed
The electrical signal is relative, therefore a reference point is needed using EEG recording which is usually behind one of the ears. Picking a wrong reference location may greatly affect the interpretation of the results. MEG would not have this kind of problem
No wet electrodes needed
MEG does not use wet electrodes, which is more convenient and cleaner.
Disadvantage of MEG
Require special space
Different from EEG, MEG has to be recorded in a room without electromagnetic waves from the environment which is not convenient and expensive
Expensive
When you read liquid nitrogen in the previous section, you know it is going to be very expensive. Not only the device is expensive, but the liquid nitrogen is expensive, also the room itself is expensive
Not special
This is my own opinion. The BCI techniques, I introduced previously, all have their specialized functions. They are either very precise, very portable, or not affected by electrical signals from other muscles. MEG has a lot of limitations and does not have corresponding powerful functions in return.
My Point of View
As said before, of course, MEG has its advantages, but is it necessary to sacrifice so many things in exchange for those little advantages? At present, its development can only stay at the stage of medical use, because all those shortcomings are very fatal for it to enter everyone’s life.
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