Functional near-infrared spectroscopy (fNIRS) is a non-invasive neuroimaging technique that measures changes in hemoglobin concentration in the brain. It utilizes near-infrared light to penetrate the scalp and skull and monitor the changes in blood oxygenation and volume in the cerebral cortex. fNIRS is often used to study brain function and activity, particularly in cognitive neuroscience and clinical research.
Key features and principles of functional near-infrared spectroscopy include:
Near-infrared light: Near-infrared light in the range of approximately 650 to 1000 nanometers is used in fNIRS. This range of light can penetrate biological tissues, allowing for monitoring of changes in blood oxygenation in the brain.
Hemodynamic response: When neurons in the brain are activated, there is an increase in local blood flow to provide the necessary oxygen and nutrients. This results in changes in the concentrations of oxygenated (oxy-Hb) and deoxygenated (deoxy-Hb) hemoglobin. fNIRS measures these changes to infer neural activity.
Optodes: fNIRS systems consist of pairs of light sources and detectors, known as optodes, which are placed on the scalp. The light sources emit near-infrared light into the tissue, and the detectors measure the light that is transmitted or diffusely reflected back.
Measurement principles: The measurement is based on the principle that oxy-Hb and deoxy-Hb absorb near-infrared light differently. By measuring the intensity of the transmitted or reflected light at multiple wavelengths, it is possible to determine the concentrations of oxy-Hb and deoxy-Hb.
Spatial and temporal resolution: fNIRS provides a moderate level of spatial and temporal resolution. It can detect changes in hemoglobin concentration at the surface of the cortex, making it suitable for studying brain activity associated with cognitive tasks.
Applications: fNIRS has a range of applications, including cognitive neuroscience research, studies of brain development in infants, monitoring brain function during tasks or stimuli, and clinical applications in fields such as neurorehabilitation and psychiatry.
Limitations: While fNIRS is a valuable tool, it has some limitations. The depth of penetration is limited, and the technique primarily captures activity from the outer layers of the cortex. Additionally, it is sensitive to motion artifacts and has lower spatial resolution compared to some other neuroimaging techniques.
fNIRS is considered a portable and relatively user-friendly neuroimaging method, making it suitable for various applications, especially in scenarios where other techniques like functional magnetic resonance imaging (fMRI) might be less practical.