Functional near infrared spectroscopy (fNIRS)

Optical principles underlying fNIRS:

  • Spectroscopy refers to the study of absorption and emission of light and other radiation

  • Bio spectroscopy applies to the principle of analysis of biological tissue, to generate a molecular fingerprint

  • Near infrared spectroscopy (NIRS) is an instance of bio spectroscopy

  • NIRS involves the emission of a light of a specific wavelength on biological tissues

  • Infrared lights provide an optical window into the brain

  • Human tissues have low absorption of near infra red lights (650-1000nm) which can travel through the skull and reach the cerebral cortex

  • The light is partly absorbed and partly reflected, detectors located nearby allow to identify light attenuation (or changes in optical density)

  • Light attenuation is the reduction in light intensity as it travels through a medium

  • NIRS spectroscopy relies on the distinct absorption spectra of oxygenated hemoglobin (hbO) and reduced or deoxygenated hemoglobin (Hb) to measure oxygenation

  • The isobestic point (the circle in the middle), 805nm, at this specific wavelength hbO and Hb absorb light equally

  • Two or more wavelengths are used, one below and one above the isosbestic point to accurately distinguish between HbO and Hb

  • Minimizing interference, using these wavelengths avoids high absorption of melanin, (400-700nm) and water (>1000nm), ensuring the signal is dominated by hemoglobin changes rather than other components

Physiological principles underlying fNIRS:

  • The left side is the resting state, light is sent into the head via near-infrared light into the scalp

  • Light travel through brain tissue, some gets absorbed, some gets scattered and some returns to the detector

  • At rest, blood flow is normal and there is a mix of HbO and Hb

  • Deoxygenated hemoglobin (Hb) absorbs more light

  • The right side is an active state, the neurons become active

  • Neurons need more oxygen, when brain cells work harder, they use up more oxygen and this increases oxygen demand

  • Neurovascular coupling happens, this is when active neurons signal nearby blood vessels to increase blood flow

  • Therefore blood flow increases and more oxygenated blood arrives

  • Oxygenated hemoglobin increases, oxygenated hemoglobin absorbs less light than deoxygenated hemoglobin-

  • Therefore more light returns to the detector and the detected light intensity increases

  • The device measures this increase

fNIRS device:

  • The system consists of optodes (sources and detectors) placed 3-5cm apart on the scalp to non invasively measure cortical hemodynamic responses

  • They detect changes in near infrared light absorption, primarily by hemoglobin in cortical blood, to map brain activity.

fNIRS as an alternative to fMRI:

  • fMRI is highly sensitive to motion, even small head movements can distort the signal

  • This makes it particularly difficult to use with young children, infants or clinical populations who may struggle to remain still

  • Some individuals also cannot undergo MRi due to claustrophobia or implanted medical devices

  • fNIRS can be portable, is quiet and far more tolerant to movement, the use of a cap allows participants to sit upright, interact with others and even move around in natural settings for testing

  • Makes fNIRS valuable for developmental research, studies with infants and toddlers work with clinical populations who are not suitable for fMRI

  • fNIRS has expanded neuroimaging into populations and settings that were previously difficult or impossible to study with traditional MRI methods

  • However, fNIRS shows reduced sensitivity in individuals with darker skin or thick hair, the light is absorbed or scattered before it can penetrate the scalp and reach underlying cortex

Principles of fNIRS:

  • Depth of penetration is related to source detector distance (longer distance = deeper penetration)

  • Increased distance leads to poorer signal to noise ratio (SNR)

  • The detected intensity is weaker, measurement variability becomes larger and the noise becomes more prominent

  • Examples of layout:

Example of fNIRS hemodynamic response:

  • The flashing checkboard task, aims to produce a strong, reliable and well localised activation of the early visual cortex

  • Flashing checkboard strongly activates the visual cortex

  • Neural activity increases

  • Brain cells use more oxygen

  • The body sends more blood to that area

  • This creates a measurable blood flow change (hemodynamic response)

  • fNIRS measures 2 things in the blood:

  • Hbo2 (oxygenated haemoglobin) and Hb (deoxygenated haemoglobin)

  • When a brain becomes active, Hbo2 increases and Hb decreases

Graph A:

  • Hbo2 and Hb going up and down repeatedly as the checkerboard flashes on and off

  • Hbo2 rises during stimulation, Hb drops

Graph B:

  • Average response across trials

  • Blood response is slow and peaks at about 5-16 seconds after stimulus starts

  • Returns to normal after the stimulus stop

  • The brain reacts quickly but blood flow changes more slowly

Graph C:

  • The coloured dots show where activation is the strongest

  • Warmer colours (yellow/red) = more Hbo2 increase

  • Cooler colours (blue) = Hb decreases

  • Shows the activation is localised to the visual cortex which is what we expect

fNIRS hemodynamic response vs BOLD fMRI:

  • Both measure blood changes caused by neural activity just in different ways

  • A visual pattern (black and white radial grid) activates the occipital cortex (visual brain area)

  • When neurons activate, they use oxygen, the brain overcompensates by sending extra oxygenated blood

  • This causes a rise in HbO and a drop in Hb

  • The graph shows that BOLD signal is positively correlated with HbO and anti correlated with HB

  • When HbO increases, BOLD increases too

  • When Hb decreases, BOLD increases

  • BOLD behaves like HbO but opposite to Hb

  • BOLD fMRI is mainly sensitive to deoxygenated haemoglobin (Hb)

  • HbR is slightly magnetic.

  • When HbR decreases: there is less magnetic distortion

  • The MRI signal increases

  • So BOLD goes up:

  • Neural activity → ↓ HbR → ↑ BOLD signal

  • FNIRS can be used in natural settings to record cortical activity during ongoing tasks:

  • Participants wearing a fNIRS system engaged in a diversity of tasks while activity of the frontal cortex was recorded.

  • In the Mirelman et al. (2014) experiment, the tasks were:

  • 1) Walking

  • 2) Walking and Counting

  • 3) Walking and Subtracting in 7s

  • 4) Standing and Subtracting in 7s

  • Higher activation was observed when participants carry out dual task: walking & counting and walking and subtracting.


fNIRS advantages:

  • Better spatial resolution than EEG, better temporal resolution than MRI

  • Safe

  • Tolerant to motion

  • Portable

  • Low cost

  • Silent

  • Suitable for long periods of continuous monitoring

  • Compatible with other electrical and magnetic devices

fNIRS disadvantages:

  • Penetration depth approx 1.5-2cm

  • Lower temporal resolution than EEG, lower spatial resolution than MRI

  • Systemic interferences

  • SNR variable

  • Poor signal quality for participants with dark or thick hair

  • Lack of standardization in data analysis

  • Acquisition of structural/anatomical images not possible

fNIRS as a neurostimulator technique:

  • Waight et al. (2023): standard fNIRS device could change brain function when turned on.Healthy adults completed several cognitive tasks (Stroop, backwards counting, delayed match-to- sample) while wearing the fNIRS over the prefrontal cortex (PFC).

  • The device was on (projected near infrared lights) in the experimental but not in the control condition.

  • The experimental group, wearing the switched-on device, showed faster reaction times and some accuracy improvements, especially in executive-function tasks.

  • How would NIR lights enhance cognitive function?

  • Near-infrared light can pass through the skull and is absorbed by an enzyme inside neurons called cytochrome c oxidase, located in mitochondria.

  • This increases cellular energy production (ATP) and nitric oxide release, improving blood flow.

  • This seems to enhance neuronal metabolism, improve efficiency of neural networks and lead to faster cognitive performance