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Robustness and Interpretability

Experiment codes for Bridging Adversarial Robustness and Gradient Interpretability. The full paper is available on arXiv.

The work is presented at 2019 ICLR Workshop on SafeML (Oral Presentation).

We summarize the results from our paper along with the links to the jupyter notebooks that can be used to reproduce the experiments.

Section 2.1. Adversarial Training Confines Gradient to Image Manifold

Section Summary

To identify why adversarial training increases the perceptual quality of loss gradients, we hypothesized adversarial examples from adversarially trained networks lie closer to the image manifold. Note that the loss gradient is the difference between the original image and the adversarial image. Hence if the adversarial image lies closer to the image manifold, the loss gradient should align better with human perception. To see this, we used VAE-GANs to indirectly calculate the distance between an image (both natural and adversarial) and its projection onto the approximated image manifold.

Experiment Procedure

  1. Notebooks 1.1, 2.1 and 3.1 were used to train the VAE-GAN.

  2. Notebooks 1.2, 2.2 and 3.2 were used to adversarially train the networks.

  3. Notebooks 1.3, 2.3 and 3.3 were used to calculate the distance of adversarial examples to their projections onto the image manifold and to visualize the loss gradients and the adversarial examples.

Results

We observed that for all datasets, adversarial examples for adversarially trained DNNs lie closer to the image manifold than those for standard DNNs. This suggests that adversarial training restricts loss gradients to the image manifold. Hence gradients from adversarially trained DNNs are more visually interpretable. Interestingly, we also observed that using stronger attacks during training reduces the distance between adversarial examples and their projections even further.

MNIST
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Fashion MNIST
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CIFAR-10
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Section 2.2. A Conjecture Under the Boundary Tilting Perspective

Section Summary

In this section, we proposed a conjecture for why adversarial training confines the gradient to the manifold. Basically, our conjecture is based on the boundary tilting perspective on the phenomenon of adversarial examples. Adversarial examples exist because the decision boundary of a standard DNN is “tilted” along directions of low variance in the data (standard decision boundary). Under certain situations, the decision boundary will lie close to the data such that a small erturbation directed toward the boundary will cause the data to cross the boundary. Moreover, since the decision boundary is tilted, the perturbed data will leave the image manifold.

We conjectured adversarial training removes such tilting along directions of low variance in the data (robust decision boundary). Intuitively, this makes sense because a network is robust when only large-epsilon attacks are able to cause a nontrivial drop in accuracy, and this appens when data points of different classes are mirror images of each other with respect to the decision boundary. Since the loss gradient is generally perpendicular to the decision boundary, it will be confined to the image manifold and thus adversarial examples stay within the image manifold.

Experiment Procedure

As a sanity check, we tested our conjecture with a 2-dimensional toy dataset. Specifically, we trained three two-layer ReLU network to classify points from two distinct bivariate Gaussian distributions. The first network is trained on original data and the latter two networks are trained against weak and strong adversaries. We then compared the resulting decision boundaries and the distribution of adversarial examples. Notebook 4.1 was used to test our conjecture with a 2-dimensional toy dataset.

Results

The decision boundary of the standard DNN is tilted along directions of low variance in the data. Naturally, the adversarial examples leave the data manifold. On the other hand, adversarial training removes the tilt. Hence adversarial perturbations move data points to the manifold of other class. We also observed training against a stronger adversary removes the tilt to a larger degree. This causes adversarial examples to align better with the data manifold. Hence the decision boundary tilting perspective may also account for why adversarial training with stronger attack reduces the distance between an adversarial example and its projection even further.

Illustration of our conjecture Results on a 2-dimensional toy dataset
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Section 3.2. Effect of Adversarial Training on Loss Gradient Interpretability

Section Summary

We established a formal framework for evaluating the quantitative interpretability of a given attribution method based on Remove and Retrain (ROAR) and Keep and Retrain (KAR). We then obsered the effect of adversarial training on the quantitative interpretability of two attribution methods Gradient and Gradient * Input.

Experiment Procedure

  1. Notebook 3.2 was used to train the network under various adversarial attack settings.

  2. Notebook 3.4 was used to run ROAR and KAR for neural networks trained under various adversarial attack settings.

  3. Notebook 3.5 was used to analyze the relation between the strength of adversary used during training and the interpretability of gradient.

Results

We observed that adversarial training generally enhances the interpretability of attributions. This result is significant because it shows adversarial training indeed causes the gradient to better reflect the internal representation of the DNN. It implies that training with an appropriate “interpretability regularizer” may be enough to produce DNNs that can be interpreted with simple attribution methods such as gradient or Gradient * Input.

ROAR KAR
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Section 3.3. Accuracy and Loss Gradient Interpretability Trade-off

Section Summary

Using the results from Section 3.2, we were also able to discover a trade-off between test accuracy and gradient interpretability under the adversarial training framework.

Experiment Procedure

We reused the codes in Section 3.2.

Results

From the results, we saw two potential approaches to resolving this trade-off. First, since the global attribution method Gradient * Input performs better than the local attribution method Gradient, we can explore combinations of adversarial training with other global attribution methods such as Layer-wise Relevance Propagation, DeepLIFT or Integrated Gradient. Second, since there is large performance gain in using l1-training over l2-training in KAR while there is only slight gain in using l2-training over l1-training in ROAR, we can seek better ways of applying l1-training.

ROAR KAR
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Citation

If you want to cite our work, please use the below .bib format.

@article{kim2019bridging,
  title={Bridging Adversarial Robustness and Gradient Interpretability},
  author={Beomsu Kim, Junghoon Seo, and Taegyun Jeon},
  journal={arXiv preprint arXiv:1903.11626},
  year={2019}
}

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