## 2 Materials and Methods
### Hemodynamic Data Acquisition and Analysis  
Eight optodes from a NIRScout system (NIRx Medical Technologies, LLC, Los Angeles, CA, USA) were utilized to measure hemodynamic activity by tracking changes in oxygenated hemoglobin (O2 Hb) and deoxygenated hemoglobin (HHb) levels. The system included four light emitters and four detectors positioned on the scalp using an fNIRS cap according to the 10/5 international standard placement system [36]. The emitter-detector distance was consistently set at 30 mm for all optode pairs, and near-infrared light at two wavelengths (760 and 850 nm) was employed. A six-channel configuration was created to cover the left and right prefrontal cortex (PFC), as illustrated in Figure 2[37].  
Figure 1: Description of the experimental procedure.  
Figure 2: The fNIRS setup included four light sources (marked in yellow) placed at AF3, AF4, F5, and F6, and four detectors (marked in orange) positioned at AFF1h, AF2h, F3, and F4. Six measurement channels (shown in violet) were established as follows: Ch1 (AF3-F3), Ch2 (AF3-AFF1h), Ch3 (F5-F3), Ch4 (AF4-F4), Ch5 (AF4-AFF2h), and Ch6 (F6-F4).  
To attribute task-related hemodynamic changes to specific brain regions, a probabilistic atlas implemented in the fOLD software (fNIRS Optodes' Location Decider, version 2.2.1) [38] was used. This approach employed the Automated Anatomical Labeling atlas and Brodmann area framework [39] to identify the most relevant functional regions. The analysis relied on data from multiple atlases and sources to accurately align the optodes and detectors with the underlying functional regions and Brodmann areas [40; 41]. The focus of this study was on the frontal brain regions, specifically on the PFC, to investigate its activity during the persuasion process.  
Using NIRStar Acquisition Software (NIRx Medical Technologies LLC, Glen Head, NY, USA), changes in O2 Hb and HHb concentrations were continuously recorded throughout the task, beginning with a 120 s resting baseline. Data from the six channels were sampled at 6.25 Hz and subsequently processed using nirsLAB software (v2014.05; NIRx Medical Technologies LLC). The signals were captured based on wavelength and optode placement and converted into mmol*mm values to represent changes in O2 Hb and HHb concentrations per channel.  
The raw signals from each channel underwent digital bandpass filtering at 0.01-0.3 Hz to remove noise. Additionally, data were visually inspected on a participant-by-participant basis during both the experimental phase and signal analysis to identify and exclude channels affected by motion artifacts or significant amplitude variations. Channels with poor optical coupling or lacking \(\sim\)1 Hz heartbeat oscillations were excluded [42]. To address breathing-related noise (\(\sim\)0.3 Hz), a linear-phase FIR filter was applied to produce a symmetric impulse response [43; 44].  
Mean concentrations from the time series of each channel were used to compute the effect sizes (Cohen's d) for each condition. Effect sizes were calculated as follows: D = (m1 \(-\) m2)/s, where m1 and m2 are the mean concentration levels for baseline and trial, respectively, and s is the baseline SD. To enhance the signal-to-noise ratio, effect sizes across the six channels were averaged. Since the raw fNIRS data were relative and could not be directly averaged across participants or channels, normalized effect size data were aggregated independently of the differential pathlength factor (DPF), as effect size calculations were not influenced by the DPF.