Near-infrared light spectroscopy and stimulation in cognitive neuroscience: the need for an integrative view?

Near-infrared spectroscopy has been largely used in neuroscience as an alternative non-invasive neuroimaging technique, primarily to measure the oxygenation levels of cerebral hemoglobin. Its portability and relative robustness against motion artifacts made it ideal for measuring cerebral blood changes during physical activity. Usually referred to as 'functional' near-infrared spectroscopy when used to monitor brain changes during motor or cognitive tasks, this technique often involves the montage of the probes on the forehead of the participants to gauge the neurophysiological underpinning of executive functioning. Other applications of near-infrared spectroscopy include other aspects of cerebral hemodynamics, such as cerebral pulsatility. More recently, it has been reported how near-infrared light can affect cognitive and psychological processes through what is known as photobiomodulation. However, 'functional' near-infrared spectroscopy studies do not seem to have taken this important bit of knowledge into account so far. Hence, drawing on a selection of near-infrared spectroscopy and photobiomodulation experiments, we suggest an integrative view for near-infrared-based neuroimaging studies, which should embrace a control for the possible effects of light stimulation, especially when 'functional' near-infrared spectroscopy is considered for testing the effect of an intervention.