The knowledge learnt by an ANN is its snaptic weights, not the hidden neurons. Hidden neurons are computation nodes that take the inputs and their associated weights from the input layer, put them through some activation function to produce the outputs for the output layer. Check this out:
AI : Neural Network for beginners (Part 1 of 3)[
^].
If you are looking for some machine learning algorithm that can produce graphical representation of its learning outcome, try SOM. Check this out:
Self-organizing Map Demystified[
^]