Labnirs

A continuous wave fNIRS system (LABNIRS; Shimadzu, Kyoto, Japan) with continuous wave laser diodes with wavelengths of 780 ± 5 nm, 805 ± 5 nm and 830 ± 5 nm was utilized to measure changes in the concentrations of oxygenated hemoglobin (HbO), deoxygenated hemoglobin (HbR), and total hemoglobin (HbT). The sampling rate was 42 Hz. Considering the measurement requirements and the number of optodes, we designed a unique 14-channels array consisting of 7 infrared optode emitters and 8 optode detectors that were separated by 30 mm.
The fNIRS optodes cap was put on the subject’s head and tightened at the chin to hold it in place (Figure 1B), and the optical optodes arrangement is shown in Figure 1A. Three-dimensional (3D) positions of the emitters and detectors were obtained using a 3D digitizer (FASTRAK, Polhemus, Vermont, USA) in reference to the nasion, central zero (Cz, respect to the International 10–20 system) and bilateral external auditory meatus.

The changes in oxygen-hemoglobin (Oxy-Hb) concentration were measured using a fNIRS optical topography system (LABNIRS, Shimadzu. Kyoto, Japan) with three wavelengths of near-infrared light (780, 805, and 830 nm). Figure 2 shows the positions of the probe and channel. The probes were set over the DLPFC, PMC, and SM. The anatomical location of each channel was determined according to the Talairach Daemon database [29 ,30 ]. The changes in Oxy-Hb concentration were recorded since they indicate representative brain activity [31 (link),32 (link)]. The block design comprised six task blocks, with three blocks in each hand. The sequences of the right- or left-hand tasks were randomly assigned (Figure 2). The six regions of interest (ROIs), three in each hemisphere, included the bilateral DLPFC (left channels: 2, 7, 8, and 14; right channels: 5, 12, 13, and 19), the bilateral PMC (left channels: 28, 29, and 35; right channels: 30, 31, and 37), and bilateral primary SM cortex (left channels: 40, 41, and 47; right channels: 44, 45, and 48). Task-related changes in Oxy-Hb concentrations in every channel in each hemispheric ROI were averaged over the time period of the task (0–60 s after task onset). Finally, the task-related Oxy-Hb concentration changes in each hemisphere were calculated by averaging the Oxy-Hb concentration changes in all channels in each hemisphere’s ROI.

For the assessment of the hemodynamic brain response, which has been shown to correlate with neural activity (Fabiani et al., 2013 (link)), a tethered fNIRS-system (LABNIRS, Shimadzu, Kyoto, Japan) was used. The fNIRS system used continuous wave laser diodes with wavelengths of 780, 805, and 830 nm to measure the HbO₂ and HHb at a sampling frequency of 23.8 Hz. Further details of this system, including HbO₂ and HHb conversion coefficients, are published in Stuart et al. (2019a (link)). The nasion, inion, and Cz anatomical landmarks for each participant were identified prior to placement of the fNIRS cap. Thirty-two optodes (16 sources, 16 receivers) were attached to the cap with a source-detector separation of ~3 cm, generating 48 channels as shown in Figure 3. To control for different head sizes, a 3D-digitizer (FASTRAK, Polhemus, VT, USA) was used to record the co-ordinates of the nasion, inion, Cz, left, and right pre-auricular points, as well as the optode locations. In addition, heart rate and oxygen saturation were monitored throughout the experiment with a wireless oximeter (Onyx II 6590, Nonin Medical Inc, MN, USA) clipped to the index finger and processed with Nonin software (OEM Evaluation Kit Program Files).