## 2 Materials and Methods
### 2.5 INIRS Data Collection  
The INIRS system used in the current study (NIRSport2 Core System Unit, NIRX Medical Technologies, LLC, Medizintechnik, Berlin, Germany) was a continuous-wave NIRS instrument with 16 LED illumination sources and 16 photodiode detectors. Each source had two LED illuminators that emitted near-infrared light with wavelengths of 760 nm and 850 nm. One of four NIRScaps, according to the participant's head scope, was positioned on the head to hold the sources and detectors and was secured by a chinar to create source-detector pairs (denoted as channels). The NIRScap sizes were 54, 56, 58, and 60 cm. Forty-one channels were used for the experimental montage, as shown in Figure 2. Thirty-three of them were separated by approximately 3.5 cm, and eight channels were used as short-distance separation channels (SSC), separated by approximately 1.2 cm. The latter were used to reduce the contamination of NIRS signals by extracerebral blood flow and improve sensitivity to cortical regions [56]. A 6-axis accelerometer, positioned on the cap was also included in order to identify and later remove, motion artifacts. By registering the positions of the sources and detectors on the NIRScap to the 10-10 system [57; 58], using the INIRS Optodes' Location Decider (fOLD) toolbox, the specificity of each channel to different neural structures was calculated. See Table 2 for a summary of specificity ratings > 30% for each anatomical area, as it relates to each source-detector position. The specificity values reported in Table 2 were obtained from the fOLD toolbox [59] using the Brodmann atlas. Note that each source-detector pair may report multiple specificity values for different anatomical structures. Because variations in the thickness of the skull and adjacent tissues can affect the inter-subject sensitivity of fNIRS [60], probe fabrication error analysis was conducted using the AtlasViewer application [61]. This analysis examined how closely the actual optode locations from individual subjects matched the original probe design (Figure 3). Analysis of variance (ANOVA) showed smaller fabrication errors for the prefrontal optodes compared to the right and left temporal and frontal optodes across all groups (NH, NHIV, CI) (\(p\) < 0.001), with no significant difference between the groups and no significant group \(\times\) brain area interaction (\(p\) > 0.05). Before commencing data collection, signal optimization was conducted to check the optode-skin contact using the Aurora 2020.7 fNIRS software. During signal optimization, NIRSport2 increased the source brightness in a stepwise manner until an optimal signal amplitude for each channel was obtained. In cases of poor signal quality, the hair under the relevant grommet was carefully pushed aside, and the signal quality was rechecked. The procedure was repeated several times until as many channels as possible showed a good signal quality. The signals were sampled at 5.4 Hz using the Aurora fNIRS software and imported to MATLAB for further processing using the Homer3 software [62]. Figure 1: Pseudorandom block design of the fNIRS stimuli presentation. In total, 21 blocks were presented, 7 from each condition. Two questions were presented, following the 1 st and 20 th blocks. SO = speech-only condition, NO = noise-only condition, SIN = speech-in-noise at SNR = 0 dB condition.