Background: Biomedical spectroscopy offers a non-invasive way to characterise disease, but the large number of spectral variables, redundancy, and limited interpretability can hinder the creation of compact diagnostic models. This study presents the Spectral Linear Series Decomposition Learner (Spectral-LSDL), an adaptive framework for
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Background: Biomedical spectroscopy offers a non-invasive way to characterise disease, but the large number of spectral variables, redundancy, and limited interpretability can hinder the creation of compact diagnostic models. This study presents the Spectral Linear Series Decomposition Learner (Spectral-LSDL), an adaptive framework for spectral information localisation and representation that combines established spectral transformations with threshold-based decomposition and morphology-based feature extraction. The framework tests whether disease-related discriminative information can be preserved within specific spectral decompositions rather than requiring representation of the full measured spectrum.
Methods: Spectral-LSDL combines complementary spectral representations, adaptive upper/lower threshold decomposition, localisation-depth optimisation, and morphology-based feature extraction to produce compact 50-feature representations of spectral structure. The framework was tested on three biomedical spectroscopy datasets: Raman spectroscopy for head and neck cancer, near-infrared (NIR) spectroscopy for skin-lesion classification, and ATR-FTIR spectroscopy for type 2 diabetes. Model training and evaluation used biological-group-aware partitioning so that spectra from the same participant or lesion did not appear in both development and hold-out sets. Spectral-LSDL was compared with Raw-LDA, PCA-LDA, PLS-DA, and a 1D-CNN. Performance was measured using balanced accuracy, macro-F1, and ROC-AUC, with paired biological-group bootstrap inference. Preferential spectral localisation was examined separately through size-matched random-region Monte Carlo analysis. The Information Localisation Index (ILI) and its dimension-normalised form (nILI) were used descriptively to assess compression efficiency and predictive retention. Results: Spectral-LSDL achieved marked dimensional reduction while maintaining competitive predictive performance across all three modalities. Group-independent hold-out ROC-AUC values were 0.739 for Raman, 0.809 for NIR, and 0.940 for ATR-FTIR, with corresponding nILI values of 1.184, 0.976, and 0.946, respectively. Paired inference showed no statistically significant differences between Spectral-LSDL and the conventional comparators for Raman or NIR after correction for multiple testing. For ATR-FTIR, Raw-LDA significantly outperformed Spectral-LSDL in balanced accuracy (ΔBA = −0.050, 95% CI −0.088 to −0.014; Holm-adjusted
p = 0.016) and ROC-AUC (ΔAUC = −0.040, 95% CI −0.073 to −0.012; Holm-adjusted
p = 0.020). Size-matched random-region analysis provided evidence of preferential localisation for Raman (balanced accuracy:
p = 0.049; ROC-AUC:
p = 0.002), whereas significant localisation enrichment was not seen for NIR or ATR-FTIR. Conclusions: Spectral-LSDL offers an adaptive framework for spectral information localisation and representation based on established analytical components, allowing compact representations of biomedical spectra while preserving predictive information. The results show that adaptive decomposition can be used to examine where predictive spectral structure is concentrated, but they do not demonstrate consistent predictive superiority, distinct biochemical regions, or clinical usefulness. These findings support further study of Spectral-LSDL as an interpretable approach to spectroscopy modelling, although external prospective validation and independent biochemical confirmation are needed to establish the reproducibility, biological specificity, and clinical relevance of the identified spectral localisations.
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