## 2 Materials and Methods
### Spectra Collection and Analysis  
The setup and procedures of the portable spectroradiometer utilization (including instrument calibration, in vivo spectral acquisition, data handling, and analytical workflow) have been previously detailed [5]. Briefly, we employed an ASD FieldSpec 4 Standard-Res spectroradiometer (ASD Inc., Boulder, CO, USA) covering a spectral range of 350-2500 nm [16]. The instrument is equipped with three separate detectors: one for the visible (VIS) spectrum (350-1000 nm), one for the first short-wave infrared (SWIR) region (SWIR1, 1001-1800 nm), and one for the second SWIR region (SWIR2, 1801-2500 nm). A contact probe with an internal halogen light source was used, delivering illumination at a 12\({}^{\circ}\) angle, with a 35\({}^{\circ}\) measurement angle and a 10 mm spot size. Spectral calibration and acquisition were performed using the ASD's proprietary RS3 software. The calibration procedure involved dark current correction and acquisition of a white reference standard (Spectralon from LabSpheretm, North Sutton, NH, USA). The collected spectra (asd format) were imported into MATLAB (MATLAB R2022a, ver. 9.12; The Mathworks, Inc., Natick, MA, USA) through a custom script. Data preprocessing and analysis were performed using the PLS_toolbox (ver.9.0; Eigenvector Research, Inc., Wenatchee, WA, USA) in combination with MATLAB's Statistics and Machine Learning Toolbox (Ver 12.3.; The Mathworks, Inc., Natick, MA, USA). Spectral files were structured into dataset objects and labeled by class accordingly. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**ID**} & \multirow{2}{*}{**Condition**} & \multirow{2}{*}{**Age** [yrs]**} & \multirow{2}{*}{**Sex**} & \multirow{2}{*}{**Weight** [kg]**} & \multirow{2}{*}{**Height** [cm]**} & \multirow{2}{*}{**BMI**} & \multicolumn{2}{c}{**DD**} & \multirow{2}{*}{**EDSS**} & \multirow{2}{*}{**FSS**} & \multirow{2}{*}{**DMT**} & \multirow{2}{*}{**ARR**} \\  & & & & & & & & & & & & \\ \hline
**NT10** & Normal & 66 & 0 & 74 & 168 & 26 & - & - & - & - & - \\ \hline
**NT11** & Normal & 53 & 1 & 75 & 156 & 31 & - & - & - & - & - \\ \hline
**NT12** & Normal & 50 & 0 & 68 & 170 & 24 & - & - & - & - & - \\ \hline
**NT13** & Normal & 58 & 0 & 83 & 173 & 28 & - & - & - & - & - \\ \hline
**NT14** & Normal & 26 & 0 & 72 & 178 & 23 & - & - & - & - & - \\ \hline
**NT15** & Normal & 27 & 0 & 80 & 127 & 26 & - & - & - & - & - \\ \hline
**NT16** & Normal & 33 & 0 & 57 & 160 & 22 & - & - & - & - & - \\ \hline
**NT17** & Normal & 64 & 1 & 62 & 159 & 25 & - & - & - & - & - \\ \hline
**NT18** & Normal & 54 & 1 & 76 & 168 & 27 & - & - & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Before data collection, both the contact probe and the participants' skin were cleaned using disposable skin wipes. Reflectance spectra in the VIS-SWIR range were then recorded from cranial and extracranial anatomical regions using the contact probe applied directly to the skin. To reduce variability in measurement conditions, all spectra were acquired in a temperature-controlled environment (22-24 \({}^{\circ}\)C), and participants were seated comfortably for 5-10 min before measurement to stabilize hemodynamics. A trained operator manually adjusted the length of the contact probe holder's extensible rod and applied the contact probe using consistent and gentle pressure across all anatomical sites, based on visual confirmation of probe-skin contact and reflectance stability. The spectroradiometer was operated remotely via a connected laptop. Participants were instructed to immediately notify the operator if they experienced any discomfort during the procedure. Cranial spectral acquisition was performed at two standardized scalp locations: a cortical site (electrode position FC3 in the 10-20 EEG system) and a brainstem-associated site at the temporal acoustic window, opposite to the most affected limb.