For the BCI technology that I’m introducing in this post. I personally think that it has a huge potential, and I will explain the reasons later.
The full name of fNIRS is Functional near-infrared spectroscopy
Based on its name we still don’t know what it does.
Without further ado, let’s take a look at what kind of technology this is.
How does fNIRS work?
As mentioned in the fMRI introduction, when the brain is functioning, the blood flow of the functioning area inside the brain will increase. By detecting the blood flow, we can indirectly know the part of the brain that is functioning.
We know that fMRI uses the difference between how the oxygenated and deoxygenated blood responds to a magnetic field to identify the different blood flows. fNIRS is also detecting the blood flow inside the brain except that it uses the characteristic that red blood cells have a higher absorption rate towards near-infrared light to differentiate the amount of blood flow.
First of all, what is near-infrared light? We all know that the human eye can only recognize colors in a certain range of the spectrum (or wavelength of light), and other light outside those wavelengths cannot be seen by the human eye.
The wavelength shorter than purple light (380~450nm) is called ultraviolet light, and the wavelength longer than red light (620~700nm) is called infrared light. These wavelengths of light are invisible to the human eye and require special instruments to see them. Near-infrared light is light that has a wavelength slightly longer than the wavelength of red light, which is about 700~900nm.
This range of light is very amazing. It has a poor absorption rate toward other body tissues, but a high absorption rate toward the red blood cells. This makes the human body look transparent to the light and only shows the location of the blood vessels. Of course, oxygenated red blood cells and deoxygenated red blood cells have different absorption rates towards near-infrared light, but they are all a lot higher than other body tissues.
When implementing fNIRS to the subjects, there will be an optical fiber that emits near-infrared light, and there will be a detector next to it to detect the reflected light intensity of the infrared light. Why is it on the side instead of on the opposite side? Good question! Because although the absorption of infrared light by other body tissue is very poor, the tissue will still scatter the light. In other words, photons will hit molecules in the tissues and then go in a random direction, so if the distance is too far, there will be no light detected since they are all scattered.
So the detector is put next to the emitter. We cannot put it inside the brain because that works against our original purpose: non-intrusive. With the light scattered back, we can still measure how much it is lowered, so we can tell which part of the brain is working.
The concept is like this:
Source
And this is how they look like on the caps
Source
The red one is the emitter and the blue one is the detector or the other way around depending on their design.
Methods
Just like other BCIs. People wear the cap to help the emitter and receiver settle at the right locations and people can focus on doing the tasks. When the blood flows inside a specific area of the brain increases, the device is able to pick up the signal. After decoding, the status of the brain will be known.
Advantages of fNIRS
Not dangerous
Other non-invasive BCI will involve using electrics, and magnetics, and we do not know if there will be some side effects after long-term usage. Lights are believed to be harmless, so from infants to the elderly, everyone can use the fNIRS device.
High mobilities
Other noninvasive BCI would always have a certain degree of movement limitation, either the image is going to be affected such as fMRI, which I have already introduced. Or the signal is going to be affected by other muscle movements such as EEG or MEG, which I will introduce later because muscle contractions also emit electrical signals.fNIRS measures blood flow inside the brain, so it is not gonna be affected by muscle contractions. As long as the light fibers stay at the position, the measurements are going to be really consistent even if there are movements.
Weightless
Other noninvasive BCI usually involves using giant machines or metals as electrodes. Things such as EEG can be portable, but it is not going to be light. fNIR uses optic fibers, which can be pretty light compared to EEG caps.
Not expensive
There are no expensive metals involved in the device. So if people are going to sell it, the price can be lower than EEG devices which should be affordable for everyone.
Disadvantages of fNIRS
Not sensitive
There must be enough blood flow for the device to detect the difference. Therefore you have to use the brain area “hard” in order for the device to detect it which may not happen frequently.
Can only get the blood flow on the surface
As I said before, lights are going to be scattered by other tissues causing the deeper activities unknown. Even if the light reaches the deeper brain, we can only record the ones that reflect, and that weak signal is going to be buried in the strong signal reflected on the surface. Therefore we can not get a complete understanding of the brain only by using fNIRS.
Not real time
Blood flow is not like neural activities, it takes time to react. As I said in the fMRI article, it takes 1-5 seconds to make the change. The human mind changes swiftly, if the device can’t follow up with the change, the application is going to be limited. Think about if you always have a 5-second delay when you use your phone, it is not going to work.
My Opinion
In fact, the application of fNIRS is still relatively scarce. Its current application is more about detecting a person’s state, such as whether the person is attentive, which part of the brain a person uses when playing a musical instrument, or the recovery of cranial nerves after surgery, etc. These activities put the brain into some kind of status for a long time, that’s when fNIRS comes in. However, the fact that it is very light and can be carried around; the signal will not be interfered with by muscle signals. These characteristics are what BCI technology has been pursuing. Once we can detect more accurate changes in blood flow, this technology will be able to get into people’s lives. The research I was doing is related to increasing the distance of light penetration in biological tissues in a non-invasive way, so there may be opportunities to change the entire ecology of BCI in the future. That’s it for a brief introduction to fNIRS. If you have any questions or supplementary information, you can leave a comment and let me know!
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