Explaining Neural Networks via Perturbing Important Learned Features

Attributing the output of a neural network to the contribution of given input elements is one way of shedding light on the black box nature of neural networks. We propose a novel input feature attribution method that finds an input perturbation that maximally changes the output neuron by exclusively perturbing important hidden neurons (i.e. learned features) on the path to output neuron. Given an input, this is achieved by 1) pruning unimportant neurons, and subsequently 2) finding a local input perturbation that maximizes the output in the pruned network. Since our method considers the importance of hidden neurons (high-level features), it inherently considers interdependencies between multiple input elements, which is vital for input feature attribution. We propose PruneGrad, an efficient gradient-based solution for the pruning and perturbation steps of our method. The efficacy of our method is evaluated by quantitatively benchmarking against other attribution methods using 1) sanity checks, 2) pixel perturbation, and 3) Remove and Retrain (ROAR). Our results show that while most of the existing attribution methods are prone to fail or get mediocre results in at least one benchmark, our proposed method achieves state of the art results in all three benchmarks. The results are further supported by comparative visual evaluation.