Fine-Grained and High-Faithfulness Explanations for Convolutional Neural Networks

Recently, explaining CNNs has become a research hotspot. CAM (Class Activation Map)-based methods and LRP (Layer-wise Relevance Propagation) method are two common explanation methods. However, due to the small spatial resolution of the last convolutional layer, the CAM-based methods can often only generate coarse-grained visual explanations that provide a coarse location of the target object. LRP and its variants, on the other hand, can generate fine-grained explanations. But the faithfulness of the explanations is too low. In this paper, we propose FG-CAM (fine-grained CAM), which extends the CAM-based methods to generate fine-grained visual explanations with high faithfulness. FG-CAM uses the relationship between two adjacent layers of feature maps with resolution difference to gradually increase the explanation resolution, while finding the contributing pixels and filtering out the pixels that do not contribute at each step. Our method not only solves the shortcoming of CAM-based methods without changing their characteristics, but also generates fine-grained explanations that have higher faithfulness than LRP and its variants. We also present FG-CAM with denoising, which is a variant of FG-CAM and is able to generate less noisy explanations with almost no change in explanation faithfulness. Experimental results show that the performance of FG-CAM is almost unaffected by the explanation resolution. FG-CAM outperforms existing CAM-based methods significantly in the both shallow and intermediate convolutional layers, and outperforms LRP and its variations significantly in the input layer.