A Unifying View of Gradient Calculations and Learning for Locally Recurrent Neural Networks

In this paper a critical review of gradient-based training methods for recurrent neural networks is presented including Back Propagation Through Time (BPTT), Real Time Recurrent Learning (RTRL) and several specific learning algorithms for different locally recurrent architectures. From this survey it comes out the need for a unifying view of all the specific procedures proposed for networks with local feedbacks, that keeps into account the general framework of recurrent networks learning: BPTT and RTRL. Therefore a learning method for local feedback network is proposed which combines together the best feature of BPTT, i.e. the lowest complexity, and of RTRL, i.e. the on-line operation, and includes as special case several specific algorithms already proposed, such as Temporal Back Propagation, Back Propagation for Sequences, Back-Tsoi algorithm and some others. In the general version, this new training method allows on-line efficient and accurate gradient calculation. It compares favourably with the previous algorithms in stability, speed/complexity trade off, accuracy.