EEG + NIRS - The Electrical Brain and the Hemodynamic Brain Do Not Change on the Same Clock

Imagine that a word appears on a screen.

Before I can even consciously say what it means, neuronal populations have already changed their electrical activity.

A few seconds later, another kind of change begins to emerge.

Local blood flow reorganizes. Oxygen delivery and consumption change. The relative concentrations of oxygenated and deoxygenated hemoglobin shift.

It is the same event.

But it is not the same clock.

Perhaps this is exactly why combining EEG and fNIRS is so interesting for W40/2026.

One technique allows us to observe changes that occur very close to the speed of electrophysiological activity.

The other follows a hemodynamic response that unfolds more slowly.

Between the two, there is a transition.

And perhaps within that temporal difference we can find part of what a stable state alone cannot reveal.

EEG encounters change early

EEG records differences in electrical potential at the scalp generated mainly by synchronized activity across large neuronal populations.

It does not “listen to a neuron thinking.”

But it can track electrophysiological changes on the scale of milliseconds.

Oscillations in theta, alpha, beta, and other frequency bands can change rapidly with attention, effort, sleep, fatigue, movement, or sensory processing.

If a transition begins now, EEG can capture part of that change very early.

It is like asking:

what is changing in electrical organization while the event is still unfolding?

This temporal resolution is one of the reasons research-grade EEG systems prioritize low-noise amplification, high sampling rates, precise synchronization, and flexible montages.

The Brain Products actiCHamp platform, for example, supports configurations of up to 160 channels, active and passive electrodes, auxiliary channels, and high sampling rates. The BrainAmp family can also integrate signals such as ECG, EMG, respiration, and EDA.

These characteristics are not merely specifications.

They make it possible to observe the brain together with part of the physiology accompanying the experience.

fNIRS finds another part of the same story

fNIRS asks a different question.

Near-infrared light travels through superficial tissues and allows us to estimate relative changes in the concentrations of HbO, oxygenated hemoglobin, and HbR, deoxygenated hemoglobin, in cortical regions.

When neural activity changes, metabolic demand, blood flow, and blood volume can also change through neurovascular coupling.

But this does not happen instantaneously.

Neural activity may change within milliseconds, while the hemodynamic response usually emerges later and can reach its peak several seconds after the beginning of an event.

So an increase in HbO is not simply:

“the EEG a few seconds later.”

And a decrease in HbR is not a delayed photograph of electrical activity.

Between neural activity and vascular response there are cells, metabolism, blood vessels, chemical mediators, pressure, respiration, CO₂, and other systemic influences.

Neurovascular coupling is a relationship.

Not a copy.

Two clocks can reveal the transition

This becomes clear in one of the studies selected for W40/2026.

Hamann and Carstengerdes progressively increased mental workload during a simulated flight while simultaneously recording EEG and fNIRS.

Increasing task difficulty was accompanied by greater frontal theta activity and reduced HbR in prefrontal regions.

More interestingly, the two modalities did not show exactly the same sensitivity. fNIRS distinguished some of the lower workload levels more effectively, while EEG showed greater sensitivity at higher levels.

The study is among the multimodal works selected for W40/2026.

Perhaps this is the fundamental advantage:

when two different signals respond differently to the same perturbation, the difference itself also contains information.

We do not need to immediately decide which one is “right.”

We can ask:

which part of the transition is each modality able to see?

When the electrical signal changes but hemodynamics does not follow in the same way

In the authors’ later study, participants completed 90 minutes of simulated flight.

Subjective fatigue increased while performance remained relatively stable.

EEG showed clear changes: parietal alpha increased early and then plateaued, while frontal theta rose more gradually throughout the task.

fNIRS, however, did not show an equally consistent progression in cortical oxygenation.

This study is also part of the W40/2026 collection.

It would be easy to interpret this as:

“EEG worked; fNIRS did not.”

But perhaps that conclusion is too quick.

The result may instead remind us that electrical activity and hemodynamic responses are not required to follow the same temporal trajectory.

Electrophysiological change may become evident while the vascular response remains more heterogeneous.

And this matters particularly in a series devoted to transitions.

Disagreement between clocks may also be data.

HbO and HbR are not simply two colors on a graph

Another risk appears when we reduce fNIRS to:

HbO goes up = activation.
HbR goes down = activation.

That pattern is common, but it is not a universal law.

The response depends on region, task, physiological state, and signal quality.

The scalp and extracerebral circulation also alter the detected light.

Respiration, blood pressure, heart rate, and CO₂ may all contribute to the measured signal.

This directly reconnects Blog 10 with Blogs 8 and 9.

Respiration does not disappear when we place an fNIRS cap on the head.

Homeostasis does not disappear when we place EEG electrodes.

We are measuring a Body-Territory, not a brain isolated from its physiology.

This is why short-separation channels matter

One important development in contemporary fNIRS systems is the use of short-separation channels.

Rather than placing every detector at distances intended to sample cortical tissue, some channels use much shorter source-detector distances to capture predominantly superficial changes.

This helps estimate what comes from the scalp and extracerebral tissue so that it can be better separated from the cortical component.

The NIRSport2 from NIRx supports short-distance detectors of approximately 8 mm, probe accelerometry, wireless or stand-alone acquisition, synchronization through LSL or physical triggers, and scalable configurations that can extend to high-density arrangements. Depending on configuration, sampling rates can reach 240 Hz.

The Artinis Brite family likewise supports flexible interoptode distances, short-separation measurements, inertial sensors for movement, and montages designed to be combined with EEG.

These characteristics matter because good multimodal measurement depends less on simply “having many sensors” than on being able to distinguish:

which change belongs to what?

Placing EEG and fNIRS on the same head is not enough

There is another problem.

Synchronization.

Imagine that EEG detects a change 300 ms after a stimulus, while the corresponding marker in the fNIRS recording was inserted with an unknown delay.

We may end up interpreting technological latency as physiology.

This is why latency and jitter remain central problems in synchronized multimodal recordings.

Mature platforms therefore invest heavily in common triggers, LSL, event synchronization, and physical integration of sensor montages.

Brain Products documents mobile configurations in which LiveAmp can be combined with both the NIRSport2 from NIRx and the Brite systems from Artinis, including strategies for mounting and synchronizing experimental markers.

NIRx likewise presents NIRSport2 combined directly with LiveAmp as a solution for concurrent EEG-fNIRS recording.

Artinis provides headcaps and mounting solutions designed so NIRS optodes and EEG electrodes can share the same physical space and emphasizes the need to synchronize EEG, fNIRS, and the experimental paradigm.

These details help explain why certain platforms repeatedly appear in advanced studies:

quality is not only sensor sensitivity; it is also the ability to let different techniques observe the same event with a known temporal relationship.

When both measurements change together

Shoaib and colleagues offer another example.

In sleep-deprived participants during simulated driving, the researchers simultaneously recorded EEG and fNIRS.

Greater fatigue was associated with lower beta power and lower HbO, together with changes in HbR, heart rate, and reported sleepiness. Exposure to blue light was associated with increases in beta activity and HbO under the conditions investigated.

The study used BrainAmp for EEG and NIRScout for fNIRS and is also part of the W40/2026 selection.

Here electrical, hemodynamic, and physiological signals pointed in convergent directions.

But convergence does not mean identity.

They remain different perspectives on the same process.

The coupling itself may also change

Perhaps the most interesting question is not only:

how much did EEG change?

or:

how much did HbO change?

But:

did the relationship between them change?

A 2024 study combined EEG, fNIRS, and transcranial Doppler together with blood pressure, capnography, and heart rate to investigate neurovascular coupling during motor and visual tasks.

Recent reviews of EEG-fNIRS fusion are also beginning to treat this relationship as something that can be modeled rather than simply placing two curves side by side.

This is especially important for W40/2026.

Perhaps a transition is not only one variable changing.

It may be the relationship between variables changing.

One event, two clocks

Within Triple-Aspect Monism, we do not need to turn EEG into “matter” and fNIRS into “energy,” nor create separate worlds for each signal.

We are observing aspects of the same living organism through different instruments.

EEG encounters electrophysiological organization.

fNIRS encounters part of the vascular and metabolic consequences associated with that activity.

Respiration participates.

Circulation participates.

Movement participates.

Feeling participates.

A single event can produce signals that arrive at our instruments on different timescales.

Perhaps, then, the W40/2026 question needs to change once again.

Not only:

“did the brain change?”

But:

which part of the change appeared first, which came later, and what happened to the relationship between them during the transition?

EEG and fNIRS together are especially powerful precisely because they do not tell the same story on the same clock.

If they did, perhaps we would not need both.

It is in the difference that the transition becomes visible.

Commented References

Li, R., Yang, D., Fang, F., Hong, K.-S., Reiss, A. L., & Zhang, Y. (2022). Concurrent fNIRS and EEG for Brain Function Investigation: A Systematic, Methodology-Focused Review. Sensors, 22, 5865. DOI: 10.3390/s22155865.
What this reference represents: a review of 92 concurrent EEG-fNIRS studies. It establishes the central complementarity of Blog 10: EEG provides high temporal resolution, while fNIRS adds hemodynamic information and comparatively better cortical spatial localization.

Yeung, M. K., & Chu, V. W. (2022). Viewing neurovascular coupling through the lens of combined EEG-fNIRS: A systematic review of current methods. Psychophysiology. DOI: 10.1111/psyp.14054.
What this reference represents: shows that EEG-fNIRS is especially well suited to investigating neurovascular interactions, while emphasizing that interpretation depends on methodological quality in acquisition and integration. Coupling cannot be inferred simply because two curves were recorded together.

Hamann, A., & Carstengerdes, N. (2022). Investigating mental workload-induced changes in cortical oxygenation and frontal theta activity during simulated flights. Scientific Reports, 12, 6449. DOI: 10.1038/s41598-022-10044-y.
What this reference represents: EEG and fNIRS distinguished levels of mental workload with different sensitivities. Two modalities can reveal different parts of the same cognitive transition.

Hamann, A., & Carstengerdes, N. (2023). Assessing the development of mental fatigue during simulated flights with concurrent EEG-fNIRS measurement. Scientific Reports, 13, 4738. DOI: 10.1038/s41598-023-31264-w.
What this reference represents: EEG showed a clear progression of fatigue while fNIRS produced less consistent results. The absence of identical trajectories is part of the message: electrophysiology and hemodynamics do not need to change on the same clock.

Shoaib, Z., Akbar, A., Kim, E. S., et al. (2023). Utilizing EEG and fNIRS for the detection of sleep-deprivation-induced fatigue and its inhibition using colored light stimulation. Scientific Reports, 13, 6465. DOI: 10.1038/s41598-023-33426-2.
What this reference represents: provides a case in which EEG, HbO/HbR, and physiological measures converged in the characterization of alertness and fatigue states. Multimodality allows us to look for convergence without assuming equivalence.

Chen, J., Yu, K., Bi, Y., Ji, X., & Zhang, D. (2024). Strategic Integration: A Cross-Disciplinary Review of the fNIRS-EEG Dual-Modality Imaging System. Brain Sciences, 14, 1022. DOI: 10.3390/brainsci14101022.
What this reference represents: a contemporary review of the applications and technical challenges of EEG-fNIRS integration. The advantage lies not simply in adding signals, but in exploring spatial, temporal, and physiological complementarity.

Physiological influences on neurovascular coupling: A systematic review of multimodal imaging approaches and recommendations for future study designs (2024). Experimental Physiology.
What this reference represents: shows that blood pressure, blood gases, heart rate, exercise, and other physiological variables remain insufficiently controlled in many studies. The hemodynamic brain is never isolated from systemic physiology.

Multimodal fNIRS–EEG sensor fusion: Review of data-driven methods and perspective for naturalistic brain imaging (2025). Imaging Neuroscience. DOI: 10.1162/IMAG.a.974.
What this reference represents: discusses multimodal fusion, HD-fNIRS, artifact correction, and current limitations, including the underuse of short-separation channels in part of the literature. The future is not merely recording two signals; it is modeling the relationship between them.

Brain Products — actiCHamp, BrainAmp and EEG-fNIRS resources.
What this reference represents: official specifications document scalable amplification, active and passive electrodes, high sampling rates, auxiliary channels, and dedicated resources for integrating EEG with both NIRSport2 and Artinis Brite systems. In transition studies, synchronization, electrophysiological quality, and peripheral physiology integration are part of the method, not accessories.

NIRx — NIRSport2 and Concurrent fNIRS-EEG.
What this reference represents: documents scalability, short-distance channels, motion sensors, trigger/LSL synchronization, mobile operation, and EEG integration. These features directly address central fNIRS challenges: extracerebral physiology, movement, and multimodal temporal alignment.

Artinis Medical Systems — Brite and fNIRS-EEG integration.
What this reference represents: documents flexible interoptode distances, short-separation channels, integrated IMUs, EEG compatibility, and combined physical montages. The quality of a multimodal recording also depends on being able to place two sensor systems over the same cortical territory without losing the experimental question.

W40/2026 - Latent Heat, Homeostasis and Metanoia