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fNIRS + EEG

Validated Integration of High-Quality EEG Recording with fNIRS

fNIRS + EEG with model

Combining functional near-infrared spectroscopy (fNIRS) and electroencephalography (EEG) provides a powerful multimodal approach to studying brain function. While fNIRS captures hemodynamic responses associated with neural activity, EEG measures neuronal electrical potentials with high temporal resolution, offering complementary insights into brain dynamics.

To support multimodal research, integrated systems enable simultaneous acquisition of fNIRS and EEG data within a single experimental setup. Using combined fNIRS + EEG holders, optodes and electrodes can be co-located, improving sensor placement flexibility and allowing for denser spatial coverage.

Although this approach enhances spatial distribution and experimental design, close proximity between fNIRS optodes and EEG electrodes can increase the likelihood of signal interference (crosstalk), which may be observed in EEG recordings. Proper system design and optimized hardware configurations are therefore essential to ensure high-quality multimodal data acquisition.

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Integrated fNIRS + EEG Solution

We offer integrated solutions designed to support seamless combination of fNIRS and EEG technologies in multimodal research environments. These solutions enable simultaneous acquisition of hemodynamic and electrophysiological signals using compatible fNIRS systems and EEG amplifiers within a single experimental setup.

To accommodate a wide range of experimental designs, our integrated headcap system supports multiple holder configurations, allowing researchers to tailor sensor placement based on their specific study requirements. Available holder options include:

Independent
optode holders

Independent optode holders

Independent
EEG ring electrode holders

Independent EEG ring electrode holders

Independent
EEG pin electrode holders

Independent EEG pin electrode holders

Hybrid optode and EEG pin
electrode holders

Hybrid optode and EEG pin electrode holders

Complete Integrated fNIRS + EEG Package

  • Supports up to 54 fNIRS channels and 64 EEG channels
  • Enables simultaneous measurement of cerebral oxygenation and blood volume changes alongside neuronal electrical activity
  • Utilizes a combined fNIRS + EEG headcap with integrated optode and electrode holders, allowing co-located placement of fNIRS optodes and EEG pin electrodes, or the use of separate holders as needed
  • Designed to minimize interference while preserving EEG signal quality
  • Allows flexible measurement from any cortical brain region

This integrated approach provides researchers with the flexibility and data quality required for advanced multimodal brain imaging studies.

Managing fNIRS + EEG Crosstalk in Integrated Systems

Simultaneous acquisition of fNIRS and EEG enables complementary measurement of cerebral hemodynamics and neuronal electrical activity. However, when fNIRS optodes and EEG electrodes are placed in close proximity, there is potential for signal interference, commonly referred to as crosstalk to appear in EEG recordings.

Crosstalk can arise from electromagnetic fields generated by the driving currents used to control fNIRS light sources. These artifacts may distort EEG data, particularly if they fall within frequency bands of interest. While separating fNIRS optodes and EEG electrodes by a small distance can mitigate this effect, many experimental designs require co-located or closely spaced sensors. The following sections summarize crosstalk evidence from the literature and validation testing demonstrating that high-quality, interference-free EEG recordings can be achieved using properly designed integrated fNIRS + EEG systems.

Evidence From the Literature

Rogers et al. (2021), in collaboration with the University of Boston Neurophotonics Center and the Technical University of Berlin, demonstrated that fNIRS optodes and EEG electrodes can be co-located within a shared holder without introducing measurable crosstalk into EEG recordings.

In this study, EEG was recorded using active electrodes with average impedances of 4 ± 1.6 kΩ while fNIRS transmitters alternated between active and inactive states. Spectral analysis showed no detectable peaks at the optode firing frequency (17.4 Hz), indicating negligible interference. EEG recordings obtained with fNIRS enabled were indistinguishable from those collected with fNIRS disabled, supporting the feasibility of shared fNIRS + EEG holders without compromising EEG signal integrity.

Validation Testing in Integrated fNIRS + EEG Configurations

To further evaluate fNIRS + EEG compatibility, we conducted a series of controlled tests using multiple holder configurations, including:

  • Independent fNIRS and EEG holders placed in close proximity.
  • Hybrid holders with EEG pin electrodes co-located on fNIRS transmitters or receivers.

EEG data were recorded using the APEX amplifier while alternating the fNIRS system (Brite) between active (“ON”) and inactive (“OFF”) states. Data were collected in multiple 30-second blocks with eyes-closed resting conditions. Power spectral density was computed using the Welch method, consistent with published methodologies, and analyzed without preprocessing.

To reflect standard research practice and published recommendations, EEG electrode impedances were maintained below 5 kΩ during combined-holder testing.

Independent Holders

When using independent fNIRS and EEG holders placed as close as 30 mm apart, no interference was observed in the EEG signal under any condition. Power spectral density plots showed no peaks at the fNIRS firing frequency (37 Hz), while expected physiological activity such as alpha rhythm around 10 Hz remained clearly visible.

These results demonstrate that both ring and pin EEG electrodes can be used near fNIRS optodes without introducing measurable crosstalk when separate holders are employed.

Crosstalk Analysis
Independent fNIRS and EEG holders placed in close proximity

Corsstalks Analysis Independent Holders fNIRS and EEG holders

Hybrid holders

In hybrid holder configurations, where fNIRS optodes and EEG electrodes are co-located, the theoretical risk of interference is higher due to reduced spatial separation. However, when using optimized fNIRS optodes and shielded EEG cables, no evidence of crosstalk was observed.

Spectral analysis showed no peaks at the fNIRS firing frequency for electrodes positioned over either transmitters or receivers. EEG recordings retained typical physiological features, confirming that signal quality was preserved even in fully integrated configurations.

Crosstalk Analysis
Hybrid holders with EEG pin electrodes co-located on fNIRS transmitters or receivers

Crosstalk Analysis Hybrid holders

Consistent with prior literature, our validation testing demonstrates that interference-free EEG recordings can be achieved across all supported fNIRS + EEG configurations, including co-localized optode and EEG electrode arrangements. Notably, these results were obtained using passive EEG electrodes, further supporting the robustness of the integrated system design.

Additional system features including flexible fNIRS sampling rates and actively shielded EEG cabling available in our devices further reduce susceptibility to electromagnetic interference and prevent overlap with EEG frequency bands of interest.

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Caution! Investigational Device. Federal (or United States) law limits device to investigational use.