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

With the further development of MRI, functional MRI or fMRI was invented. Like PET, the principle of fMRI is based on the oxygen supply to the brain. This oxygenation takes place by bringing more oxygen-rich blood to a particular brain area. In oxygenated blood, oxygen is bound to haemoglobin. When a certain area is active, oxygen-rich blood is brought to this area within a few seconds. This natural response of the body is called the haemodynamic response, and can be measured using fMRI.

Oxygenated and deoxygenated blood have different magnetic properties, with oxygenated blood being diamagnetic and deoxygenated blood paramagnetic. Oxygen-rich blood gives a high fMRI signal, and deoxygenated blood gives a low fMRI signal. When a lot of oxygen-rich blood flows to a certain brain area, the fMRI signal suddenly becomes stronger. This leads to a typical signal from the brain, the BOLD signal. BOLD stands for Blood Oxygenation Level Dependent, and basically represents the ratio of oxygenated to deoxygenated blood.

There are several characteristics by which the BOLD signal can be recognised.

The signal always starts with a small, initial, dip, which means that there is a reduction in oxygenated blood. This is caused by the fact that the active brain area immediately needs oxygen. However, it takes a few seconds for the extra oxygen-rich blood to reach this brain area.

Approximately three seconds after the start of the activity, the extra blood supply can be seen in the signal, there is extra oxygen-rich blood coming to this brain area. This extra amount is explained by both an increasing blood flow, and an increasing volume of blood flow. So there is more blood coming one after the other, but also more blood at the same time, so the elastic blood vessels expand slightly.

After a while, the brain area has received enough oxygen to perform the task properly, and the oxygen supply will decrease again to the baseline level. The blood vessels, however, need a little more time to return to their original state. Therefore, a lot of deoxygenated blood remains in these vessels for a while. This causes the BOLD signal to drop below the baseline.

Only when the blood vessels have returned to their original state will the BOLD signal be equal to that at rest.

Two types of tasks are distinguished that can be used in fMRI.

In a blocked design, several trials of the same kind are clustered in blocks. First, for example, there is a block in which only numbers are presented as stimuli. This can be followed by a block with only words as stimuli. During the analysis of this design, the average activity during the blocks is compared with each other.

In an event-related design, trials of different conditions are no longer clustered, but offered in a random order. After this, the individual trials of one condition are put together and an average is calculated. This mean is compared to the mean of the other condition. The analysis of event-related designs is basically the same as for ERP. However, one has to take into account that fMRI shows activity much slower than EEG. This can be done by allowing more time between stimuli, so that the signal can return to baseline level. The ideal time interval for this is 12 seconds.

On cognitive tasks, an event-related design is preferred. To begin with, subjects cannot develop a strategy to solve the problem, as they do not know what type of stimulus will occur. In a blocked design, this is possible, because the subjects know what is coming (after all, it is all the same). In addition, in an event-related design, you can easily link the activity to the stimuli offered. This means that the basic activity, or the activity between two trials, is not included in the average. Another advantage of event-related design is that you can separate correct reactions from incorrect reactions. You can therefore calculate the difference between them.

Author: Myrthe Princen (translated by Thomas von Rein)
Images: Marcel Loeffen

Summary

The Fusiform Face Area (FFA) is what enables us to recognise faces. When the FFA becomes damaged, people cannot recognise faces properly: this is called prosopagnosia. The FFA lies at the bottom of the temporal lobe.

Function

The FFA is an area of the brain that is needed to recognise faces. A face has three different components that enable you to recognise it:

1. Character features, for example eyes, nose and mouth.
2. Overall picture, e.g. shape and hairline.
3. General features, e.g. vertical and horizontal lines centred on eyes, mouth and nose.

The FFA reacts equally to all these characteristics. This is also the reason why in some pictures we see a face, while there is actually no face depicted.

People with autism sometimes have difficulty in having social relationships with others. Much research has been done to explain this, for example by looking at activity in the FFA. It turns out that in people with autism the FFA reacts just as strongly to objects as to faces. These people are therefore probably not very good at recognising faces and facial expressions. This makes it difficult for them to link people's characteristics to a face, making social interaction less obvious.

Location

The FFA is located at the bottom of the brain, in the temporal lobe. The FFA on the right side is more developed than the FFA in the left hemisphere.

Fact

The FFA is not yet fully developed when you are born, although babies quickly recognise faces and enjoy looking at them more than anything else. During the first ten years of life your brain gets better and better at recognising familiar faces. This is probably due to the experiences you gain during these years.

Patients

In some people, the FFA becomes damaged and they develop an abnormality called Prosopagnosia. People with Prosopagnosia can no longer recognise faces, even if they are the faces of family members or very close friends. Sometimes people with Prosopagnosia cannot even recognise their own face in the mirror. In order to still know who is facing them, these people use other senses. For example, they can recognise others much more easily by their voice.

Author: Myrthe Princen (translated by Thomas von Rein)

Summary

Frontal Eye Field (FEF) is responsible for controlling your eyes when following a moving object, or looking for a new fixation point.

Function

FEF is involved in visual attention and in moving your eyes. It involves two different kinds of eye movements. The first kind consists of following moving images or objects. The second type of eye movements are voluntary movements to find a new fixation point. These movements take place when, for example, reading a book or looking at a photograph.

Location

The FEF is part of the premotor area (PMA). This is part of Brodmann area 8 and is located between the primary motor cortex and the prefrontal cortex. The FEF works together with the supplementary eye fields (SEF), the intraparietal sulcus (IPS) and the superior colliculus (SC).

Fact:

The FEF has a topographic layout. This means that the representations are arranged according to a retinotopic map. It means that areas that are close together on the retina (the back of the eye) are also close together in the brain.

Patients

When the FEF is damaged, the eyes can no longer be directed to move quickly. It is then slower to scan the environment, reading is slower and it is difficult to follow a moving object.

Author: Myrthe Princen (translated by Thomas von Rein)

The frontal lobe is the anterior brain area, and thus lies anterior to the parietal lobe. The area of the frontal lobe that borders the parietal lobe is the primary motor cortex. This is the most posterior part of the frontal lobe. The frontal lobe is the last part of the brain to be fully developed. This area of the brain continues to grow up to the age of 20.

The frontal lobe is very sensitive to dopamine, the brain's "reward" chemical. Dopamine increases or decreases the amount of sensory information passed from the thalamus to other areas of the brain. Because of this, the frontal lobe is also often associated with motivation, attention, planning and working memory tasks. Processes responsible for executive functioning take place in the frontal lobe. This includes recognising the consequences of certain actions, choosing between two different actions, suppressing socially undesirable reactions and identifying differences and similarities between different situations.

There are many different theories about how the frontal lobe functions. These theories can be divided into four categories:

• Single-process theories, that say that different structures in the frontal lobe are responsible for different processes.
• Multiprocess theories, that assume that different structures in the frontal lobe work together to be able to function
• Construct-led theories, that attempt to explain all frontal lobe functions by a single mechanism, such as working memory.
• Single-symptom theories, that suggest that a specific frontal lobe dysfunction is related to underlying structures

Damage to the frontal lobe can have many consequences, such as problems with flexibility and spontaneity. In addition, this damage can lead to extreme risk-taking and increased or decreased talking. Creativity and sexual feelings can also be diminished when there is damage 

Author: Myrthe Princen (translated by Pauline van Gils)

Image: Marcel Loeffen