Constructive role of plasticity rules in reservoir computing

Barrios, Guillermo (Supervisors: Miguel C. Soriano, Claudio Mirasso)
Master Thesis (2019)

Over the last 15 years, Reservoir Computing (RC) has emerged as an appealing approach in Machine Learning, combining the high computational capabilities of Recurrent Neural Networks with a fast and easy training. By mapping the inputs into a high-dimensional space of non-linear neurons, this class of algorithms have shown their utility in a wide range of tasks from speech recognition to time series prediction. With their popularity on the rise, new works have pointed to the possibility of RC as an existing learning paradigm within the actual brain. Likewise, successful implementation of biologically based plasticity rules into RC artificial networks has boosted the performance of the original models. Within these nature-inspired approaches, most research lines focus on improving the performance achieved by previous works on prediction or classification tasks. In this thesis however, we will address the problem from a different perspective: instead on focusing on the results of the improved models, we will analyze the role of plasticity rules on the changes that lead to a better performance. To this end, we implement synaptic and non-synaptic plasticity rules in a standard Echo State Network , which is a paradigmatic example of an RC model. Testing on temporal series prediction tasks, we show evidence that improved performance in all plastic models may be linked to a decrease in spatio-temporal correlations in the reservoir, as well as a significant increase on individual neurons ability to separate similar inputs in their activity space. From the perspective of the reservoir dynamics, optimal performance is suggested to occur at the edge of instability. This is a hypothesis previously suggested in literature, but we hope to provide new insight on the matter through the study of different stages on the plastic learning. Finally, we show that it is possible to combine different forms of plasticity (namely synaptic and non-synaptic rules) to further improve the performance on prediction tasks, obtaining better results than those achieved with single-plasticity models.

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