Now that we have introduced some basic brain monitoring methods. Let’s take a look at what signals we can record, and what kind of information can these signals give us!
First of all, let’s understand what brain waves are. Although there is probably only one application in the field of BCI using brain waves so far, I believe it is quite important for you to know it. And maybe you can think of other applications.
Brain waves, also known as neural oscillations, are an activity detected in the central nervous system, which includes the brain and the spinal cord.
Nerves regularly release neural signals together with different frequencies, which can be observed in individual nerves or groups of nerves. The actual reason why the nerve does this is still unknown.
The following is the classification of various brain waves. I will briefly introduce them. If you are interested in a certain wave, I will put the link on the Wikipedia page and you can read it later.
Classification of brain waves
Delta wave (0.5-4Hz)
Detected: Deep Sleep (Stage 3: Non-REM Period)
Theta wave (4-7Hz)
Detected: meditating, slightly groggy, hypnotized, and non-tertiary sleep
Alpha wave (8-12Hz)
Detected: awake, eyes closed (opening eyes will reduce its amplitude)
There seems to be a lot of information about alpha waves. In fact, there are low (8-9Hz) and high (9-12Hz) alpha waves. Low is more related to sleep and high is more related to inspiration and creativity. Some people also say that it has something to do with self-healing and relaxation. I think it depends on different individuals. There is a lot of alpha wave music on the Internet, which they said can inspire and arouse your subconscious. If you like it and it is effective for you, listen to it. But I think a more scientific explanation is that this wave may be inhibiting a certain phenomenon in the brain, and this phenomenon should not appear when the eyes are opened. Also, the alpha wave has the strongest signal in the occipital lobe, and not so much elsewhere. In short, it can only be said that the research is not enough, and we do not have a good conclusion about the alpha waves.
Mu wave (7.5-12.5Hz)
Detected: when voluntary movement or imaginary movement or even observed movement, it will decrease, and for other timing, it can be detected.
Mu waves are also called SMR (Sensorimotor rhythm), which are in the same band as alpha waves but in a different location. Mu waves mainly appear near the sensorimotor cortex, which is near where you put your overhead headphones. It is the wave that the BCI will use as mentioned earlier. When humans initiate movement, imagine movement, or even observe movement, this waveform will also disappear (this phenomenon is called ERD, event-related desynchronization). If you move on the left side, the waves on the right side of the brain will disappear; if you move on the right side, the waves on the left side of the brain will disappear. Each region is also responsible for different organs, you can refer to the following pictures. Red is the motor cortex, blue is the sensory cortex
Source
It can be found that the part of the hand is particularly large and occupies the most surface. Therefore, for BCI applications, the users are often asked to imagine the movement of their right or left hand, so that the signal will be more obvious, and some people will be asked to imagine the movement of the tongue or something like that. The signal of actual movement will be the largest, but the muscle signal of body movement will affect the detection of EEG, so most of the time it is better to use imagination. We can then ask the subject to control the flight direction of the drone by imagining holding their left or right hand. If the Mu wave on the left is gone, we know that he wants to go to the right, and vice versa. While imagining both hands holding tight is moving forward, imagining letting go of both hands is moving backward. So the following applications emerged:
Cool, isn’t it? Of course, the subjects have to undergo a long time of training, they cannot do it this well if it is their first time.
Beta wave (12.5-30Hz)
Detected: Awake, Busy, Nervous
Beta waves can be divided into three parts: low (12.5-16Hz), medium (16-20Hz), and high (20-30Hz)
Low: Relaxed and focused
Middle: Thinking
High: Anxiety
Gamma wave (30-140Hz)
Detected: When cognitive functions are functioning, instantaneous memory, high concentration, etc.
The most typical gamma wave is around 40Hz.
My Point of View
Among all these waves, the most important wave for BCI is probably the Mu wave, because it seems that it is the only one that is autonomous, and there are left and right. It is commonly used in the BCI. Other waves are related to sleep and attention, some BCI devices would also use them. The problem is that it is hard to control your attention or sleep very well. Sometimes the more you want to pay attention, the less you are able to concentrate, so these waves are more difficult to be used. And if you read it carefully, you may find that brain waves seem to be a continuous phenomenon. For example, low frequencies belong to sleep, drowsiness, and other phenomena, while high frequencies are more towards focused and awake, so you don’t have to care too much about the actual frequency of each wave. All in all, it seems that we don’t know much about the brain in the end. We can only say what kind of state this person is in when we observe what frequency. As for the reason, there seems to be no good explanation.
Comments