分类训练的ViT能自发形成空间层次结构,类似灵长类视觉皮层。
From Edges to Depth: Probing the Spatial Hierarchy in Vision Transformers

- 逐层探查模型编码,发现边界信息在第5-6层可解码,深度信息在第8层最强。
- 深度信号需全局整合,在第8层达到最小平均误差0.0875,最终层完全消失。
- 空间信息是主动维持而非被动传递,适合研究视觉表征机制的研究者。
仅通过图像分类训练的Vision Transformers能够迁移到需要空间理解的任务,但预训练中未接受空间监督。我们逐层探测冻结的ViT-B/16模型,考察两种互补属性:局部块边界(BSDS500)与每块深度(NYU Depth V2)。结果显示清晰的空间层次:边界结构在第5-6层即可线性解码(AP = 0.833),而需整合全局线索的深度信号在第8层达到峰值(MAE = 0.0875)。两者在最终分类层均崩溃,随机权重对照验证了编码为学习所得。因果干预表明:仅移除深度探测器读取方向,解码性能下降高达165%;其他方向影响小于1%。沿该方向的激活修补显示,深度信号在每一层部分重构,而非仅通过残差流传递,中层干预对下游影响最持久。结果表明,分类训练的ViT会自发建立一个主动维护的空间层次结构,其发展路径与灵长类视觉皮层的早期到晚期进程一致。
原文摘要 · Abstract (English)
Vision Transformers trained only on image classification routinely transfer to tasks that demand spatial understanding, yet they receive no spatial supervision during pretraining. We ask where and how robustly such structure is encoded. Probing a frozen ViT-B/16 layerwise for two complementary properties, local patch boundaries (BSDS500) and per-patch depth (NYU Depth V2), reveals a clear hierarchy: boundary structure becomes linearly decodable at layers 5-6 (AP = 0.833), while depth, which requires integrating global cues, peaks two to three layers later at layer 8 (MAE = 0.0875). Both signals collapse at the final classification layer, and random-weight controls confirm the encodings are learned rather than architectural. Causal interventions add specificity: ablating the single direction a linear depth probe reads degrades depth decoding by up to 165%, while ablating any other direction changes it by less than 1%. Targeted activation patching along that direction shows the depth signal is partially re-derived at each layer rather than passively carried in the residual stream, with mid-layer interventions persisting most strongly downstream. The result is that a classification-trained ViT develops an actively maintained spatial hierarchy that mirrors the early-to-late progression observed in the primate visual cortex.
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