Quantified Semantic Comparison of Convolutional Neural Networks

The state-of-the-art in convolutional neural networks (CNNs) for computer vision excels in performance, while remaining opaque. But due to safety regulations for safety-critical applications, like perception for automated driving, the choice of model should also take into account how candidate models represent semantic information for model transparency reasons. To tackle this yet unsolved problem, our work proposes two methods for quantifying the similarity between semantic information in CNN latent spaces. These allow insights into both the flow and similarity of semantic information within CNN layers, and into the degree of their similitude between different networks. As a basis, we use renown techniques from the field of explainable artificial intelligence (XAI), which are used to obtain global vector representations of semantic concepts in each latent space. These are compared with respect to their activation on test inputs. When applied to three diverse object detectors and two datasets, our methods reveal the findings that (1) similar semantic concepts are learned \emph{regardless of the CNN architecture}, and (2) similar concepts emerge in similar \emph{relative} layer depth, independent of the total number of layers. Finally, our approach poses a promising step towards informed model selection and comprehension of how CNNs process semantic information.