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Overcoming Generic Knowledge Loss with Selective Parameter Update
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Proposes Selective Parameter Update (SPU), which localizes continual updates of CLIP to the first MLP layer of each transformer block and sparsely updates the ~10% of those parameters with highest gradient-based scores, preserving generic zero-shot knowledge.
Problem
Continually updating foundation vision-language models (CLIP) on new class-incremental tasks causes loss of their generic pre-trained knowledge, which existing continual learning methods largely ignore.
Approach
Localize updates to the first MLP layer of each transformer block (viewed as key-value memories), then select a sparse subset of parameters by a training-free gradient-magnitude score computed before training, and update only those with replay.
Key results
- SPU achieves the best new-task average accuracy increment (21.34%) among compared methods, versus 18.48% for FLYP+ER.
- SPU has a control-set (ImageNet) accuracy drop of only 0.94%, the smallest among methods without external data, comparable to ZSCL which uses CC12M.
- Updating about 10% of the first MLP layer parameters (≈2.7–3.15% of total) gives the best trade-off; a 0.01 selection rate gives C. Drop 1.11 and 0.5 gives Avg. F. 7.76.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| selectively update first MLP-layer parameters using gradient-based scores | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | Acc. In.: 21.34 vs. 18.48 | Helped |
| selectively update first MLP-layer parameters | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | C. Drop: 0.94 vs. 17.56 | Helped |
| update attention layers instead of first MLP layers (ablation) | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | Acc. In.: 21.34 -> 20.21 | Hurt |
| update second MLP layers instead of first MLP layers (ablation) | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | C. Drop: 0.94 -> 1.85 | Hurt |
| update both MLP layers instead of only first MLP layers (ablation) | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | C. Drop: 0.94 -> 1.51 | Hurt |
| select parameters randomly instead of using the gradient-based score (ablation) | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | Acc. In.: 21.34 -> 17.73 | Hurt |
| use selection rate 0.01 instead of 0.10 (ablation) | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | Acc. In.: 21.34 -> 17.70 | Hurt |
| use selection rate 0.50 instead of 0.10 (ablation) | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | Avg. F.: 4.51 -> 7.76 | Hurt |
| reduce replay-buffer size from 4% to 1% (ablation) | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | Acc. In.: 21.34 -> 16.18 | Hurt |
| reduce replay-buffer size from 4% to 2% (ablation) | SPU on CLIP-ViT/B-16 · 6 CIL sequences (average) | Acc. In.: 21.34 -> 18.63 | Hurt |
Surprising findings
- Random parameter selection achieves nearly the same control-set preservation (C. Drop 0.04) but lower new-task gains, while the gradient-based selection improves over full fine-tuning on new tasks.
- Updating attention layers gives less forgetting but worse new-task accuracy than updating the first MLP layer.
Limitations
- Evaluated only on CLIP ViT-B/16 vision-language classification; generative tasks not explored.
- Relies on a replay buffer (~4% of data) for forgetting mitigation of new tasks.
- Control set accuracy drop (0.94%) is still nonzero and the method is unidirectional across domains.
Experimental setup
Datasets: Birdsnap CUB-200-2011 FGVC-Aircraft Stanford Cars CIFAR100 GTSRB ImageNet
Benchmarks: Birdsnap CUB-200-2011 FGVC-Aircraft Stanford Cars CIFAR100 GTSRB ImageNet (control set)
Metrics: Average accuracy increment (Acc. In.) Average forgetting (Avg. F.) Control set accuracy drop (C. Drop) Accuracy
Baselines: FLYP FLYP+ER MAS LwF PRD LoRA-EWC L2P DualPrompt SLCA ZSCL SparseCL SPG
Builds on: FLYP Experience replay (ER) Causal tracing (ROME-style analysis) Key-value memory view of MLP layers Piggyback
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