Functional near-infrared spectroscopy (fNIRS) is a tool used to indirectly measure brain activity by tracing the blood flow in the brain. However, unlike the popular fMRI, which measures the blood’s magnetic properties, fNIRS measures the blood’s optical properties. 

Biophysical foundations
The optical properties of a material refer to how the material interacts with light. The ones that are important for fNIRS are transmission and absorption: For instance, glass transmits light by allowing it to pass through. On the other hand, a material like coal appears dark because it absorbs light of nearly all wavelengths, meaning there is little light coming from the coal for us to see.

These properties become relevant in neuroscience when we consider the infrared wavelength. Infrared light (the IR in fNIRS) is invisible to the naked eye and can pass through human tissue including skin, bone, or brain matter. The important thing to realise is that, just like glass and coal, different kinds of human tissue also absorb and transmit light in their own unique ways. For example, if you sent some infrared light separately through skin (low absorption like glass) and bone (high absorption like coal), you could measure how much light reaches the other side to identify which is which. The weaker of the two light signals (i.e. more absorbed light) must be bone.

Now, in order to use this procedure to measure brain activity, we can use blood cells. Within blood cells there is a protein called haemoglobin that can carry oxygen to wherever the body needs it. Hence, it exists in two states: 1) carrying oxygen (oxygenated haemoglobin) and 2) carrying no oxygen (deoxygenated haemoglobin). Crucially, these two states of haemoglobin absorb different amounts of infrared light, so they can be differentiated using fNIRS. 

The full process looks as follows: When a brain area becomes activated, it requires more oxygen, causing an influx of oxygenated haemoglobin in that area. As a result, the active area will have a higher ratio between oxygenated and deoxygenated haemoglobin. When the ratio changes, we can detect this by measuring with fNIRS how much infrared light got absorbed before the brain area became active (before oxygen inflow) compared to how much is getting absorbed during activation (after oxygen inflow). In summary, what fNIRS measures is the change of the ratio of oxygenated/deoxygenated haemoglobin in blood cells over time by detecting the change in how much infrared light is absorbed.

Practical implementation
Here is how fNIRS works in practice: Similar to how electroencephalography (EEG) uses electrodes on the scalp to measure electricity, fNIRS uses so-called optodes on the scalp to measure light. Optodes are small cylindrical buds that can be either emitters or detectors. An emitter contains an LED sending infrared light through the skin and skull to the brain matter where the blood vessels are located. A detector contains a detection device measuring the intensity of the light that has successfully passed through the tissue between the emitter and the detector without being absorbed along the way (see part of the Figure labelled “fNIRS”). To ensure good coverage of the brain, emitters and detectors are commonly placed on the scalp in a more or less alternating fashion.

(Source: Wikimedia Commons)

Each emitter-detector pair is called a channel and measures one location in the brain. Channels are distributed across the scalp to measure different regions of the brain. For this, they are attached to a cap (depicted below) pressing them tightly to the head to ensure that the light enters directly through the scalp without air or hair in-between. Due to scattering, the light’s path from the emitter to the detector is curved rather than straight, looking more like a banana. Furthermore, the distance between the emitter and detector affects the concrete shape of the light’s path, which is crucial for the depth of the measurement. For instance, if the emitter and detector are too close to each other, the light will travel along a highly curved and very short path, only passing through skin or bone without reaching the brain tissue. In other words, in this case we do not obtain any brain data. If the emitter and detector are too far apart, the light will travel along a weakly curved and very long path that reaches deep into the brain. This case is problematic because the detected signal could have originated at any point along the light’s long trail. In other words, the signal will become spatially unspecific (see spatial resolution).

(Source: Wikimedia Commons)

Strengths and weaknesses
Although fNIRS is less popular than other neuroimaging methods, such as fMRI, it actually has some attractive key strengths. Firstly, starting at around 10,000€, fNIRS sets are fairly affordable compared to an MRI scanner which costs millions of euros. A second major advantage of fNIRS is its portability. Some fNIRS machines are built for portability, meaning all necessary equipment is small enough to be transported in a car trunk, in contrast to the very large MRI machine. Because of this portability, fNIRS is suited well for settings outside the traditional laboratory. Especially because the optodes are pressed very tightly against the scalp, there is a much lower risk of movement interfering with the signal compared to EEG. This overall flexibility allows researchers to measure brain activity in settings where participants move more freely and naturally. This way it becomes possible to study, for instance, the effects of going for a walk around the neighbourhood or of making consumer choices in a real-life supermarket. Interestingly, studies of this kind are only possible because the cap pressing the optodes onto the scalp makes them resistant to slipping. This way, we can be sure that the measured signal comes from the same location at all times. Another use case of fNIRS is to visit patients in their homes, so that they do not have to travel to a research facility to have brain measurements taken. 

The main  drawback of fNIRS is that it can only reach the superficial layers of the brain (~1.5 cm deep into the cortex). Since the optodes have to be in a certain distance from each other, the signal cannot travel to deep subcortical structures, such as the hippocampus, which are more easily accessible with fMRI. Moreover, because optodes must be placed far enough apart to even reach into the brain, each channel represents a sensitivity region in the brain of ~1 cm, which is fairly large. While not as coarse as EEG data, it is still inferior to fMRI’s much better resolution in the range of a few millimetres or even less. With respect to time resolution, being able to detect events on the order of tenths of seconds, fNIRS is in the intermediate range. It is slower than EEG, which resolves events on the order of milliseconds, and faster than fMRI, which in most cases resolves events on the order of seconds. Another interesting consideration is that participants with thick, dark hair are not preferred for fNIRS research. This is because dark hair between the skin and the emitter absorbs more infrared light, which leads to a weaker signal. 

Summary
In summary, fNIRS is a relatively affordable and portable tool that tracks changes in blood oxygenation by measuring infrared light absorption to draw conclusions about brain activity. Its affordability, portability, and suitability for different environments make it an attractive tool for neuroscientists, especially those who want to conduct research outside of the confines of a research lab, like in the homes of (immobile) patients or in everyday situations. It is also possible to combine fNIRS with other methods, such as EEG. This way, fNIRS can compensate for EEG’s very poor spatial resolution and EEG can in turn compensate for fNIRS moderate temporal resolution.

Author: Emil Stroecker