The human brain is what they call a recurrent neural network which means that it contains directed cycles. Classic neural networks are strictly hierarchical and

The human brain is what they call a recurrent neural network which means that it contains directed cycles. Classic neural networks are strictly hierarchical and one-way. If you take into account that signals can keep resonating in the brain (firing in circles) then the amount of layers is infinite. Recurrent neural networks are incredibly hard to train with the exception of liquid state machines (also known as echo state networks or reservoir computing).