Beyond Anti-Forgetting: Multimodal Continual Instruction Tuning with Positive Forward Transfer

Multimodal Continual Instruction Tuning (MCIT) enables Multimodal LLMs (MLLMs) to meet continuously emerging requirements without expensive retraining. MCIT faces two major obstacles: catastrophic forgetting (where old knowledge is forgotten) and negative forward transfer (where the performance of future tasks is degraded). Although existing methods have greatly alleviated catastrophic forgetting, they still suffer from negative forward transfer. By performing singular value decomposition (SVD) on input embeddings, we discover a large discrepancy in different input embeddings. The discrepancy results in the model learning irrelevant information for old and pre-trained tasks, which leads to catastrophic forgetting and negative forward transfer. To address these issues, we propose Fwd-Prompt, a prompt-based method projecting prompt gradient to the residual space to minimize the interference between tasks and to the pre-trained subspace for reusing pre-trained knowledge. Our experiments demonstrate that Fwd-Prompt achieves state-of-the-art performance while updating fewer parameters and requiring no old samples. Our research sheds light on the potential of continuously adapting MLLMs to new tasks under the instruction tuning paradigm and encourages future studies to explore MCIT. The code will soon be publicly available.