Functional Near-Infrared Spectroscopy (fNIRS) is a non-invasive method for observing brain function by measuring changes in blood oxygen levels. This neuroimaging technique is portable, allowing for its use in a variety of settings. It is similar to a sophisticated pulse oximeter but is engineered to monitor these changes within the brain’s outer layers.
How fNIRS Technology Works
fNIRS technology centers on a wearable cap or headband fitted with light emitters and detectors. The emitters are specialized diodes that generate near-infrared light, a spectrum just beyond what the human eye can see. This light is directed onto the person’s scalp.
A property of near-infrared light is its ability to pass through skin, bone, and tissue with relative ease. This allows the light to penetrate the skull and reach the cerebral cortex, the brain’s outermost layer. As the light travels through this brain tissue, it is partially absorbed and scattered by the substances it encounters.
Light detectors on the scalp, adjacent to the emitters, measure the light reflected back from the cortex. Emitter-detector pairs are arranged in an array to map activity across different brain regions. By comparing the intensity of the returned light to the initial light, researchers calculate how much was absorbed by the brain tissue. This measurement process is safe and can be performed while a person is active.
What fNIRS Actually Measures in the Brain
Data gathered by an fNIRS system provides an indirect look at neural function by tracking blood flow. When a brain region increases its activity, its neurons require more oxygen. To meet this demand, the body increases blood flow to that active area, a process known as the hemodynamic response, delivering oxygen-rich blood.
Near-infrared light is sensitive to hemoglobin, the protein in red blood cells that carries oxygen. The two states of this molecule—oxygenated hemoglobin (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb)—absorb different wavelengths of near-infrared light. Oxygenated blood absorbs light with longer wavelengths, while deoxygenated blood absorbs shorter wavelengths.
By using multiple wavelengths of light, the fNIRS system can detect the relative concentrations of both oxy-Hb and deoxy-Hb. An increase in brain activity in a particular region leads to a spike in oxy-Hb and a subsequent dip in deoxy-Hb. Tracking these changes allows researchers to infer which parts of the brain were active, though this is a proxy for neural firing, not a direct measurement of the brain’s electrical signals.
Practical Uses of fNIRS
The portability and non-invasive nature of fNIRS allow for applications outside of traditional labs. Its tolerance for movement makes it useful for research with infants and children. Scientists can study how a baby’s brain processes language or social cues without requiring them to be still.
This technology also allows for the study of brain activity in complex, real-world environments. Researchers have used fNIRS to measure the cognitive workload of individuals performing demanding tasks, such as pilots in flight simulators, surgeons during procedures, or students in a classroom. These studies help in designing better systems and training protocols by understanding the mental demands of such professions.
In clinical settings, fNIRS is used for monitoring brain health and function. It can be applied in neonatal intensive care units (NICUs) to track the cerebral oxygenation of premature infants, helping to detect potential brain injuries. For adults, it is used to assess brain activity during rehabilitation for conditions like stroke or traumatic brain injury. There is also growing interest in its application in psychiatry to investigate the neural patterns associated with various mental health conditions.
Comparing fNIRS to fMRI and EEG
fNIRS is often compared to other techniques like functional magnetic resonance imaging (fMRI). Both fNIRS and fMRI measure the same biological process: the hemodynamic response. However, fMRI does so with much higher spatial resolution, while fNIRS is limited to imaging the cortical surface. The advantages of fNIRS are its portability, lower cost, and tolerance for participant movement, which are major constraints for fMRI machines.
The comparison with EEG highlights different trade-offs. EEG directly measures the electrical activity of neurons, giving it excellent temporal resolution—detecting changes in milliseconds. fNIRS, relying on the slower process of blood flow, has a much lower temporal resolution. However, fNIRS generally offers better spatial resolution than EEG.
These technologies are not necessarily competitors; they are often complementary. Researchers can use fNIRS and EEG together in multimodal studies to get a richer picture of brain function. This approach combines the “when” of brain activity from EEG with the “where” from fNIRS.