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

The post delves into the synergy between fNIRS (functional Near-Infrared Spectroscopy) and EEG (Electroencephalography) technologies for advancing brain activity research. It highlights the unique strengths of each: fNIRS measures hemodynamic changes, while EEG captures neuronal electrical activity. Through their integration, researchers gain a deeper understanding of brain function, facilitating studies on cognitive processes and neurological disorders. The post emphasizes the benefits of fusion techniques, including enhanced spatial and temporal resolution, and offers examples of their applications in fields like neuroergonomics and neurofeedback.

Introduction

The growing interest in multimodal recording setups in research, fueled by technological advancements, has recently gained significant attention. This approach offers a key advantage: the ability to gather various biosignals in a single study, streamlining research processes across fields like human behavior, BCI (Brain-Computer Interface), and neurorehabilitation.

Several factors influence the selection of technology for establishing a multimodal laboratory, including the specific use case, cost considerations, and device compatibility.

Given that the brain provides rich insights into unconscious emotional responses, measuring associated brain activity is pivotal for understanding human behavior comprehensively. This necessity has led to the development of a bimodal combination of EEG and fNIRS techniques. EEG excels in assessing electrical brain activity, while fNIRS evaluates hemodynamic changes. These distinct physiological processes offer a more comprehensive view of neural activation. Despite their differences, both methods share similarities: they are non-invasive and offer precise monitoring of brain activity.

What makes this combination particularly compelling is its potential to address each other's limitations in spatial and temporal resolution, enhancing overall research outcomes.

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 captures millisecond-level changes in electrical brain activity, offering excellent temporal resolution. 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 excels in temporal resolution, its spatial resolution is limited. fNIRS, on the other hand, offers insights into the spatial distribution of hemodynamic changes in the brain, providing better spatial resolution when combined with EEG.
3. Multimodal Brain Imaging: Combining fNIRS and EEG leverages the strengths of both techniques while mitigating their limitations. This approach enables a more accurate and detailed characterization of brain activity than using either method alone.
4. Localization of EEG Signals: fNIRS supplements EEG by helping to localize the origin of neural activity within the brain. By examining hemodynamic responses associated with electrical signals, researchers can better pinpoint the source of EEG signals.
5. Artifact Identification and Correction: Simultaneous recording of fNIRS data aids in identifying and correcting artifacts in EEG recordings, such as eye blinks, muscle activity or motion artifacts. This improves the quality of EEG data by mitigating various sources of interference.
6. Cognitive and Clinical Insights: Combining fNIRS and EEG offers deeper insights into cognitive processes and clinical conditions. Researchers can examine neural activity from multiple angles, leading to a more comprehensive understanding of cognitive function and brain disorders.
7. Developmental and Clinical Studies: This combination is valuable for studying 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 provides insights into electrical brain activity.
8. Neurofeedback and Brain-Computer Interfaces: fNIRS and EEG integration can be applied to real-time neurofeedback and brain-computer interface applications. This enables increased signal classification accuracy, potentially aiding in therapeutic and training applications.

In summary, combining fNIRS and EEG in experiments offers a 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, leading to an increase in blood flow to those regions.

This increased blood flow results in changes 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.

In this method, near-infrared light with wavelengths between 650 and 1000 nanometers is shone into the scalp. Oxyhemoglobin and deoxyhemoglobin absorb light differently, allowing estimation of their concentrations 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 undergone significant evolution, resulting in various types of fNIRS systems with differences in hardware, methodologies, and applications. Here are some common types:

• Continuous-Wave (CW) fNIRS: Continuous-wave systems emit a steady light source and measure changes in light intensity across multiple wavelengths. They are relatively straightforward and cost-effective but offer limited depth information.
• Time-Domain (TD) fNIRS: Time-domain systems send short light pulses into the tissue and analyze the time it takes for the light to traverse the tissue. By examining the time-of-flight of photons, these systems provide depth information, distinguishing between superficial and deep tissue.
• Frequency-Domain (FD) fNIRS: Frequency-domain fNIRS utilizes modulated light sources and measures phase shift and amplitude changes as light passes through tissue. This technique offers insights into the optical properties of tissue and can differentiate between different chromophores.

Each type of fNIRS technology has its own advantages and limitations, with researchers selecting the most appropriate system based on their research questions and experimental needs. However, continuous-wave fNIRS systems are currently popular due to their accessibility, portability, 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?

Electroencephalography (EEG) is a valuable tool in neuroscience for non-invasively recording electrical brain activity. By distributing electrical sensors on the scalp, EEG captures brain activity through the contact between electrodes and the skin. These sensors detect and amplify the electrical signals produced by the brain's neurons, commonly referred to as "brain waves," offering crucial insights into brain function.

The versatility of EEG extends to various fields, including medicine, research, and brain-computer interfaces. Its applications range from diagnosing neurological disorders to studying cognitive processes and developing innovative technologies.

EEG devices for real-world application

Bitbrain specializes in developing innovative devices with excellent usability for multimodal monitoring, encompassing semi-dry EEG, dry-EEG, and textile-EEG systems, as well as biosignals (ExG, GSR, RESP, TEMP, IMUs, etc.), and eye-tracking solutions (screen-based and mobile platforms). The software tools facilitate the design of experiments, effortless data gathering with over 35 synchronized sensor types, and extensive data analysis covering a broad spectrum of emotional and cognitive biometrics.

Our Versatile EEG 16 and 32 channels is a technology conveived for real-world research. It combines excellent signal quality with user-friendly design, benefiting both researchers and study subjects. Its ergonomic design ensures easy handling for researchers, while providing a comfortable experience for subjects. This not only simplifies the researcher's work but also enhances the comfort of study participants, contributing to smoother research processes.

Combined solution of Cortivision and Bitbrain

Bitrain and Cortivision share a common vision: to democratize neurotechnology by creating portable, user-friendly, and wireless brain monitoring devices, enabling researchers to conduct studies in diverse settings, both within and beyond the laboratory.

While Cortivision focuses on developing fNIRS devices, Bitbrain specializes in EEG equipment. By integrating these technologies, we can capture two distinct physiological processes, leading to a more comprehensive analysis of brain activity than what each technique could achieve individually. This synergy enhances our ability to understand brain function and cognition across various contexts.

Conclusion

In this post, we've conducted a review of fNIRS technology, covering its key technical features, functionality, and the various types available. We've also provided a brief overview of the Photon Cap, a commercial fNIRS device designed for real-world applications. When combined with the Versatile EEG 32ch, it offers a bimodal recording solution capable of capturing brain activity at different levels.

The growing importance of such technology within the scientific community is undeniable, given the benefits it offers. This innovative approach to non-invasive measurement enables researchers to integrate multiple streams of information, yielding new insights and overcoming the limitations of individual techniques. The emergence of equipment combining EEG and fNIRS technologies has already become a reality, with significant studies being conducted across various fields, including human factors and Brain-Computer Interfaces.

At Bitbrain and Cortivision, we're actively developing new hardware based on two of our most popular systems, the Photon Cap and the Versatile EEG. Through their integration, we aim to provide researchers with a comprehensive toolset for studying brain activity at a deeper and more complete level, further advancing the capabilities of neuroscientific research.

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.

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