Caching a video as text versus caching the model's own state changes the answers, and token counts hide it
AspireAI community · arXiv
Video-understanding systems watch a recording once, build a memory, and answer many later questions from it - but what to keep is decided before anyone knows what will be asked, and whatever is dropped cannot be recovered. REEL compares three saved representations built from exactly the same observed frames, so any change in answers comes only from what the system remembered: Text writes a short record of what happened, Visual keeps the encoder outputs, and Prefix State also keeps the language-model prefix, each compared against full reprocessing. Across six vision-language models and two benchmarks, Text and Frames differ in correctness on 20% to 29% of questions even though their mean accuracy is nearly identical, while Visual and Prefix State stay much closer to Frames. Prefix State is cheapest in total GPU time among memories that keep almost all the answers Frames gets right, but it can hold about 21 GiB per video where Text needs kilobytes. The authors' broader point is that reusing computation and discarding information both look like efficiency, and neither mean accuracy nor token count distinguishes them.
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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.