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LLM Enhancers for GNNs: An Analysis from the Perspective of Causal Mechanism Identification
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Analyzes the LLM-enhancer-plus-GNN paradigm via interchange interventions on a synthetic causal graph dataset (CCSG), derives three empirical findings, and proposes an attention-based transmission (AT) module that selects LLM output token features, improving GCN/GAT/GraphSAGE on Cora, Pubmed, and Instagram.
Problem
The fundamental properties and internal mechanisms of the LLM-enhancer-plus-GNN paradigm, where LLM-generated node features feed a GNN, remain underexplored.
Approach
Applies interchange intervention (causal abstraction) with a controllable causal synthetic graph dataset (CCSG) to align GNN hidden variables with a high-order causal model, and uses the findings to motivate an attention-based selection of LLM output token features passed to the GNN.
Key results
- The AT module improves accuracy across all tested LLM/GNN combinations, e.g., +2.03 on Cora with Qwen2+GCN and +3.09 on Pubmed with Llama3+GraphSAGE.
- Node- and graph-level interchange interventions show the LLM enhancer's features align with node-level and raw-data-level variables, with GNN alignments appearing in shallow layers.
- Optimal alignment loss L_II is lower for models with higher accuracy, suggesting L_II partially reflects model capability.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Add the AT module to Qwen2 + GCN | Qwen2 + GCN · Cora | accuracy: +2.03 | Helped |
| Add the AT module to Llama3 + GraphSAGE | Llama3 + GraphSAGE · Pubmed | accuracy: +3.09 | Helped |
| Add the AT module to Llama2 + GraphSAGE | Llama2 + GraphSAGE · Instagram | accuracy: +2.23 | Helped |
Surprising findings
- Scaling up the GNN only enlarges its structure without improving causal relation modeling, according to the authors' hypothesis.
- Changing which LLM token positions are passed to the GNN affects performance more than model scale or LLM choice in their analysis.
Limitations
- Analysis focuses on fixed-parameter LLM enhancers and mainly on GNN internals; LLM causal modeling not analyzed.
- Assumes node as smallest variable unit, ignoring internal node interactions.
- Theoretical results rely on assumption that both models map graphs to outputs exactly, which holds only approximately in practice.
Experimental setup
Datasets: CCSG Cora Pubmed Instagram
Benchmarks: Cora Pubmed Instagram
Metrics: accuracy L_II interchange intervention loss
Baselines: Llama2 Qwen2 Llama3
Builds on: interchange intervention causal abstraction TAPE
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