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PRISM: Topology-Aware Cross-Modal Imputation for Modality-Deficient Federated Graph Learning

Zekai Chen, Miao Zhang, Jiayang Xing, Xunkai Li, Xun Wu, Rong-Hua Li, Guoren Wang

cs.LG
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#3679 of 5669 · cs.LG
Tournament Score
1365±42
10501750
45%
Win Rate
10
Wins
12
Losses
22
Matches
Rating
6.8/ 10
Significance7
Rigor6.5
Novelty7.5
Clarity7

Abstract

Multimodal federated graph learning (MM-FGL) aims to collaboratively learn from decentralized graphs with text and images. However, real-world clients may not share a common modality basis: a visual-search client may contain image--interaction graphs but no seller descriptions, while a catalog client may provide text but no product images. We refer to this practical setting as client-level modality deficiency. Unlike random instance-wise missingness, a deficient client lacks the local semantic basis needed to reconstruct the absent modality. More importantly, in graph learning, incomplete representations initialize message passing, so imputation errors can be filtered, mixed, and amplified by the receiving topology. To address this gap, we propose \textbf{PRISM} (\textbf{P}roactive \textbf{R}etrieval and \textbf{I}mputation via \textbf{S}tructural \textbf{M}eta-prompting), a topology-aware federated cross-modal imputation framework. Rather than reconstructing the missing modality solely from local observations, PRISM recovers missing-modality semantics from the federation and introduces them into local graph propagation under topology-aware control. Experiments on six multimodal graph datasets across graph-centric and modality-centric tasks show that PRISM consistently improves modality-deficient clients, outperforming state-of-the-art baselines by \textbf{4.48}\% on average.

AI Impact Assessments

(1 models)

Scientific Impact Assessment: PRISM

1. Core Contribution

PRISM addresses a genuinely underexplored problem at the intersection of federated learning, multimodal learning, and graph neural networks: client-level modality deficiency, where entire clients (not just individual nodes) permanently lack one or more modalities. The paper's key insight is that in graph-structured data, imputation errors don't stay localized—they propagate, mix, and amplify through message passing. This "semantic boundary error" concept is the paper's most distinctive intellectual contribution.

The framework follows a "retrieve globally, inject structurally" principle with three main components: (1) a server-side Global Modality Bank storing multimodal prototypes, (2) proactive cross-modal retrieval using cluster-level queries conditioned on topology, and (3) virtual anchor injection with confidence-gated, topology-aware control. The design cleanly separates *what* semantics to borrow from *how* they enter graph propagation.

2. Methodological Rigor

Strengths in methodology:

  • The empirical study in Section III is well-designed, progressively building the case through three diagnostic questions (Q1–Q3) that directly motivate each design component. The correlation between error amplification ratio R_k^(L) and structural metrics (ρ=1.00 with average degree, ρ=0.97 with homophily) is compelling.
  • The theoretical analysis (Section V) provides useful formalization: Theorem 1 establishes local irrecoverability, Theorem 2 factorizes boundary risk into semantic residual × injection strength × structural exposure, and Theorem 3 bounds the risk under sparse retrieval. These are mechanistic analyses rather than convergence proofs, which the authors honestly acknowledge.
  • Experiments span six datasets, three tasks (node classification, modality matching, cross-modal retrieval), and 15 baselines across six categories, providing comprehensive coverage.
  • Concerns:

  • The ρ=1.00 correlation between R_k^(2) and average degree across only 5 clients per dataset is suspiciously perfect and statistically unreliable with such small sample sizes. This deserves more scrutiny.
  • The 40%:30%:30% modality split (full:image-only:text-only) is somewhat arbitrary. While the paper does vary missing ratios in Fig. 7, the main results rely on this single configuration.
  • The assumption that "at least a subset of clients observe paired multimodal evidence" is critical but not deeply examined—how does performance degrade as the fraction of full-modal clients decreases toward zero?
  • Privacy analysis is acknowledged as out-of-scope, but the prototype bank could leak aggregate information. The paper waves this away by saying differential privacy is "orthogonal," but in practice the interaction between noise injection and retrieval quality could be significant.
  • 3. Potential Impact

    Direct applications: The problem setting is practically motivated—e-commerce platforms where visual-search clients lack text descriptions while catalog clients lack images is a real scenario. IoT and V2X systems mentioned in the introduction also face analogous modality heterogeneity.

    Broader influence: The concept of topology-aware imputation control could influence:

  • General federated missing-data methods to consider downstream propagation effects
  • Graph augmentation strategies that account for structural amplification of synthetic features
  • Multimodal learning pipelines that incorporate graph structure awareness
  • The semantic boundary error concept, while intuitive, provides a useful vocabulary for discussing imputation risks in graph-structured settings. However, the practical deployment barrier remains high—the framework adds considerable complexity (spectral descriptors, motif statistics, meta-prompting, confidence gating) on top of standard FGL pipelines.

    4. Timeliness & Relevance

    The paper addresses a timely gap. Multimodal federated learning and federated graph learning are both active areas, but their intersection—particularly under realistic modality heterogeneity—is underexplored. The 2025-2026 references (FedSPA, FedIIH, S2FGL, MM-OpenFGL) indicate the authors are well-connected to the cutting edge. The problem will likely grow in importance as edge devices with heterogeneous sensor suites increasingly generate graph-structured data.

    5. Strengths & Limitations

    Key Strengths:

  • Problem formulation: Client-level modality deficiency is clearly distinguished from instance-level missingness, with concrete motivation.
  • Principled design: Each component (retrieval, structural prompting, confidence gating) is motivated by a specific diagnostic finding.
  • Comprehensive evaluation: Six datasets, three tasks, 15+ baselines, plus ablations, sensitivity analysis, backbone variations, asynchronous settings, and communication efficiency analysis.
  • Average improvement of 4.48% across missing-modality settings, with particularly strong gains on modality-centric tasks (10+ points on QB and Cartoon retrieval).
  • Notable Limitations:

  • Scalability questions: The spectral decomposition for topology descriptors could become expensive for large graphs. The paper doesn't test on graphs beyond the moderate scale of the benchmark datasets.
  • Limited modality types: Only text and images are considered. Extension to more modalities (audio, time-series) is unexplored.
  • No formal privacy guarantees: The prototype bank exchanges cluster-level summaries, but membership inference or attribute inference risks are not quantified.
  • Warm-up sensitivity: The framework requires Tw=30 warm-up rounds before activation, during which deficient clients train with incomplete features—a practical limitation.
  • Reproducibility: While implementation details are provided, the code availability is not mentioned. The complexity of the full pipeline (spectral descriptors, motif counting, meta-prompting, confidence gating, two-phase training) raises reproducibility concerns.
  • Statistical reporting: Results are reported with standard deviations, but some improvements over second-best methods fall within overlapping confidence intervals (e.g., Toys Full: 80.65±0.2 vs 80.25±0.2).
  • 6. Additional Observations

    The paper's writing is generally clear, though dense. The diagnostic study (Section III) is the paper's strongest methodological contribution—it could stand alone as an empirical finding about error propagation in graphs with imputed features. The theoretical analysis, while not providing convergence guarantees, effectively formalizes the intuitions. The comparison with graph augmentation methods (FedSage, FedDEP, FedC4) and topology-aware FGL methods (FedSPA, FedIIH) strengthens the evaluation by showing these related approaches are insufficient for the modality deficiency setting.

    Rating:6.8/ 10
    Significance 7Rigor 6.5Novelty 7.5Clarity 7

    Generated Jun 9, 2026

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