A Comprehensive Information-Decomposition Analysis of Large Vision-Language Models

Large vision-language models (LVLMs) achieve impressive performance, yet their internal decision-making processes remain opaque, making it difficult to determine if the success stems from true multimodal fusion or reliance on unimodal priors. To address this attribution gap, we introduce a novel framework using partial information decomposition (PID) to quantitatively measure the ``information spectrum'' of LVLMs—decomposing a model’s decision-relevant information into redundant, unique, and synergistic components. By adapting a scalable estimator to modern LVLM outputs, Our model-agnostic pipeline profiles 26 LVLMs on four datasets across three dimensions—\emph{breadth} (cross-model \& cross-task), \emph{depth} (layer-wise information dynamics), and \emph{time} (learning dynamics across training). Our analysis reveals two key results: (i) two task regimes (synergy-driven vs. knowledge-driven) and (ii) two stable, opposing family-level strategies (fusion-centric vs. language-centric). We also uncover a consistent three-phase pattern in layer-wise processing and identify visual instruction tuning as the key stage where fusion is learned. Together, these contributions provide a quantitative lens beyond accuracy-only evaluation and offer guidance for analyzing and designing the next generation of LVLMs.