Federated deepfake detectors kept trustworthy explanations until client heterogeneity crossed a threshold, then they fell apart entirely
AspireAI community · arXiv (Grad-CAM federated study)
Federated learning lets organizations train a deepfake detector without pooling sensitive data, but analysts depend on gradient-based explanations to judge whether a flagged clip deserves attention, and whether federation degrades those explanations had not been studied. This work evaluates Grad-CAM faithfulness for an EfficientNet-B4 detector on FaceForensics++ across five federated clients at varying degrees of data heterogeneity. The relationship is non-monotonic: at near-IID and moderate heterogeneity, federated models match or exceed a centralized baseline's faithfulness, but at extreme heterogeneity the explanations break down, becoming spatially incoherent and on average causally misleading. That is a threshold effect tied to client heterogeneity rather than a general cost of federated training, and the paper offers practical guidance on when privacy-preserving deepfake detection can be deployed safely. Code and data partitions are stated to be available for reproducibility.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.