Two signals, two relationships to brain activity
EEG measures rapidly changing scalp potential differences. fNIRS estimates oxygenated and deoxygenated haemoglobin changes from optical measurements. Its haemodynamic response develops more slowly. A high sampling rate does not turn that response into an instantaneous electrical measurement.
fNIRS is sensitive to superficial tissue and contact conditions. EEG is also sensitive to ocular, muscular and movement artefacts. Both require interpretation in the context of the protocol and planned controls.
Compare methods before devices
| Dimension | EEG | fNIRS |
|---|---|---|
| Signal | Scalp potential differences | Absorption changes associated with HbO and HbR |
| Timing | Fast electrical dynamics | Slower haemodynamic response |
| Setup | Electrodes and contact quality | Sources, detectors and optical coupling quality |
| Challenges | Electrical, ocular and muscular artefacts | Movement, hair and superficial contributions |
| Spatial analysis | Depends on setup and models used | Depends on optode arrangement and channel sensitivity |
Start with the effect you want to observe
For the timing of responses to a stimulus, examine EEG requirements first. For a cortical haemodynamic response during a task, fNIRS may be appropriate. This guides discussion but does not replace a review of methods in your field.
Also consider movement, duration, participant numbers, repeated sessions and team resources. Portable systems must remain stable during the task. Studies with children need appropriate fitting, preparation and procedures.
What simultaneous EEG–fNIRS acquisition involves
The first challenge is sharing scalp space. Electrodes and optodes must be positioned without compromising contact, and locations documented. The setup must remain acceptable to the participant throughout recording.
The second challenge is timing. Both modalities need identifiable events and a controlled time base. Acquisition rates may differ; comparing file rows is insufficient. Define in advance how signals will be analysed separately and then related.
Prepare a useful pilot session
- Choose a representative task rather than only a resting test.
- Check contact and coverage over regions of interest.
- Record events and monitor data loss.
- Examine each modality's signals before calculating relationships.
- Keep settings, positions and observations needed to reproduce the session.
Define an expandable configuration
For an initial installation, separate essential needs from potential future requirements. Expansion must be checked against the actual model and interfaces purchased. “Multimodal” does not guarantee the same accessories suit every setup.
The pages for fNIRS systems and EGI high-density EEG present equipment families. To connect acquisition to experiment software, see our synchronisation and StreamLab guide.
Sources and documentation
Technical documentation and further reading.