Functional near-infrared spectroscopy

Functional near-infrared spectroscopy (fNIRS) is a non-invasive optical imaging technique used to measure brain activity. It operates on the principle that neuronal activation and cerebral blood flow are coupled. When an area of the brain is more active, it consumes more oxygen, and the local cerebral blood flow to that area increases accordingly. fNIRS measures these changes in blood flow by detecting variations in the absorption of near-infrared light by the blood, allowing for the inference of neural activity.

Principle

fNIRS utilizes near-infrared light (typically wavelengths between 650 and 950 nm) which can penetrate the skull and other biological tissues with relatively low absorption. Hemoglobin, the oxygen-carrying component of blood, absorbs near-infrared light differently depending on its oxygenation status: oxygenated hemoglobin (HbO) and deoxygenated hemoglobin (HbR) have distinct absorption spectra in the near-infrared range. By emitting light into the scalp and measuring the light that is scattered back, fNIRS can quantify changes in the concentrations of HbO and HbR, thus providing indirect measures of neural activity.

Applications

fNIRS is used in various research and clinical settings. In neuroscience, it is applied to study cognitive functions, such as language processing, memory, and attention in both healthy individuals and patients with neurological disorders. In psychology, fNIRS is used to investigate social and emotional processes. Its non-invasiveness and portability make it particularly suitable for studying populations that are difficult to examine with other imaging techniques, such as infants and children, or for use in naturalistic settings outside the laboratory.

Advantages and Limitations

One of the main advantages of fNIRS is its non-invasiveness, which allows for repeated measurements over time without exposing subjects to ionizing radiation. It is also relatively inexpensive and portable compared to other brain imaging techniques like functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). However, fNIRS has limitations, including a relatively low spatial resolution and the inability to directly measure neural activity deep within the brain. The technique is also susceptible to artifacts from scalp blood flow, motion, and ambient light.

Technical Aspects

fNIRS systems consist of light sources (LEDs or laser diodes) that emit near-infrared light and detectors (photodiodes) that measure the intensity of light that has traveled through the tissue. The placement of these sources and detectors on the scalp defines the measurement channels, each of which samples a specific region of the underlying cortex. Advances in fNIRS technology, including the development of high-density arrays and the integration with other neuroimaging methods, continue to improve its spatial resolution and utility.

Future Directions

Research in fNIRS technology and its applications is rapidly expanding. Future directions include the development of wearable fNIRS systems for monitoring brain activity in real-world environments, the integration of fNIRS with other neuroimaging and neurostimulation techniques for a more comprehensive understanding of brain function, and the application of machine learning algorithms to fNIRS data for improved analysis and interpretation.