Searching Discriminative Regions for Convolutional Neural Networks in Fundus Image Classification With Genetic Algorithms

Deep convolutional neural networks (CNNs) have been widely used for fundus image classification and have achieved very impressive performance. However, the explainability of CNNs is poor because of their black-box nature, which limits their application in clinical practice. In this paper, we propose a novel method to search for discriminative regions to increase the confidence of CNNs in the classification of features in specific category, thereby helping users understand which regions in an image are important for a CNN to make a particular prediction. In the proposed method, a set of superpixels is selected in an evolutionary process, such that discriminative regions can be found automatically. Many experiments are conducted to verify the effectiveness of the proposed method. The average drop and average increase obtained with the proposed method are 0 and 77.8%, respectively, in fundus image classification, indicating that the proposed method is very effective in identifying discriminative regions. Additionally, several interesting findings are reported: 1) Some superpixels, which contain the evidence used by humans to make a certain decision in practice, can be identified as discriminative regions via the proposed method; 2) The superpixels identified as discriminative regions are distributed in different locations in an image rather than focusing on regions with a specific instance; and 3) The number of discriminative superpixels obtained via the proposed method is relatively small. In other words, a CNN model can employ a small portion of the pixels in an image to increase the confidence for a specific category.