Today, I’m going to introduce a signal that should be considered the most applicable brain signal so far. There are already related products on the market. Although the price is a bit high, it is actually functional. I will introduce it to you later.
The signal is called SSVEP (Steady State Visual Evoked Potential).
Other than the BCI using SMR introduced in the previous article, what I’m going to introduce now and later are all passive.
Passive means you can’t imagine going left to right, users can only look at the signal that has been designed, and by detecting that signal from users’ brains we know what the users are looking at. If the signal is not designed, we are not able to analyze anything.
Let’s see how it works!
Mechanism of SSVEP
When the retina receives a light signal, the brain will produce electrical signals in the occipital lobe. When the light flickers, the electrical signals in the occipital lobe will change according to the frequency as well.
Source
We use EEG or other methods to detect this signal and use the frequency detected to know which target the users are staring at, which can achieve the effect that seems like users are controlling it.
Note that this signal can only be about 3.5Hz to 75Hz, if it is too fast or too slow, the signal in the brain cannot react to it accordingly. The best effect falls around 8Hz.
The distance between the screen and the eyes is also a problem. Too far away or the tiny targets may cause the signal to be undetectable. The viewing angle of the target should be maintained at an angle of more than 2.9 degrees. If it is too small, the effect will be very poor. There are a lot of limitations!
Advantage of SSVEP
SNR is High
As mentioned earlier, SNR is the signal-to-noise ratio. The higher the value, the more useful information is in the received signal. SSVEP has a high SNR, which may be why many products on the market use this signal. This is a very important advantage. If the SMR brainwave signal mentioned in the previous article has such a high SNR, the BCI technology today will be very prosperous.
Disadvantage of SSVEP
Not Autonomous
Only the designed function will be executed, that is to say, we can only know what the users are staring at but do not know what users actually want to do. And no matter what the users want to do, it can only execute the designed functions. For example, if users want to turn on the TV, they have to stare at the designated position. There will probably be a flashing light at that position, and the wearable device detects the SSVEP signal at the user’s occipital lobe to know that the users want to turn on the TV. On the other hand, if users want to increase the volume, because there is no such pre-designed function, there is no way to achieve it using the SSVEP signal.
Visual fatigue
The users who have been in the experiment report that their eyes feel very tired after the experiment. Yes, it is true that the eyes will be tired after staring at a flashing object for a while which also makes SSVEP unable to be used as a long-term application, which may affect the eyesight or some other damages.
My point of View
Because of its high SNR, dry EEG electrodes can be used and still get good brain signals. The advantage of dry EEG electrodes is that they can be used in daily life without spending a lot of time implementing them. But the lack of autonomy makes its application very limited. At first, it may feel very interesting to use it every day. If the home appliance has corresponding functions, you may be able to use it to change the channels, select songs, and turn on and off the lights. After a long time, you may feel that I might as well do these things directly with my hands since it is so extra to do those simple things with a brain recording device. It is also limited by its frequency and distance. In general, although it may be the most likely signal to be applied in daily life at the moment, it is still not that convenient.
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