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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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