Towards Understanding the Regularization of Adversarial Robustness on Neural Networks
Authors: Yuxin Wen, Shuai Li, Kui Jia
Keywords: Adversarial robustness, Statistical Learning, Regularization
TL;DR: We study the accuracy degradation in adversarial training through regularization perspective and find that such training induces diffident NNs that concentrate prediction around decision boundary which leads to worse standard performance.
Abstract:   The problem of adversarial examples has shown that modern Neural Network (NN) models could be rather fragile. Among the most promising techniques to solve the problem, one is to require the model to be {\it $\epsilon$-adversarially robust} (AR); that is, to require the model not to change predicted labels when any given input examples are perturbed within a certain range. However, it is widely observed that such methods would lead to standard performance degradation, i.e., the degradation on natural examples.  In this work, we study the degradation through the regularization perspective.  We identify quantities from generalization analysis of NNs; with the identified quantities we empirically find that AR is achieved by regularizing/biasing NNs towards less confident solutions by making the changes in the feature space (induced by changes in the instance space) of most layers smoother uniformly in all directions; so to a certain extent, it prevents sudden change in prediction w.r.t.  perturbations. However, the end result of such smoothing concentrates samples around decision boundaries, resulting in less confident solutions, and leads to worse standard performance.  Our studies suggest that one might consider ways that build AR into NNs in a gentler way to avoid the problematic regularization.