Entropy Is Not Enough: Unlocking Effective Reinforcement Learning for Visual Reasoning via Vision-Anchored Token Selection

Senjie Jin, Peixin Wang, Boyang Liu, Xiaoran Fan, Shuo Li, Zhiheng Xi, Jiazheng Zhang, Yuhao Zhou

#1482 of 3355 · Artificial Intelligence
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Tournament Score
1418±46
10501800
57%
Win Rate
12
Wins
9
Losses
21
Matches
Rating
6.5/ 10
Significance
Rigor
Novelty
Clarity

Abstract

While token-level entropy is commonly recognized as effective for credit assignment in text-only reinforcement learning with verifiable rewards (RLVR), it remains unclear whether this mechanism still holds in visual reasoning. Our controlled study shows that this mechanism collapses in visual reasoning due to the omission of vision-sensitive tokens with naturally low entropy. Although existing multimodal RL methods increasingly acknowledge the importance of visual perception, they struggle to satisfy the inherent demand for interleaving precise perceptual grounding with semantic reasoning, either lacking systematic visual measurements or overlooking that token entropy primarily drives semantic exploration. To address this, we introduce VEPO (Vision-Entropy token-selection for Policy Optimization), an effective RL framework explicitly integrating visual sensitivity with token entropy via a principled multiplicative coupling, where VEPO redirects gradient credit toward tokens which are simultaneously visually grounded and highly informative. Extensive experiments demonstrate VEPO's leading performance, significantly outperforming the entropy-only baseline by 2.28 points at 7B-scale and 3.15 points at 3B-scale. Ablations further substantiate the soundness of our method.

AI Impact Assessments

(1 models)

Scientific Impact Assessment: "Entropy Is Not Enough: Unlocking Effective Reinforcement Learning for Visual Reasoning via Vision-Anchored Token Selection"

1. Core Contribution

This paper identifies and diagnoses a specific failure mode: the entropy-based token selection mechanism that works well for text-only RLVR (reinforcement learning with verifiable rewards) collapses in visual reasoning tasks. The key insight is that vision-sensitive tokens naturally exhibit low entropy (because visual evidence disambiguates predictions), causing them to be systematically excluded by entropy-only selection. The proposed solution, VEPO, introduces a multiplicative coupling of visual sensitivity signals (Jensen-Shannon divergence and absolute entropy gap from counterfactual image perturbation) with token entropy, creating a joint scoring function that identifies "visual forking tokens"—tokens that are simultaneously visually grounded and informationally rich.

The problem formulation is clean: a counterfactual forward pass with a noise-perturbed image produces paired distributions, from which JSD captures distributional disagreement and |ΔH_t| captures uncertainty shift magnitude. These are combined via a noisy-OR-style aggregation and then modulated by entropy, selecting the top-k fraction for policy gradient updates.

2. Methodological Rigor

Strengths in experimental design:

  • The preliminary experiments (Section 2) are well-controlled, with three independent runs reported in appendix tables, providing error bars that strengthen credibility.
  • The diagnostic analysis (Figure 2) is compelling: at k=0.2, top-entropy selection recovers only 59% of top-JSD tokens, directly quantifying the mechanism failure.
  • Fair comparisons with baselines (VPPO, PAPO-DAPO, NoisyRollout, R1-ShareVL) are conducted on the same 4.2K training set.
  • Ablation studies systematically vary each component (JSD, |ΔH_t|, entropy), hyperparameters (α, k), perturbation type, and fusion mechanism.
  • Concerns:

  • The training set is relatively small (4.2K samples), which limits conclusions about scalability to larger training regimes.
  • Only Qwen2.5-VL models (3B and 7B) are tested; generalization to other architectures remains unverified.
  • The improvements, while consistent, are modest in absolute terms (2.28 points at 7B, 3.15 at 3B on average across benchmarks).
  • The additional forward pass for counterfactual perturbation adds ~16% overhead versus top-entropy selection, though it's ~10% faster than full GRPO due to sparse updates. This overhead may compound at scale.
  • The theoretical interpretation via aleatoric-epistemic decomposition (Appendix G) is well-constructed but serves as post-hoc justification rather than derivation—the authors acknowledge lacking rigorous theoretical foundations for why this improves optimization.
  • 3. Potential Impact

    Direct applications: The method is immediately applicable to any VLM training pipeline using RLVR, particularly for visual math reasoning, diagram interpretation, and visual grounding tasks. The framework is modular—the token selection mechanism can be integrated into existing RL frameworks with minimal modification.

    Broader influence: The paper contributes a conceptual insight that may influence how the community thinks about credit assignment in multimodal settings. The observation that different modalities contribute tokens with fundamentally different entropy characteristics could extend to audio-language, video-language, or other multimodal reasoning contexts. The counterfactual perturbation approach for measuring token-level visual dependency is a reusable measurement tool.

    Limitations on impact: The method is specific to the training phase and requires an additional forward pass per batch, which may limit adoption in resource-constrained settings. The reliance on Gaussian noise as the perturbation method is somewhat ad hoc, and the paper acknowledges limited exploration of alternatives.

    4. Timeliness & Relevance

    This paper addresses a timely bottleneck. The community has been rapidly scaling RLVR from text-only to multimodal settings, with multiple concurrent works (VPPO, PAPO, NoisyRollout) tackling visual perception in RL. The "80/20 rule" for entropy-based token selection has become a widely referenced finding, and demonstrating its failure in the visual domain is a valuable corrective. The paper positions itself well within a very active research front (multiple 2025-2026 citations), making it highly relevant.

    5. Strengths & Limitations

    Key Strengths:

  • Clear problem identification: The diagnostic analysis is the strongest contribution—demonstrating that high-JSD/|ΔH| tokens cluster in low-entropy regions (Figure 2a) is visually intuitive and empirically convincing.
  • Principled design: The noisy-OR aggregation with information-theoretic grounding (JSD as conditional mutual information, |ΔH| as aleatoric change) provides solid justification.
  • Comprehensive evaluation: Seven benchmarks, two model scales, multiple ablations, and qualitative analysis.
  • Reproducibility: Algorithm pseudocode, hyperparameter tables, and code release support reproducibility.
  • Notable Weaknesses:

  • Scale limitations: Only tested at 3B/7B with 4.2K training samples. The paper doesn't explore whether the phenomenon persists at 32B+ scale or with larger training sets.
  • Marginal gains on some benchmarks: MathVision shows a -0.64 drop versus the entropy baseline, and several out-of-domain gains are small.
  • Perturbation sensitivity: The method depends on Gaussian noise at a specific diffusion step (500), and the paper doesn't thoroughly explore sensitivity to this choice.
  • No theoretical convergence guarantees: The paper provides intuitive interpretations but no formal analysis of how sparse, modality-aware token selection affects policy optimization convergence.
  • Limited architectural diversity: Testing only on Qwen2.5-VL family leaves open whether the phenomenon and solution generalize to other VLM architectures (e.g., LLaVA, InternVL natively).
  • Additional Observations

    The qualitative cases (Figures 7-8) effectively illustrate the difference: VEPO selects content tokens tied to visual elements ("a", "c+b", "greater", "when") while entropy-only selects LaTeX formatting and discourse markers. This provides compelling evidence for the mechanism's face validity. The training dynamics comparison (Figure 5a-b) showing VEPO's stability versus competitors' premature convergence is also notable.

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

    Generated Jun 3, 2026

    Comparison History (21)

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