Title: Video Reasoning without Training
Primary area: foundation or frontier models, including LLMs
Keywords: Efficient Video Reasoning, Reasoning, Exploration-Exploitation

Abstract: Video reasoning using Large Multimodal Models (LMMs) relies on costly reinforcement learning (RL) and verbose chain-of-thought, resulting in substantial computational overhead during both training and inference. Moreover, the mechanisms that control the thinking process in these reasoning models are very limited. In this paper, using entropy of the model's output as a signal, we discover that the high-quality models go through a series of $\textit{micro-explorations}$ and $\textit{micro-exploitations}$ which keep the reasoning process grounded (i.e., avoid excessive randomness while the model is exploring or thinking through an answer). We further observe that once this ``thinking'' process is over, more accurate models demonstrate a better convergence by reducing the entropy significantly via a final exploitation phase (i.e., a more certain convergence towards a solution trajectory). We then use these novel, theoretically-grounded insights to tune the model's behavior directly at inference, without using any RL or supervised fine-tuning. Specifically, during inference, our proposed approach called V-Reason ($\underline{V}ideo-\underline{Reason}$) adapts the value cache of the LMM via a few optimization steps on a small, trainable controller using an entropy-based objective, i.e., no supervision from any dataset or RL is necessary. This tuning improves the model's micro-exploration and exploitation behavior during inference. Our experiments show that our proposed method achieves significant improvements over the base instruction-tuned models across several video reasoning datasets, narrowing the gap with RL-trained models to within $\textbf{0.6}$% average accuracy without any training, while offering massive efficiency benefits: output tokens are reduced by $\textbf{58.6}$% compared to the RL model.

TL;DR: We introduce a training-free method for enhancing video reasoning via value cache optimization during inference that narrows the gap to RL-trained models with fewer output tokens.