Circadian variability is negligible in primary visual cortices as measured by fNIRS

Neural activation leads to an increase of regional cerebral blood flow. Most of the functional imaging studies implicitly assume that variability of the hemodynamic response throughout a single day is negligible. To test this assumption we measured brain activation by functional near-infrared spectroscopy (fNIRS) in the visual cortex of ten subjects six times throughout the day, from 0800-1800 h, during an event-related checkerboard paradigm. Concentration of oxygenated hemoglobin increased, whereas concentration of deoxygenated hemoglobin decreased at each time point examined, without significant influences of daytime. Variability of the hemodynamic response was higher across subjects than for single subjects across day. In conclusion, our study is the first one supporting the common practice of ignoring circadian variability in functional imaging studies.