Biological plausibility of C1-C3 and φ

We thank all reviewers for their comments. We will correct all typos and address all minor comments in the final paper. Other criticisms for BP (spiking and recurrent neural networks) remain? This work was published on 7 Jun 2020 (after the NeurIPS'20 It includes another approximation to BP but no equivalence; we will add it to the related work. Please see line 8 in the response to Reviewer #1.