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Title: Eligibility traces provide a data-inspired alternative to backpropagation through time

Paper content:
Learning in recurrent neural networks (RNNs) is most often implemented by gradient descent using backpropagation through time (BPTT), but BPTT does not model accurately how the brain learns.
Instead, many experimental results on synaptic plasticity can be summarized as three-factor learning rules involving eligibility traces of the local neural activity and a third factor.
We present here eligibility propagation (e-prop), a new factorization of the loss gradients in RNNs that fits the framework of three factor learning rules when derived for biophysical spiking neuron models.
When tested on the TIMIT speech recognition benchmark, it is competitive with BPTT both for training artificial LSTM networks and spiking RNNs.
Further analysis suggests that the diversity of learning signals and the consideration of slow internal neural dynamics are decisive to the learning efficiency of e-prop.
The brain seems to be able to solve tasks such as counting, memorizing and reasoning which require efficient temporal processing capabilities.
It is natural to model this with recurrent neural networks (RNNs), but their canonical training algorithm called backpropagation through time (BPTT) does not appear to be compatible with learning mechanisms observed in the brain.
There, long-term changes of synaptic efficacies depend on the local neural activity.
It was found that the precise timing of the electric pulses (i.e. spikes) emitted by the pre-and post-synaptic neurons matters, and these spike-timing dependent plasticity (STDP) changes can be conditioned or modulated by a third factor that is often thought to be a neuromodulator (see [1, 2] for reviews).
Looking closely at the relative timing, the third factor affects the plasticity even if it arrives with a delay.
This suggests the existence of local mechanisms that retain traces of the recent neural activity during this temporal gap and they are often referred to as eligibility traces [2] .
To verify whether three factor learning rules can implement functional learning algorithms, researchers have simulated how interesting learnt behaviours can emerge from them [1, 3, 4] .
The third factor is often considered as a global signal emitted when a reward is received or predicted, and this alone can solve learning tasks of moderate difficulty, even in RNNs [4] .
Yet in feed-forward networks, it was already shown that plausible learning algorithms inspired by backpropagation and resulting in neuron-specific learning signals largely outperform the rules based on a global third factor [5, 6, 7] .
This suggests that backpropagation provides important details that are not captured by all three factor learning rules.
Here we aim at a learning algorithm for RNNs that is general and efficient like BPTT but remains plausible.
A major plausibility issue of BPTT is that it requires to propagate errors backwards in time or to store the entire state space trajectory raising questions on how and where this is performed in the brain [8] .
We suggest instead a rigorous re-analysis of gradient descent in RNNs that leads to a gradient computation relying on a diversity of learning signals (i.e. neuron-specific third factors) and a few eligibility traces per synapse.
We refer to this algorithm as eligibility propagation (e-prop).
When derived with spiking neurons, e-prop fits under the three factor learning rule framework and is qualitatively compatible with experimental data [2] .
To test its learning efficiency, we applied e-prop to artificial Long Short-Term Memory (LSTM) networks [9] , and Long short-term memory Spiking Neural Networks (LSNNs) [10] (spiking RNNs combining short and long realistic time constants).
We found that (1) it is competitive with BPTT on the TIMIT speech recognition benchmark, and (2) it can solve nontrivial temporal credit assignment problems with long delays.
We are not aware of any comparable achievements with previous three factor learning rules.
Real-time recurrent learning (RTRL) [11] computes the same loss gradients as BPTT in an online fashion but requires many more operations.
Eventhough the method is online, one may wonder where can it be implemented in the brain if it requires a machinery bigger than the network itself.
Recent works [12, 13, 6] have suggested that eligibility traces can be used to approximate RTRL.
This was shown to be feasible if the neurons do not have recurrent connections [6] , if the recurrent connections are ignored during learning [12] or if the network dynamics are approximated with a trained estimator [13] .
However these algorithms were derived for specific neuron models without long-short term memory, making it harder to tackle challenging RNN benchmark tasks (no machine learning benchmarks were considered in [6, 12] ).
Other mathematical methods [14, 15] , have suggested approximations to RTRL which are compatible with complex neuron models.
Yet those methods lead to gradient estimates with a high variance [15] or requiring heavier computations when the network becomes large [14, 11] .
This issue was solved in e-prop, as the computational and memory costs are the same (up to constant factor) as for running any computation with the RNN.
This reduction of the computational load arises from an essential difference between e-prop and RTRL: e-prop computes the same loss gradients but only propagates forward in time the terms that can be computed locally.
This provides a new interpretation of eligibility traces that is mathematically grounded and generalizes to a broad class of RNNs.
Our empirical results show that such traces are sufficient to approach the performance of BPTT despite a simplification of the non-local learning signal, but we believe that more complex strategies for computing a learning signals can be combined with e-prop to yield even more powerful online algorithms.
A separate paper presents one such example to enable one-shot learning in recurrent spiking neural networks [8] .

assistant: text: We present eligibility propagation an alternative to BPTT that is compatible with experimental data on synaptic plasticity and competes with BPTT on machine learning benchmarks.