Differential temporal activation of oxy- and deoxy-hemodynamic signals in optical imaging using functional near-infrared spectroscopy (fNIRS)
Background Optical imaging of the brain based on near-infrared spectroscopy (NIRS) can provide real-time measurements of the hemodynamic signals that represent metabolic demands of the underlying neural tissues. Functional imaging based on NIRS (fNIRS) can detect both oxyhemoglobin (oxy-Hb) and deoxy-hemoglobin (deoxy-Hb) levels related to neural metabolic activity, whereas BOLD fMRI (blood-oxygen-level dependent functional magnetic resonance imaging) can only detect signals related to deoxy-Hb. Thus, during task execution, only fNIRS can determine the differential temporal activation/deactivation of oxy-Hb and deoxy-Hb hemodynamic signals as the blood-oxygen demand changes. We have previously shown that as metabolic demand increases, temporal changes in oxy-Hb and deoxy-Hb levels can be temporally decoupled (i.e., oxy-Hb level can decrease while deoxy-Hb level increases) rather than being coupled, in which case both would increase or decrease simultaneously [1-5]. In order to account for the observed differential temporal decoupling of oxy-Hb and deoxy-Hb levels, we hypothesize that as oxygen demand increases, the delivery of blood oxygen cannot keep up with the demand of the neural tissues, resulting in decreased oxy-Hb and increased deoxy-Hb levels. This study provides experimental evidence that validates the above hypothesis.