Reversion, Retention, and Semantic Intervention: Measuring Functional Content in CLIP Fine-Tuning Deltas

Overview & Synopsis

Fine-tuning adapts pretrained models to new tasks, but identifying which weight changes support the resulting behavior remains difficult. Weight-space interpretability assigns semantic descriptions to these changes, called fine-tuning deltas. Such descriptions leave open how much task performance depends on the identified updates, particularly when other updates can support the same predictions. We address this gap by pairing reversion, which restores selected parameters to their pretrained values, with retention, which keeps only selected fine-tuning updates. We evaluate Contrastive Language-Image Pretraining (CLIP) models on synthetic financial charts, Flowers102, EuroSAT, and FGVCAircraft with ViT-L/14, and on Flowers102 with ViT-B/16, using a shared recipe that tunes the final six visual blocks. Our contributions are threefold. First, attention value-output (VO) updates alone recover 43-87% of the accuracy improvement, although reverting them leaves chart accuracy unchanged. Cross-entropy loss preserves this contrast on Charts and EuroSAT, while correct-class score margins reveal additional contributions. Second, VO weights recover more loss improvement than all ten equally sized, layer-matched random subsets in each setting. VO and multilayer perceptron (MLP) retentions jointly correct 25-81% of pretrained errors, demonstrating overlapping recovered behavior, which we call operational redundancy. Third, removing semantically selected directions reduces VO-retained recovery more than matched-random removal in all five settings. Together, these results distinguish semantic alignment, removal cost, and retained recovery as complementary evidence about the functional role of fine-tuning updates within the evaluated CLIP recipe.