Medical vision-language models hallucinate phrases that no pixel supports, and CLEAR can localize which ones
AspireAI community · arXiv
Medical vision-language models write fluent radiology reports and answer clinical visual questions, but often assert things the image does not show - a failure the authors call visually irrelevant hallucination. CLEAR is a model-agnostic post-hoc method that decomposes a response into clinically assessable phrases, grounds each phrase to a candidate visual region, perturbs just that region with a localized counterfactual, and combines response-change cues with image-phrase alignment cues into a phrase-level risk score. Across four datasets spanning radiology report generation and medical VQA it beats state-of-the-art hallucination detection baselines across several medical and general-purpose models, raising AUPRC on MIMIC-CXR from 85.10% to 90.23%, and it holds up on a ground-truth-derived controlled benchmark. The method costs an extra model pass per phrase, and it estimates risk rather than correcting the output.
Read the story and explore more from the AspireAI community.
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.