Fusion of fNIRS and EEG: a step further in brain activity research

Introduction

The increasing relevance of multimodal recording scenarios in research as technology advances has recently become a hot topic.

The most significant advantage is the ability to record different biosignals in a single study so that complete research can be carried out in less time. This is marking a turning point in the study of fields such as human behavior, BCI, and neurorehabilitation.

The factors that influence the choice of technology to create a multimodal laboratory are the use case, the cost, and the compatibility of the devices.

The brain is the organ that provides the most information about the unconscious emotional responses of human beings. Being able to measure the brain activity associated with these responses is fundamental to better understanding aspects of human behaviour.

For this reason, the bimodal combination of EEG and fNIRS techniques has recently been developed. On one hand, EEG is the most common method for assessing electrical activity in the brain. On the other hand, the fNIRS technique evaluates changes in the hemodynamic activity of the brain. These are two completely different physiological processes, which allows for a more complete representation of brain activity. As well as having differences, they also have similarities: both are non-invasive and able to accurately monitoring brain activity.

But the most important factor on why this combination is so interesting is that they have the potential to compensate for each other's limitations in terms of spatial and temporal resolution.

What are the benefits of synchronizing EEG and fNIRS?

Combining functional near-infrared spectroscopy (fNIRS) with electroencephalography (EEG) in experiments offers several advantages, making it a powerful approach for studying brain function and cognition. This combination provides complementary information about brain activity and can enhance the overall understanding of neural processes. Here are some key reasons to combine fNIRS with EEG in experiments:

1. Improved Temporal Resolution: EEG is known for its excellent temporal resolution, allowing researchers to capture millisecond-level changes in electrical brain activity. By integrating EEG with fNIRS, which provides relatively slower but spatially precise information, researchers can obtain a more comprehensive picture of the timing of neural events.
2. Enhanced Spatial Resolution: While EEG provides high temporal resolution, its spatial resolution is limited. It detects electrical activity on the scalp's surface but doesn't offer precise localization of brain activity. fNIRS, on the other hand, can provide information about the spatial distribution of hemodynamic changes in the brain, offering better spatial resolution
3. Multimodal Brain Imaging: Combining fNIRS and EEG allows researchers to take advantage of the strengths of both techniques while mitigating their individual limitations. This multimodal approach enables a more accurate and detailed characterization of brain activity than using either method alone.
4. Localization of EEG Signals: EEG alone often cannot precisely localize the source of neural activity within the brain due to the so-called "inverse problem." fNIRS can provide supplementary information that helps to better localize the origin of EEG signals by offering insights into the hemodynamic responses associated with those electrical signals.
5. Artifact Identification and Correction: EEG recordings can be susceptible to various artifacts, such as eye blinks, muscle activity, and electrical interference. By simultaneously recording fNIRS data, researchers can use the hemodynamic measures to help identify and correct these artifacts in the EEG signal, improving data quality.
6. Cognitive and Clinical Insights: Combining fNIRS and EEG can provide deeper insights into cognitive processes and clinical conditions. For example, in studies of cognitive load, working memory, or brain disorders, researchers can use both techniques to examine neural activity from multiple angles, leading to a more comprehensive understanding of the underlying mechanisms.
7. Developmental and Clinical Studies: This combination is particularly valuable in studies involving infants, children, and clinical populations. fNIRS is well-suited for use with these groups due to its non-invasiveness and tolerance for motion, while EEG offers insights into electrical brain activity. Together, they can be used to study brain development, cognitive development, and neurological disorders.
8. Neurofeedback and Brain-Computer Interfaces: Combining fNIRS and EEG can be applied to real-time neurofeedback and brain-computer interface applications. The combination allows increased signal classification accuracy, helping individuals to control their brain activity in real-time, with potential therapeutic and training applications.

In summary, combining fNIRS and EEG in experiments offers a powerful and versatile approach to studying brain function, particularly when researchers require both high temporal resolution and improved spatial resolution. This multimodal approach has applications in various fields, including cognitive neuroscience, clinical research, neurorehabilitation, and brain-computer interface development.

What is functional Near-Infrared Spectroscopy?

Functional near-infrared spectroscopy (fNIRS) is a non-invasive neuroimaging technique used to measure brain activity by detecting changes in hemoglobin concentration in the blood. When neurons in the brain become active, they require more oxygen and glucose. Blood flow to the active regions of the brain increases to meet this demand.

As a result, there is a change in the concentration of oxyhemoglobin (HbO) and deoxyhemoglobin (HbR) in the blood in those areas. fNIRS detects these changes and provides a measure of localized brain activity. 

Near-infrared light (wavelengths between 650 and 1000 nanometers) is shone into the scalp in this method. HbO and HbR absorb light differently, so it's possible to estimate the concentrations of these two types of hemoglobin by measuring the amount of light absorbed and scattered after passing through the brain tissue. 

Types of fNIRS technology

Functional near-infrared spectroscopy (fNIRS) technology has evolved over the years, leading to the development of various types of fNIRS systems. These systems differ in terms of their hardware, methodologies, and applications. Here are some of the common types of fNIRS technology:

• Continuous-Wave (CW) fNIRS:
  •  Continuous-wave fNIRS systems emit a continuous light source and measure the changes in light intensity at multiple wavelengths. They are relatively simple and cost-effective but provide limited depth information.
• Time-Domain (TD) fNIRS:
  • Time-domain fNIRS systems send short pulses of light into the tissue and measure the time it takes for the light to travel through the tissue. By analyzing the time-of-flight of photons, these systems can provide depth information and help distinguish between superficial and deep tissue.
• Frequency-Domain (FD) fNIRS:

  • o   Frequency-Domain fNIRS utilizes modulated light sources and measures the phase shift and amplitude of the light as it passes through the tissue. This technique provides information about tissue's optical properties and can help distinguish between different chromophores.

Each type of fNIRS technology has advantages and limitations, and researchers choose the most suitable system based on their research questions and experimental requirements (Fig. 1.). However, it can be argued that CW-fNIRS are currently the most popular systems on the market due to their accessibility, mobility, and cost-effectiveness

Fig 1. CW-NIRS gives us information about the relative changes in oxygenated and deoxygenated hemoglobin concentrations. However, its disadvantage is its inability to calculate the optical properties of tissue, absorption, and scattering coefficient, making it impossible to obtain absolute values of HbO and HbR. The TD-NIRS and FD-NIRS, on the other hand, provide absolute measurements of these two chromophores, but the devices itself is large, expensive, and not portable. As a result, it is mainly used in medical or clinical applications. 

fNIRS devices for real-world application.

The Cortivision Photon Cap device is an example of a Continuous-Wave (CW) fNIRS that is fully mobile and allows non-invasive measurement of the activity of selected areas on the cerebral cortex. It can be integrated with other devices using the Lab Streaming Layer protocol.

What is EEG?

EEG is a valuable tool used in the field of neuroscience to record electrical brain activity in a non invasive way. This brain activity can be measured thanks to the distribution of electrical sensors on the scalp. These sensors can pick up and amplify the electrical signals generated by the brain's billions of neurons, thanks to the contact between their electrodes and the skin. These electrical signals, often referred to as "brain waves," provide valuable insights into brain function. This information can be adapted to perform in different fields, as medical, research, brain to computer interfaces... If you want to explore this technique in more depth, you can visit this post: What is EEG and what is it used for? | Bitbrain 

EEG devices for real-world application

We are a company specializing in the development of EEG technology. What we seek with our devices is the combination of two important factors:

• Practicality: they are wireless devices that allow controlled movement of the subject.
• Convenience: they are lightweight devices that ensure good contact between the sensors and the scalp without resorting to gels or saline solutions, by simply using tap water.

Our Versatile EEG 16 and 32 channel equipment is an example of an EEG measurement headset that brings together all these characteristics. While maintaining an excellent signal quality, its design allows for an easy handling from the researcher, as well as a comfortable experience for the subject, making the researcher's life easier and the study subject's life more comfortable.

Combined solution of Cortivision and Bitbrain

Bitrain and Cortivision are companies focused on developing technologies that capture brain activity.

We have the same vision: to bring neurotechnology closer to society by developing portable, simple, and wireless equipment, allowing researchers to carry out studies both inside and outside the laboratory.

Cortivision specializes in developing fNIRS devices while we are specializing in EEG equipment. The combination of these technologies allows us to measure two different physiologycal processes, making it possible to analyze brain activity in a more complete manner than if these techniques were used separately.

Conclusion

In this post we have made a review of the fNIRS technology: its main technical features, what it is, and the different types of fNIRS and what they measure. We also provided a brief description of the Photon Cap, a comercial fNIRS device for real-world applications that, combined with Versatile EEG 32ch, offers a bimodal recording solution that can capture brain activity in different levels.

It is undeniable that this type of technology is becoming increasingly important for the scientific community because of the benefits that it offers. This new way of non-invasive measurement will allow researchers to combine more streams of information, thus getting new insights and reducing the limitations of their separate uses. Equipment that combines EEG and fNIRS technology is already a reality and relevant studies are being carried out with it in different fields, such as human factors, Brain-Computer Interfaces...

From Bitbrain and Cortivision we are working in a new hardware based on two of our most popular systems, the Photon Cap and the Versatiele EEG, whose integration combines both techniques and will allow studying brain activity at a more complete and deeper level.

The combination of fNIRS and EEG is one of the many solutions developed in recent years for multimodal research. This post "Beyond Research Horizons: The Synergy of Technologies in Multimodal Labs" explores deeply the value of multimodal recording and the combination of eye tracking, EEG, and other technologies in research studies.

Bibliography

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