What are Volterra classifiers

I want to learn about Volterra classifiers in-depth to understand the following papers
Papers

  1. https://openaccess.thecvf.com/content_cvpr_2014/papers/Kang_Convolutional_Neural_Networks_2014_CVPR_paper.pdf
  2. https://www.researchgate.net/publication/51855543_Trainable_Convolution_Filters_and_Their_Application_to_Face_Recognition#:~:text=Each%20Volterra%20classifier%20is,has%20a%20three%2Dtiered%20architecture.
    I planning to do a mini project and want to understand these papers.
    Need introductory materials for better understanding