Functional Near-Infrared Spectroscopy (fNIRS): Uses and Configurations

Near-infrared light can pass through scalp and skull tissue, while oxygenated and deoxygenated haemoglobin absorb the light differently. Source–detector pairs placed on the head are used to estimate haemodynamic changes in superficial cortical regions. [1,2]

The fNIRS signal is linked to neurovascular coupling—the process by which local neural activity is followed by changes in blood flow and oxygenation—and therefore reflects vascular responses associated with neural activity rather than direct electrophysiological activity. The haemodynamic response develops over seconds, so its temporal resolution is substantially slower than EEG.

Compared with functional MRI, fNIRS can be used in more mobile and natural environments. It has been applied to social interaction, walking, rehabilitation tasks, infant development and other naturalistic research settings. [2,3]

Hyperscanning means recording brain activity from two or more people at the same time while they interact or perform a shared task. It allows researchers to quantify inter-brain coupling metrics, but interpretation requires careful controls for common task structure, chance correlations and systemic physiology. [6]

fNIRS is especially useful when cortical haemodynamics need to be measured outside the constraints of an MRI scanner or during interaction and movement.

Changes in blood pressure, respiration, heart rate and superficial scalp blood flow can contaminate fNIRS signals. Poor optode contact, hair and motion can create additional artefacts. Best-practice guidance therefore emphasizes optode geometry, signal quality, preprocessing, reporting and methods such as short-separation channels, in which source–detector pairs are placed close together to capture mainly superficial scalp physiology or concurrent systemic physiology. [3–5]

This is especially important when claims concern subtle cognitive effects. Accordingly, haemoglobin changes require analysis that separates cortical responses from systemic and extracerebral contributions.

EEG and fNIRS are complementary: EEG captures fast electrical dynamics; fNIRS captures slower haemodynamic dynamics. Simultaneous systems can be valuable when a study needs both, and hybrid EEG–fNIRS BCIs are an active area of development. [7]

The trade-off is complexity. Headgear layout, timing, shared triggers, motion artefacts and analysis pipelines must be designed as one system rather than as two independent devices.

fNIRS systems differ in optical architecture, number and arrangement of sources and detectors, portability and whether short-separation measurements or multimodal integration are available.