🧠 Biologically plausible learning without backpropagation fails on deep networks, and the real culprit is gradient rank collapse. Fixing it with geometric interventions lifts accuracy from 1.4% to 46.1%.
Title: Overcoming Rank Collapse in Feedback Alignment
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📝 Overview
Feedback alignment (FA) is a biologically plausible alternative to backpropagation. It avoids symmetric weight transport, using the same forward weights in the backward pass, and instead learns with fixed random feedback weights. This paper identifies why FA fails to scale to deep architectures, gradient rank collapse, and proposes how to overcome it.
❓ Challenges Solved
The FA error has considerably lower rank than backpropagation and is confined to a lower-dimensional subspace. This rank collapse severely limits parameter-space exploration and is the root cause of weight alignment failing to progress during training.
💡 Methodology & Proposed Approach
It proposes two complementary mechanisms to raise gradient dimensionality.
・The Muon optimizer, based on orthogonalization, sets every singular value to 1. It flattens the spectrum of momentum updates to emphasize previously neglected directions, rewriting the update geometry itself
・Batch normalization normalizes hidden-layer activations to promote activation orthogonality and maintain high-dimensional representations across layers
・Muon equalizes update geometry while batch norm preserves representational dimensionality; together they maintain richer learning directions throughout training, so alignment emerges even in deeper networks
🎯 Use Cases
It is relevant to backprop-free learning, brain-inspired neuromorphic computing, and research on optimization trajectory dimensionality in general.
📊 Experimental Results
・It was evaluated on CIFAR-10, CIFAR-100, STL-10, and Tiny ImageNet with AlexNet and ResNet-18
・On CIFAR-100 with ResNet-18, baseline FA reached just 1.4%, while FA plus batch norm hit 37.1% and FA plus Muon hit 25.3%
・FA plus Muon plus batch norm reached 46.1%, about a 9-point improvement over batch norm alone
・Similar gains were confirmed across all datasets and architectures
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DeepLearning# #
Neuroscience#