  Short-term plasticity (STP) is a mechanism that stores decaying memories in
synapses of the cerebral cortex. In computing practice, STP has been used, but
mostly in the niche of spiking neurons, even though theory predicts that it is
the optimal solution to certain dynamic tasks. Here we present a new type of
recurrent neural unit, the STP Neuron (STPN), which indeed turns out strikingly
powerful. Its key mechanism is that synapses have a state, propagated through
time by a self-recurrent connection-within-the-synapse. This formulation
enables training the plasticity with backpropagation through time, resulting in
a form of learning to learn and forget in the short term. The STPN outperforms
all tested alternatives, i.e. RNNs, LSTMs, other models with fast weights, and
differentiable plasticity. We confirm this in both supervised and reinforcement
learning (RL), and in tasks such as Associative Retrieval, Maze Exploration,
Atari video games, and MuJoCo robotics. Moreover, we calculate that, in
neuromorphic or biological circuits, the STPN minimizes energy consumption
across models, as it depresses individual synapses dynamically. Based on these,
biological STP may have been a strong evolutionary attractor that maximizes
both efficiency and computational power. The STPN now brings these neuromorphic
advantages also to a broad spectrum of machine learning practice. Code is
available at https://github.com/NeuromorphicComputing/stpn
