Vision-Language Models (VLMs) provide a unified framework to process both text-only tasks and vision-language tasks. However, finetuning VLMs on vision-language data has degraded language capabilities. In this paper, we prove that as the training loss declines during finetuning, the visual representation and textual representation move closer to each other, a phenomenon we term “representation mixing.” We prove that the representation mixing occurring within the post-representation layers causes the degradation of language capabilities. Post-representation layers refer to the first few layers in LLMs that are involved in representation learning. To preserve the language capabilities, we propose the Representation Regulation for VLM Training (RRVLM), which introduces a Representation Distribution Difference (RDD) loss to reduce the distance between these representations. Extensive experiments on various benchmarks and VLM frameworks show that our method can effectively preserve the language capabilities and achieve superior vision-language performance.

Open-source Large Language Models (LLMs) often employ safety alignment methods to resist harmful instructions. However, recent research shows that maliciously fine-tuning these LLMs on harmful data can easily bypass these safeguards. To counter this, we theoretically uncover why malicious fine-tuning succeeds and identify potential defense strategies. Building on the theoretical analysis, we introduce the Self-Degraded Defense (SDD) framework. SDD encourages LLMs to produce high-quality but irrelevant responses to harmful prompts. When attackers attempt malicious fine-tuning, the general capability of the LLM aligned by SDD will significantly decrease, rendering it incapable of following harmful instructions. Our experimental results confirm SDD’s effectiveness against such attacks.Our code is available at https://github.com/ZeroNLP/SDD.