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Video RLVR post-training quietly taught models to guess from text priors, and a gradient analysis traced the collapse to perceptual uncertainty

AspireAI community · arXiv
A study of reinforcement learning with verifiable rewards on video reasoning found that the reward signal produces shortcut amplification: models increasingly answer from text priors even when the visual evidence does not support the answer. The mechanism, perceptual uncertainty collapse, is that RL keeps sharpening the answer distribution while the share of uncertainty reduction actually attributable to the video shrinks. A gradient-direction analysis shows the growing confidence becomes less aligned with visually grounded uncertainty and more dominated by text-prior compression. The proposed fix, BSPO, adds shortcut-aware visual perturbation that penalises predictions which stay the same when the relevant visual evidence is weakened, while keeping the task-solving gains of RLVR.
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