fNIRS recording

We used a commercial continuous-wave (CW) fNIRS system (Light NIRS, Shimadzu Corporation, Kyoto, Japan) to measure PFC activity. The absorption values of the three wavelengths (780 nm, 805 nm, and 830 nm) of near-infrared light were measured at a sampling rate of 13.3 Hz and were then transformed into concentration changes in oxyhemoglobin (HbO), deoxyhemoglobin (HbR), and total hemoglobin (HbT) as per the modified Beer–Lambert law (Owen-Reece et al., 1999). Eight emitters and 8 detectors were placed at alternate points on a 2 × 8 grid, enabling us to detect signals from 22 channels (Figure 2). The distance between the two kinds of probes was 3 cm. The center of the probe matrix was placed on Fpz. We applied a three-dimensional digitizer to record the exact spatial coordinates of the four reference points of the 10–10 system (NZ, CZ, AL, RL) as well as the 16 optical probes. We converted these coordinates into locations of the 22 channels in an estimated Montreal Neurological Institute (MNI) space through NIRS-SPM, which calculates the NIRS location maximum likelihood estimate to the Brodman area location (Ye et al., 2009). NIRS-SPM provided a set of probability values for the measured anatomical region of each channel, and a higher probability value represents a more accurate anatomical region. Activity in the right vlPFC was monitored with ch1, ch8, and ch16. Activity in the right dlPFC was monitored with ch2 and ch9. Activity in the frontopolar area was monitored with ch3, ch4, ch5, ch10, ch11, ch12, ch13, ch17, and ch20. Activity in the orbitofrontal area was monitored with ch18, ch19, and ch21. Finally, activity in the left dlPFC was monitored with ch6, ch14, and ch22, while activity in the left vlPFC was monitored with ch7 and ch15 (Figure 3).

fNIRS setup. We conduct 2 × 8 optode placement for one subject. The red ones are the emitters, and the blue ones are the detectors. The channels between pairs of emitters and detectors are measured.

Frontal view of the corresponding anatomical regions of channel configuration. fNIRS data are recorded from the prefrontal cortex, including the right dorsolateral prefrontal cortex, left dorsolateral prefrontal cortex and frontopolar area, localized by the MNI 10-10 system and 3D digitizer.