About fNIRS
Functional near-infrared spectroscopy and imaging (fNIRS/fNIRI) is a tool that detects neuroactivation by measuring oxygenated and deoxygenated blood flow. This is accomplished through spectroscopically measuring absorbance of the chromophore hemoglobin in blood that flows to neurally activated regions. Figure 1 shows rat brain vasculature that is constantly modifying itself in response to nutrient and oxygen demands, pruning and sprouting new vessel branches within days.
Figure 1
Brain vasculature. Red arrows indicate areas of vessel pruning determined by blood flow, and white arrowheads indicate vessel sprouting. Obtained from http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001375
BASED ON THE HEMODYNAMIC RESPONSE
Neuroactivation is coupled with the hemodynamic response. Activated neurons increase metabolism and oxygen consumption, a change that is reflected in the local abundance of oxygenated hemoglobin (HbO) and deoxygenated hemoglobin (HHb). This process is called the hemodynamic response and can be measured noninvasvely by fNIRI. Figure 2 shows the hemodynamic response curve and shows the rapid increase in blood flow followed by the return to a homeostatic level once the need is met.
Figure 2
The hemodynamic response with relative blood flow in a certain location of the brain over time. Obtained from http://www.jarrodmillman.com/rcsds/lectures/convolution_background.html
MEASURED BY SPECTROSCOPY
Optical imaging relies on the property of light scattering and also requires a modification to the Beer-Lambert Law. As photons penetrate the opaque medium of scalp, skull, meninges, and several centimeters of brain cortical tissue, they are scattered in an infinite number of paths. Although it is impossible to predict the paths of individual photons, bulk photon movements can be predicted. One particular sensitivity profile is called 'the photon banana' and describes how some photons are scattered in a direction that archs back toward the scalp where a photodetector is positioned several centimeters away from the source. Along the way, photons are absorbed by chromophores, HbO and HHb. The detector measures the change in absorbance over time as an electrical signal.
Detectors "hear" signals from their nearest neighbor sources. Varying the distance between sources and detectors alters the depth of light penetration into the cortical tissue. Furthermore, depth measurements vary with scalp and skull thickness which are unique to every individual.
THE PARTS
OPTODES/PROBES
These terms are used interchangeably. They are sensor devices that use light to measure a substance and are the general terms for sources and detectors.
Ensure complete and proper contact with the scalp.
SOURCES
Emits light at 760 nm and 850 nm, the optimum wavelengths at which HHb and HbO absorb respectively.
HANDLE WITH CARE! Do not scratch the bulb surface because this will skew absorbance readings.
Clean with alcohol wipes following each trial.
DETECTORS
Propagates the transmitted light along a fiber-optic cable.
HANDLE WITH CARE! Do not bend or fold into sharp corners as this will break the glass mirrors inside the cable.
Clean with alcohol wipes following each trial.
CAP
Two sizes are available in 56" and 58" and are assigned according to subject head circumference.
DOGBONES
Plastic loops that secure a 3 cm distance between a source and a detector. Their positions represent channels and are the purple lines that connect sources and detectors on the montage images.
MONTAGES
A montage is a unique arrangement of sources and detectors that targets a specific region of the cortex. The two used montages of interest to us are the motor cortex 8x4 (sources x dectors) and the prefrontal cortex 8x4. Go to [link] to view other montages.
