Blood flow affects light transmission but its impact on NIRS remains unclear

While most investigators that use near infrared spectroscopy (NIRS)-based techniques do not believe that changes in light transmission are exclusively attributable to hemoglobin changes, most only consider the various hemoglobin moieties when decomposing the source signal. In their commentary, Tomita and colleagues challenge this assumption and speculate that the majority of the NIRS signal is due to flow effects rather than changes in hemoglobin. While the authors present convincing preliminary evidence that flow can affect light transmission, the authors stop short of providing conclusive evidence that the flow effect is indeed significant when using spectroscopic techniques. Nevertheless, the authors raise sufficient concern regarding the potential contribution of the "flow-effect" to warrant further investigations.