用动态系统重新审视视觉模型的令牌顺序,发现排序能显著提升性能。
Rethink MAE with Linear Time-Invariant Dynamics

- 引入状态空间模型作为有序探针,捕捉令牌序列的时间依赖性。
- 在局部特征上表现远超固定扫描,实现与全监督相当的分类精度。
- 揭示预训练目标影响令牌结构,适合分析视觉表征的内在机制。
标准视觉模型表征探测依赖全局平均等置换不变操作,将补丁表示视为无序集合。本文挑战这一范式,证明在冻结视觉表征(如MAE、BEiT、DINOv2和ViT)中,令牌顺序是关键且可利用的维度。提出SSMProbe框架,基于状态空间模型(SSM),以离散线性时不变(LTI)动力系统为探针,其状态严格依赖于序列顺序,因具有固有记忆衰减特性。将令牌顺序建模为信息调度问题,对比固定扫描策略与可微软排列(基于Sinkhorn学习)。在标准及细粒度分类基准上评估显示:固定扫描在高度局部化补丁特征上表现极差,而学习的软排列则从原本高度局部化的序列中提取出极具竞争力的性能。发现预训练目标从根本上塑造了令牌结构:DINOv2将全局语义集中于优化后的CLS令牌,使补丁高度专业化;纯MAE保持分布式表示,补丁信息量异质性强;ViT则代表监督下的CLS主导极端。BEiT处于中间位置。这种异质性具有顺序依赖性——探针性能高度依赖于令牌在时间位置上的安排,而非仅空间网格拓扑。SSMProbe学习的路由有效发现了并利用了这一异质性,为视觉表征分析提供了强大新视角。
原文摘要 · Abstract (English)
Standard representation probing for visual models relies on mathematically permutation-invariant operations like Global Average Pooling (GAP) or CLS tokens, treating patch representations as an unstructured bag-of-words. We challenge this paradigm by demonstrating that token order is a critical, exploitable dimension in frozen visual representations (e.g., MAE, BEiT, DINOv2, and ViT as CLS-ablation extreme). We propose SSMProbe, a probing framework driven by a State Space Model (SSM). Operating as discrete Linear Time-Invariant (LTI) dynamical systems, SSMs act as permutation-sensitive probes where sequence order strictly dictates the final state due to inherent memory decay. Formulating token ordering as an information scheduling problem, we compare fixed scan heuristics against a differentiable soft permutation (Sinkhorn-based) learned from downstream supervision. Evaluations on standard and fine-grained classification benchmarks reveal a striking order gap: while fixed scans fail dramatically on highly localized patch features, our learned soft permutation successfully extracts highly competitive performance from otherwise heavily localized patch sequences. We find that pre-training objectives fundamentally shape token structure: DINOv2 concentrates global semantics in optimized CLS tokens leaving patches hyperspecialized, pure MAE preserves distributed representations with heterogeneous patch informativeness, and ViT represents a supervised CLS-dominated extreme. BEiT occupies middle ground. This heterogeneity is order-dependent -- meaning the SSM probe's performance depends critically on which tokens are placed at which temporal positions -- and is not merely a topological property of the spatial grid. SSMProbe's learned routing effectively discovers and exploits this heterogeneity, offering a powerful new diagnostic lens for visual representation analysis.
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