Shedding Near-Infrared Light on Brain Networks

Near-infrared spectroscopy is a novel and promising technology for cost effective and noninvasive brain imaging in research and clinical practice. Utilizing the fact that near-infrared light is mostly absorbed by tissue hemoglobin, one can measure the intensity of light scattered and reflected by tissue (e.g., brain) to track local changes in hemoglobin concentrations within cortical layers (near-infrared spectroscopy, NIRS). Moreover, with multiple source-detector pairs, one can perform spatial reconstruction of an activation map for both oxygenated (HbO) and de-oxygenated forms of hemoglobin (in this case, the term ‘Diffuse Optical Tomography’ is used). Conceptually, NIRS detects hemodynamic modulations as an indirect measure of neuronal activity similar to the blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal. Although spatial resolution of NIRS is lower than that of fMRI (about 1 cm), NIRS provides an imaging method with excellent temporal resolution (up to a few ms as found in electrophysiological methods such as EEG and MEG). Moreover, its low cost, portability and the ease of use make it ideally suitable for those subject and patient populations which are not easily amenable to the gold-standard imaging techniques of fMRI and positron emission tomography (PET). The modern NIRS instruments provide high density multi-channel recordings which allow for larger coverage of the head and it becomes possible to measure not only brain activation but also dynamic interactions between the brain areas. Those interactions can be assessed through temporal correlations of optical signals simultaneously recorded from multiple brain regions and thus a NIRS-based ‘functional connectivity’ similar to the functional connectivity measured by the fMRI BOLD signal [1] can be derived.