A spike-based long short-term memory on a neurosynaptic processor.

Low-power brain-inspired hardware systems have gained significant traction in recent years. They offer high energy efficiency and massive parallelism due to the distributed and asynchronous nature of neural computation through low-energy spikes. One such platform is the IBM TrueNorth Neurosynaptic System. Recently TrueNorth compatible representation learning algorithms have emerged, achieving close to state-of-the-art performance in various datasets. An exception is its application in temporal sequence processing models such as recurrent neural networks (RNNs), which is still at the proof of concept level. This is partly due to the hardware constraints in connectivity and synaptic weight resolution, and the inherent difficulty in capturing temporal dynamics of an RNN using spiking neurons. This work presents a design flow that overcomes the aforementioned difficulties and maps a special case of recurrent networks called Long Short-Term Memory (LSTM) onto a spike-based platform. The framework is built on top of various approximation techniques, weight and activation discretization, spiking neuron sub-circuits that implements the complex gating mechanisms and a store-and-release technique to enable neuron synchronization and faithful storage. While many of the techniques can be applied to map LSTM to any SNN simulator/emulator, here we demonstrate this approach on the TrueNorth chip adhering to its constraints. Two benchmark LSTM applications, parity check and Extended Reber Grammar, are evaluated and their accuracy, energy and speed tradeoffs are analyzed.