让卷积网络的特征图直接指向分类结果,提升可解释性。
End-to-end Feature Alignment: A Simple CNN with Intrinsic Class Attribution
- 通过保留顺序的层设计,实现输入到输出的特征对齐。
- 特征图与Grad-CAM热力图完全一致,且逐层平滑演化。
- 适合需要模型可解释性的图像分类任务。
我们提出特征对齐卷积神经网络(FA-CNN),一种具有内在类别归属能力的端到端卷积架构。其核心思想是:线性层和二维卷积层等无序操作会导致语义概念的混乱混合,使原始特征图难以理解。为此,我们引入两种保持顺序的新层——阻尼跳跃连接和全局平均池化分类头,强制模型从原始像素到最终类别逻辑值全程保持特征对齐。这种对齐显著提升了模型可解释性,使原始特征图能内在地表现出类别归属。理论证明,FA-CNN的倒数第二层特征图与Grad-CAM显著性图完全相同。同时,这些特征图随网络深度逐层缓慢演变,展示了特征向类别激活演化的全过程。FA-CNN在基准图像分类数据集上表现良好。我们还通过移除像素百分比的任务,对比了平均特征图与Grad-CAM及置换方法的可解释性,验证了其优势。最后讨论了局限性与向混合模型扩展的未来方向。
原文摘要 · Abstract (English)
We present Feature-Align CNN (FA-CNN), a prototype CNN architecture with intrinsic class attribution through end-to-end feature alignment. Our intuition is that the use of unordered operations such as Linear and Conv2D layers cause unnecessary shuffling and mixing of semantic concepts, thereby making raw feature maps difficult to understand. We introduce two new order preserving layers, the dampened skip connection, and the global average pooling classifier head. These layers force the model to maintain an end-to-end feature alignment from the raw input pixels all the way to final class logits. This end-to-end alignment enhances the interpretability of the model by allowing the raw feature maps to intrinsically exhibit class attribution. We prove theoretically that FA-CNN penultimate feature maps are identical to Grad-CAM saliency maps. Moreover, we prove that these feature maps slowly morph layer-by-layer over network depth, showing the evolution of features through network depth toward penultimate class activations. FA-CNN performs well on benchmark image classification datasets. Moreover, we compare the averaged FA-CNN raw feature maps against Grad-CAM and permutation methods in a percent pixels removed interpretability task. We conclude this work with a discussion and future, including limitations and extensions toward hybrid models.
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