fNIRS data acquisition

Cortical activation was detected using an fNIRS system (LABNIRS, Shimadzu Corp., Kyoto, Japan). This system is based on the modified Beer–Lambert law²⁶ and used 780, 805, and 830 nm continuous near-infrared light. Relative oxygenated hemoglobin (oxy-Hb), deoxygenated hemo-globin (deoxy-Hb), and total hemoglobin (total-Hb) signal changes were measured by the optodes. From the fNIRS measurements, we used the oxy-Hb which was the most sensitive indicator.²⁷ The sensitivity of fNIRS to detect spatially specific activations during the performance of executive tasks was supported using functional magnetic resonance imaging^28–30 and positron emission tomography studies³¹ that investigated frontal cortex activation with the Stroop effect. Thus, we converted the oxy-Hb signal to a numerical value (cM-mm) for statistical analyses. In addition, we performed a baseline correlation using LABNIRS prior to the analysis. Before the initiation of the task period, the optional data range area, with points of 0 and 5.0 s for the rest period, was set as the baseline.

We used 27 optodes that consisted of 14 light emitters and 13 light detectors to obtain 42 channel measurements of cortical hemodynamic changes. The optodes on the skull covered the frontal area. The optodes were set based on the lower center probe, which was anchored at Fpz according to the international 10–20 system. The distance between pairs of detector probes was set to 3.0 cm. Previous studies have shown that the verbal response influenced skin blood flow in fNIRS signals.³² Therefore, to obtain three channel measurements of skin blood flow, we added three optodes that consisted of light detectors directed to the right, middle, and left of PFC. The distance between these detector probes was set to 1.5 cm. In this study, skin blood flow was removed using segment-independent component analysis. The post-experimental SCWT was performed 5 min after both vacuuming and the motion of vacuuming experiments. The skin blood flow increased within 10 min of low-intensity exercise and then returned to basal levels within 5 min of stopping the exercise.⁶ The motion of pressing a button only in the SCWT periods may have contaminated fNIRS data during the task periods compared with during rest periods. Therefore, during the rest periods, subjects tapped next to the “Enter” key with the same finger and at the same pace as that during the task periods. Furthermore, to alleviate data noise in hemodynamics due to physical activities, subjects placed their chin on a holder in a sitting position and performed SCWT. On the other hand, to exclude the noise due to heart beat and respiration, changes in the oxy-Hb signals were smoothed with a moving average.³³

Channel coordinates on the Montreal Neurological Institute standard template were localized from the probabilistic registration method using a 3D digitizer (FASTRAK; Polhemus, Colchester, VT, USA).³⁴ This included NIRS-SPM,³⁵ which enabled us to localize a probabilistic estimate of structural labels of Talairach Daemon³⁶ that is based on Brodmann areas. We used six regions of interest (ROIs): the right fron-topolar PFC (channels 21, 29, and 38), left frontopolar PFC (channels 22, 31, and 39), right dorsolateral PCF (channels 2, 3, 11, and 12), left dorsolateral PFC (channels 6, 7, 14, and 15), right ventrolateral PFC (R-VLPFC, channels 18, 35, and 36), and left ventrolateral PFC (L-VLPFC, channels 25, 41, and 42) (Figure 3). This procedure is considered to be valid because optical properties of neighboring channels were similar.³⁷

The positions of the fNIRS channels.

Notes: Red shows the frontopolar prefrontal cortex. Blue shows the dorsolateral prefrontal cortex. Green shows the ventrolateral prefrontal cortex.

Abbreviation: fNIRS, functional near-infrared spectroscopy.