## 2 Materials and Methods
### MRI Acquisition and CVR Processing  
Due to limitations in the quality of the acquired EtCO2 data, dynamic analysis of the BOLD response using EtCO2 as a regressor was not feasible in this study. The MRI CO2 experiment employed a block design with a total duration of 10 min. This design alternated between 2 min blocks of room air inhalation (repeated three times) and 2 min blocks of  
Figure 1: Diagram of the gas delivery system. (**A**) Components of the gas delivery system, including various tubes for breathing used in the study. (**B**) A representative participant comfortably positioned supine in the SPECT scanner gantry with the gas delivery system connected. SPECT, single photon emission computed tomography; EtCO2, end-tidal CO2. 5% CO\({}_{2}\) inhalation (repeated two times). Participants were instructed to lie still in the MRI scanner, focus on the experiment, and avoid active thinking or meditation. MRI data were acquired using a 3T scanner (Magnetom Verio, Siemens, Berlin, Germany) with a 12-channel head matrix coil. To minimize head movement during the MRI scan, sponges were placed between the head coil and the participant's head. MRI images were acquired using a two dimensional (2D) gradient echo planar imaging (2D GE-EPI) sequence with the following parameters: repetition time (TR) 2500 ms; echo time (TE) 25 ms; slice thickness 3.5 mm; number of slices 42; matrix size 64 \(\times\) 64; flip angle 90\({}^{\circ}\); field of view (FOV) 220 \(\times\) 220 mm\({}^{2}\); acquisition time (TA) 10 min 10 s; volumes 244 (including four discarded volumes); voxel size 3.44 \(\times\) 3.44 \(\times\) 3.5 mm\({}^{3}\). High resolution T1-weighted anatomical images were acquired using the three dimensional (3D) magnetization prepared rapid acquisition of gradient echo sequence with the following parameters: TR 1900 ms; TE 2.93 ms; slice thickness 1 mm; inversion time (TI) 900 ms; number of slices 176; matrix size 256 \(\times\) 224; flip angle 9\({}^{\circ}\); FOV 256 \(\times\) 224 mm\({}^{2}\); TA 3 min 29 s; voxel size 1 \(\times\) 1 \(\times\) 1 mm\({}^{3}\); phase partial Fourier 7/8; integrated parallel acquisition technique mode: generalized auto calibrating partially parallel acquisition (GRAPPA) [27]. GRAPPA is a parallel imaging technique that accelerates MRI acquisition by reducing the amount of data collected, utilizing data from multiple receiver coils and calibration lines (specifically, 24 reference lines acquired in the center of k-space in this study) to reconstruct the image. This process allows for faster scans while maintaining acceptable image quality. An acceleration factor of 2 was used along the phase encoding direction. Data processing of the BOLD images was performed using statistical parametric mapping software (SPM12; Wellcome Trust Centre for Neuroimaging, London, UK) implemented in MATLAB R2022a (MathWorks, Natick, MA, USA), as illustrated in Figure 2. Realignment initiated the preprocessing pipeline for BOLD images, which consisted of several standard steps. To correct subject specific motion during fMRI acquisition, realignment was implemented, employing affine transformations to spatially register functional volumes to a mean reference image. Slice timing correction followed realignment to address temporal variations from sequential slice acquisition, improving time-series analysis fidelity. Spatial normalization to Montreal Neurological Institute (MNI) space for group analysis and inter subject comparison was then performed using non-linear warping. Spatial smoothing was subsequently applied using a 6 mm full width at half-maximum (FWHM) gaussian kernel. Following these preprocessing steps, BOLD time-series were extracted. Subsequently, temporal detrending was performed on the extracted time-series using the detrend function in MATLAB to mitigate low frequency drifts inherent in BOLD signals. The automated anatomical labeling atlas (AAL2) was then utilized to delineate regions of interest (ROIs) within the gray matter (GM) for subsequent analysis [28].