分析图语言模型中图标记的内部机制,发现激活强不代表信息有用。
When Graph Tokens Sink: A Mechanistic Analysis of Graph Language Models

- 通过激活值异常识别图标记中的'下沉节点'
- 实验证明这些强激活标记并非关键信息载体
- 揭示当前图标记方法存在表征与语义脱节问题
图语言模型(GLMs)将图结构和节点信息转换为图标记,使大语言模型能联合处理结构化图数据与文本指令。然而,尚不清楚大语言模型如何内部理解这些图标记,以及它们是否真正承载图结构信息。本文分析了代表性GLM架构中图标记的行为。发现:图下沉标记在少数隐藏状态维度上持续表现出巨大激活值,且集中在早期图标记位置,但其激活强度并不等同于信息利用率。与语言和视觉-语言模型中的注意力中心不同,这些下沉标记不必然吸引查询标记的最大注意力权重。通过剪枝、重定位和替换干预实验,证实图下沉标记并非下游预测中最关键的语义或结构标记。结果表明,现有图标记映射机制未能形成有效的拓扑感知内部表示,而是导致激活显著性与图语义效用之间的解耦,暴露出图标记构建、放置与对齐机制的局限性。
原文摘要 · Abstract (English)
Graph Language Models (GLMs) have become a promising direction for adapting Large Language Models (LLMs) to graph learning tasks. By transforming graph topology and node information into graph tokens, GLMs allow LLMs to jointly process structured graph inputs and textual instructions. Yet, it remains unclear how LLMs internally interpret these graph tokens and whether graph tokens act as meaningful carriers of graph structure. In this work, we analyze how LLMs process graph information through graph-token behavior in representative GLM architectures. Findings. We find that the internal saliency of graph tokens in GLMs is not equivalent to graph information utilization. Graph sink tokens consistently emerge as activation-level outliers: they can be identified by massive activation values along a small set of hidden-state dimensions and are biased toward early graph-token positions. However, this activation-level saliency does not imply that these tokens are the main carriers of graph information. Unlike classical attention sinks in language and vision-language models, graph sink tokens do not necessarily attract the largest attention weights from query tokens. Through pruning, repositioning, and swapping interventions, we show that graph sink tokens are not the most important semantic or structural tokens for downstream prediction. Implications. Together, these results suggest that after current GLMs map graph structure into the LLM token space, the resulting graph-token representations do not naturally form a fully usable topology-aware internal representation; instead, they exhibit a decoupling between activation-level saliency and graph-semantic utility. This decoupling points to limitations in existing graph-token construction, placement, and alignment mechanisms.
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