2.6. NIRS Data Acquisition, Online Preprocessing, and Neurofeedback

Hemodynamic response (HR) signals were assessed using a NIRSport fNIRS Systems (eight sources/eight detectors, NIRx Medizintechnik, GmbH, Berlin, Germany) operating at two wavelengths (760 and 850 nm) at a sampling rate of 20.83 Hz. An optode set of 3 sources and 8 detectors was used leading to 12 source–detector pairs (channels; see Fig. 1, optode placement in line with Ieong and Yuan⁴³). Optodes were accurately placed according to the International 10–20 system,⁴⁴ and the source–detector distance was 30 mm. In line with a previous fNIRS study, the channels were allocated to the medial and lateral regions of the OFC (in line with Leong and Yuan,⁴³ channels 1–4 of the current study would be allocated to the medial OFC, whereas the other channels would be allocated to the lateral OFC). Channel 7 was designated as right lateral OFC feedback channel and the position of the channel was further validated by MRI scans in two independent participants (see Fig. S1 in Supplementary Material).

In line with previous fNIRS NF studies, the feedback was based on the oxy-Hb signal.^45–47 Online preprocessing of the oxy-Hb NIRS signal was performed by the built-in real-time output solution implemented in the NIRSport system. Next, the real-time output was computed and visually displayed via a previously evaluated real-time fNIRS NF platform.⁴⁸ The raw oxy-Hb NIRS signal was smoothed using a 2-s moving average window. A baseline was calculated by taking the average of signals 2 s before each regulation block and was subsequently subtracted from the smoothed signal. The feedback signal was computed in real-time from the signal of the a priori target channel (No. 7) using the equation “feedback signal = preprocessed oxy-hemoglobin signal/difficult coefficient.”

The “difficulty coefficient” served to calibrate the feedback visualization according to the signal range in the current study to promote learning success (e.g., feedback visualization that only spans a small range of the thermometer used for feedback display may not provide sufficient information for the participants to learn the regulation). The study-specific “difficulty coefficient” was therefore initially determined in a pre-experiment that was conducted before the start of the primary experiment that examined the feasibility of OFC NF. During this pre-experiment, an independent sample of n=6 participants underwent four runs of the OFC NF training. Each run used a separate difficulty coefficient (0.003, 0.006, 0.008, 0.01 values based on our experience during implementing the paradigm). The order of the four runs with the separate difficulty coefficient was randomized across participants. Subsequently, participants were asked to designate the run with the most suitable difficulty to allow regulation. Four (from six participants) designated the run that used 0.008; consequently, this coefficient was employed to visualize the online feedback in terms of the height of the stone for all participants in the subsequent study. Participants in the sham NF group received oxy-Hb signal changes from a participant in the real-time OFC NF group who had previously completed the training. Participants in both groups were blinded for the training condition they received and the experimenter operating the feedback platform was separated from the participant by a partition wall to minimize any interaction during pretraining and training.