Functional Near-Infrared Spectroscopy (fNIRS) is a non-invasive brain imaging technique that measures brain activity. It uses near-infrared light to estimate changes in cortical hemodynamic activity, observing how blood flow and oxygenation in the brain change in response to different tasks or stimuli. This allows researchers and clinicians to gain insights into brain function.
The Science Behind fNIRS
fNIRS operates by utilizing near-infrared light, typically within the 700 to 900 nanometer spectral range, which can penetrate biological tissues like skin, skull, and brain tissue. This light interacts with chromophores, primarily oxygenated hemoglobin (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb). The different absorption properties of oxy-Hb and deoxy-Hb allow fNIRS to detect changes in their concentrations.
The foundation of fNIRS measurements lies in neurovascular coupling. This principle describes the link between increased neuronal activity and a subsequent localized increase in cerebral blood flow and oxygenation. When neurons become active, they require more oxygen and nutrients, leading to a rapid increase in local blood flow. This surge results in a localized increase of oxygenated hemoglobin and a decrease in deoxygenated hemoglobin.
fNIRS systems detect these changes using pairs of light sources and detectors, called optodes, placed on the scalp. The light sources emit near-infrared light into the tissue, and the detectors measure the light that is either transmitted through or diffusely reflected back from the brain. By measuring the intensity of the light at multiple wavelengths, fNIRS can calculate the relative changes in the concentrations of these two forms of hemoglobin. These calculated changes serve as indirect indicators of underlying neural activity.
Where fNIRS is Applied
fNIRS has diverse applications across scientific and clinical domains. In brain research, it is employed in cognitive neuroscience to investigate higher cognitive functions in adults and infants. Researchers use fNIRS to study processes such as language development, attention, memory, and decision-making, providing insights into how these functions are supported by brain activity, including object, face, and language processing.
In clinical settings, fNIRS plays a role in studying neurological conditions and psychiatric disorders. It has been used to assess regional oxygenation in pediatric intensive care units and to characterize task-based cortical function in patients. The technique is also applied in neurorehabilitation, allowing for the monitoring of brain activity during recovery from conditions such as stroke or in individuals with epilepsy.
fNIRS is particularly useful in developmental studies, offering a child-friendly method to examine cognitive development in infants and children. It is suitable for investigating brain function in younger populations, contributing to an understanding of neural changes during the acquisition of academic skills like mathematics and language. This technique can even be used in schools to collect data from children in their natural learning environments, enhancing the ecological validity of the research.
Beyond laboratory and clinical environments, fNIRS is increasingly applied in real-world settings. Its portability allows researchers to study brain activity during natural tasks and movements, such as walking or driving simulations. This enables investigations into brain function during everyday activities, providing a more naturalistic understanding of cognitive processes than traditional stationary neuroimaging methods.
Unique Characteristics and Capabilities
fNIRS offers distinct characteristics that make it a valuable tool in specific research and clinical contexts. It is a non-invasive technique that uses safe, non-ionizing near-infrared light, making it a safe option for repeated measurements and suitable for a wide range of populations.
The portability and flexibility of fNIRS systems are significant advantages, as they can be used in various settings outside of a traditional laboratory. Unlike large, stationary scanners, fNIRS devices are often smaller and can be worn by participants, allowing for data collection in environments like homes, classrooms, or during physical movement. This capability supports more naturalistic studies of brain function during active behaviors.
fNIRS is particularly well-suited for specific populations who may not be able to tolerate other neuroimaging methods. Its tolerance to movement and less restrictive setup make it useful for studying infants, children, and individuals with conditions that preclude the use of fMRI, such as those with metal implants or claustrophobia. The minimal preparation time required also benefits populations with low frustration tolerance.
fNIRS can also complement other neuroimaging techniques by providing different types of information or enabling studies where other methods are impractical. While it measures the same hemodynamic response as fMRI, it offers better temporal resolution than fMRI and superior spatial resolution compared to EEG for cortical activity. This allows for a more comprehensive understanding of brain activity when used alongside other modalities.