# Credit Assignment in Spiking Neural Networks

The problem of spatial and temporal credit assignment in RNNs are solved through backpropagating errors in the unrolled RNN.

Algorithmic solutions to RNNs have 2 challenges in SNNs. First, spiking neurons have \$S(U(t)) = $$\Theta(U(t) - \theta)$$. Their derivative is zero everywhere except at $$U = \theta$$, where it is ill-defined. This binary spiking non-linearity stops gradients from flowing, and makes gradient-based optimization unsuitable. The same issues occur in binary neurons.

Second, BP is expensive in terms of computation and memory. These restrictions may be poorly suited to the hardware that implements it. For example, non-von Neumann architectures have specific locality requirements. The forward propagation approach may be more favourable.

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