Significance-based pruning for reservoir’s neurons in echo state networks

Echo State Networks (ESNs) are a family of Recurrent Neural Networks (RNNs), that can be trained efficiently and robustly. Their main characteristic is the partitioning of the recurrent part of the network, the reservoir, from the non-recurrent part, the latter being the only component which is explicitly trained. To ensure good generalization capabilities, the reservoir is generally built from a large number of neurons, whose connectivity should be designed in a sparse pattern. Recently, we proposed an unsupervised online criterion for performing this sparsification process, based on the idea of significance of a synapse, i.e., an approximate measure of its importance in the network. In this paper, we extend our criterion to the direct pruning of neurons inside the reservoir, by defining the significance of a neuron in terms of the significance of its neighboring synapses. Our experimental validation shows that, by combining pruning of neurons and synapses, we are able to obtain an optimally sparse ESN in an efficient way. In addition, we briefly investigate the resulting reservoir’s topologies deriving from the application of our procedure.