The origin of infra-slow oscillations of oxygenated hemoglobin observed in functional near-infrared spectroscopy

There is increasing interest in the intrinsic activity of the resting brain, especially the activity slower than 0.1Hz (i.e., low-frequency oscillations, or LFOs). To investigate the origin of LFOs observed in functional near-infrared spectroscopy (fNIRS), we recorded multichannel fNIRS and electroencephalography (EEG) from the frontal cortex of 11 healthy young volunteers in the resting state. Electrocardiography (ECG), electro-oculography and respiration were also measured. Synchronous oscillations of oxy-hemoglobin (oxy-Hb) around 1.0Hz were detected in all fNIRS channels, and their frequency was consistent with a peak frequency of ECG, suggesting the changes of cerebral blood flow due to heart beats. In addition, oxy-Hb oscillations around 0.1Hz (i.e., LFOs) appeared in the fNIRS. The channels where LFOs appeared differed among the subjects, and the LFOs appeared or disappeared even in the same fNIRS channels. The appearance of LFOs in fNIRS channels was significantly higher when the LFOs appeared on the EEG in the adjacent EEG electrodes compared to when LFOs did not appear on EEG. The amplitude and coherence (synchronicity) of the LFOs were increased by changing the subjects' position from dorsal to the sitting position in both fNIRS and EEG, and the coherence in particular was increased in the homologous fNIRS channels on the bilateral hemispheres. These results suggest that LFOs of oxy-Hb couple with resting-state EEG activity