“The backpropagation algorithm has three steps.

1. Flow information forward through a network to compute a prediction.
2. Compute an error by comparing the prediction to a target value.
3. Flow the error backward through the network to update the weights.

The backpropagation algorithm requires information to flow forward and backward along the network. But biological neurons are one-directional.

[…]

The authors have unified predictive coding and backpropagation into a single theory of neural networks. Predictive coding and backpropagation are separate hardware implementations of what is ultimately the same algorithm.”