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Predictive Coding Can Do Exact Backpropagation on Any Neural Network (arxiv.org)
113 points by MindGods on Jun 3, 2021 | hide | past | pdf | 9 comments on HN

In plain words: A brain-inspired learning rule is defined directly on a network's step-by-step calculation graph, so each weight gets the same update as standard backpropagation. Unlike earlier brain-inspired rules that only matched it on simple networks, this one matches exactly on any network.

Abstract · Reverse Differentiation via Predictive Coding

Deep learning has redefined the field of artificial intelligence (AI) thanks to the rise of artificial neural networks, which are architectures inspired by their neurological counterpart in the brain. Through the years, this dualism between AI and neuroscience has brought immense benefits to both fields, allowing neural networks to be used in dozens of applications. These networks use an efficient implementation of reverse differentiation, called backpropagation (BP). This algorithm, however, is often criticized for its biological implausibility (e.g., lack of local update rules for the parameters). Therefore, biologically plausible learning methods that rely on predictive coding (PC), a framework for describing information processing in the brain, are increasingly studied. Recent works prove that these methods can approximate BP up to a certain margin on multilayer perceptrons (MLPs), and asymptotically on any other complex model, and that zero-divergence inference learning (Z-IL), a variant of PC, is able to exactly implement BP on MLPs. However, the recent literature shows also that there is no biologically plausible method yet that can exactly replicate the weight update of BP on complex models. To fill this gap, in this paper, we generalize (PC and) Z-IL by directly defining them on computational graphs, and show that it can perform exact reverse differentiation. What results is the first biologically plausible algorithm that is equivalent to BP in the way of updating parameters on any neural network, providing a bridge between the interdisciplinary research of neuroscience and deep learning.

Tommaso Salvatori, Yuhang Song, Thomas Lukasiewicz, Rafal Bogacz, Zhenghua Xu
arXiv:2103.04689 · cs.LG · submitted Mar 8, 2021 · updated Feb 20, 2023
abstract · pdf · html · 17 pages, 5 figures

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Different paper but yeah it's basically the same thing.
That’s what I thought-sounds much like the great stuff coming out of Chris Buckley‘s group.

The authors even refer to that work in the abstract. The “breakthrough“ seems to consist of showing that the method produces the same results as vanilla backprop.

I'm not sure this is that important for understanding biological neural systems. But it might enable mass parallelism!
Is it cool to call your own work a "breakthrough", or does that just engender skepticism?
Unfortunately that is what is often recommended to win grants and get considered for the best journals. But yes, it should draw scepticism. But then again, scepticism is anyway part of the game in science, always.
What does this suggest about biological neural systems?
Predictive coding is based on a theory for brain operation that grounds in the ubiquity of recurrent connection between functional brain regions, where the forward pass transmits the prediction, and the backward pass transmits the error. Unlike backprop, this theory uses only local information, and is therefore somewhat more compatible with how the brain works.

There is a huge body of work about this. The current paper seems to show that the result of predictive coding is equivalent to the result one gets with backprop.

I think it says more about predictive coding as a way to train neural networks rather than biological systems.

There's been an argument that schemes relying on backpropagation can't provide insight into biological neural systems, but this argument is weakened by the existence of predictive coding equivalents.