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Prompt-tuned vision-language models are underconfident on known classes and overconfident on new ones

AspireAI community · arXiv
Prompt-tuning a VLM while keeping the pretrained weights frozen produces a dual miscalibration: underconfidence on base classes and overconfidence on novel ones. Existing calibration work addresses the two separately or jointly but lacks a geometric account of the hyperspherical embedding space. M-PFT adds an angular margin to prompt tuning with two objectives - minimising the maximum cosine similarity between tunable class embeddings to reduce embedding collapse, and minimising the angular distance between each tunable embedding and its zero-shot embedding to preserve semantic geometry. Across 9 prompt-tuning methods, several backbones and 15 datasets it reduces average calibration error on novel classes while staying competitive on base classes and keeping comparable accuracy, at negligible computational overhead. The analyses attribute the gain to adjusting the distorted decision boundaries prompt tuning induces while preserving the pretrained embedding space. WACV 2027.
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One distance function matched sparse-autoencoder features across layers and compressed feature circuits into supernodes
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.
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