Here is a very good video about understanding neural networks if you know a little digital signal processing / wavelets.
https://youtu.be/nHXO43BqeQw
I was also able to find a couple of papers about decision regions in neural networks:
https://arxiv.org/abs/1705.09552
https://drive.google.com/open?id=1X4XfnO1dVnRzVmjBCgnm6yblmKLvlFsv