4. Deep neural networks (DNN)

The main feature of deep neural networks is their large number of hidden layers. There are three main classes of deep neural networks:

• fully connected unidirectional neural networks, also known as artificial neural networks (ANN) or multi-layer perceptrons (MLP);

• convolutional neural networks (CNNs) are unidirectional, with specific layers that perform pre-processing;

• in recurrent neural networks, information can flow in both directions.